The Reliability and Validity of Pain Interference Measures in Persons with Multiple Sclerosis

Transcription

1 Vol. 32 No. 3 September 2006 Journal of Pain and Symptom Management 217 Original Article The Reliability and Validity of Pain Interference Measures in Persons with Multiple Sclerosis Travis L. Osborne, PhD, Katherine A. Raichle, PhD, Mark P. Jensen, PhD, Dawn M. Ehde, PhD, and George Kraft, MD Department of Rehabilitation Medicine (T.L.O., K.A.R., M.P.J., D.M.E., G.K.), University of Washington School of Medicine; and Multidisciplinary Pain Center (M.P.J.), University of Washington Medical Center, Seattle, Washington, USA Abstract Valid measures of pain-related interference with functioning could serve as useful outcome measures in much needed clinical trials of pain treatments for persons with multiple sclerosis (MS). The purpose of this study was to examine the psychometric properties of two pain interference measures in persons with MS and chronic pain. Modified versions of the Interference scale of the Brief Pain Inventory (BPI) and the Disability scale of the Graded Chronic Pain Scale were administered via a mailed survey to 187 community-dwelling persons with MS. Data from the 125 participants who reported pain were analyzed. Although both measures demonstrated excellent internal consistency, in the current sample, evidence regarding the construct and concurrent validity was stronger for the modified versions of the BPI Interference scale. These results provide preliminary support for the reliability and validity of modified versions of the BPI Interference scale in persons with MS and chronic pain. J Pain Symptom Manage 2006;32:217e229. Ó 2006 U.S. Cancer Pain Relief Committee. Published by Elsevier Inc. All rights reserved. Key Words Multiple sclerosis, Brief Pain Inventory, Graded Chronic Pain Scale, pain interference, reliability, validity, psychometric This research was supported by a grant (Management of Chronic Pain in Rehabilitation, PO1 HD/NS33988) from the National Institutes of Health, National Institute of Child Health and Human Development (National Center for Medical Rehabilitation Research), and the National Institute of Neurological Disorders and Stroke. Address reprint requests to: Travis L. Osborne, PhD, Department of Rehabilitation Medicine, Box , 1959 NE Pacific St., University of Washington School of Medicine, Seattle, WA , USA. osbornet@u.washington.edu. Accepted for publication: March 3, Ó 2006 U.S. Cancer Pain Relief Committee Published by Elsevier Inc. All rights reserved. Introduction Pain is a common problem among persons with multiple sclerosis (MS), with the majority of studies indicating prevalence rates of pain between 43% and 80%. 1e10 MS-related pain can be acute or chronic 2,3,5,6 and can be neuropathic or musculoskeletal in nature. 11 Persons with MS and pain typically describe their pain intensity (or severity) as mild to moderate, 4,8,12,13 on average, and frequently experience pain in more than one bodily location. 4,10 Evidence suggests that pain in MS may be associated with decreased rates of /06/$esee front matter doi: /j.jpainsymman

2 218 Osborne et al. Vol. 32 No. 3 September 2006 employment, 14 poorer general health, 2 and poorer psychological functioning. 2,4 However, pain is not the only symptom associated with MS. Other common symptoms include fatigue, problems with mobility, bowel and bladder problems, visual disturbances or blindness, cognitive problems, and tremors. 15 In short, persons with MS are at risk for experiencing functional impairment associated with MS itself, as well as additional impairment associated with a large number of secondary symptoms, including pain. Although the presence of pain symptoms can contribute to greater functional impairment in persons with MS, few studies have examined pain-related interference with functioning in this population. Those studies that have examined this issue have tended to use single-item measures that focus broadly on pain interference with daily activities 2,8 or nonstandardized questions to assess the impact of pain on various aspects of functioning (e.g., ability to work, ability to fulfill various social roles). 4 To our knowledge, only one published study has used a standardized measure that assesses pain-related interference with a range of functional domains (e.g., mood, mobility, sleep, recreational activities, work). 16 This study found that greater overall pain interference, as measured by the Medical Outcomes Study Pain Effects Scale (PES), 17 was associated with increased fatigue, poorer selfreported global health, and more severe depressive symptom severity in a large, community-dwelling sample of veterans with MS. The PES is a component of the MS Quality of Life Inventory (MSQLI) 18,19 and has established reliability and validity in several samples of persons with MS. 20e22 Although psychometric support for use of the PES in persons with MS has been available for several years, the scale does not appear to have been widely used in research on MS and pain. Instead, the scale is usually administered as part of the larger MSQLI in research examining quality of life in persons with MS. One potential reason for the lack of use of the PES in MS pain research is that the measure has not been widely used in the broader chronic pain literature. Instead, the research on pain interference in persons with chronic pain has mostly used the Brief Pain Inventory (BPI) Interference scale. 23,24 The BPI was initially developed to assess pain severity and interference in persons with cancer and has been extensively validated in samples of persons with cancer pain 23e31 and noncancer pain. 32e35 The seven-item BPI Interference scale assesses the extent to which pain interferes with general activity, mood, walking, work, relationship with others, sleep, and enjoyment of life. Thus, like the PES, it can provide useful information about the impact of pain on a broad range of functional domains. Modified versions of the BPI Interference scale have been found to be valid and reliable for assessing pain secondary to several types of physical disabilities. More specifically, the measure has demonstrated good psychometric properties in samples of persons with spinal cord injury and pain, 36 as well as cerebral palsy and pain. 37 To our knowledge, however, no prior studies have examined the psychometric properties of the BPI Interference scale in a sample of persons with MS and pain. Although psychometric support is available for the PES in this population and the PES was adapted from an early version of the BPI, there are several advantages to using the BPI Interference scale instead of the PES in research on MS-related pain. First, the scale has been widely used in research on chronic pain in other populations. Therefore, the use of this scale in MS research would allow for comparisons to samples of persons with other types of pain problems. Second, the BPI Interference scale has been shown to be sensitive to changes in pain in response to treatment. 33 As such, the scale could serve as a useful outcome measure in research examining the efficacy of pharmacological and psychosocial treatments for pain in persons with MS. Given the dearth of clinical trial research in this area, 38 such outcome measures are greatly needed. In addition to the BPI Interference scale, the psychometric properties of the Disability scale of the Graded Chronic Pain Scale (GCP) 39 have also been examined in several samples of persons with pain secondary to a physical disability. 36,37 The GCP scale was developed as a method to grade chronic pain severity along two dimensions (pain intensity and pain interference) and was originally validated on a large sample of primary care

3 Vol. 32 No. 3 September 2006 Pain Interference Measures in Persons with MS 219 patients with back pain, headache, and temporomandibular disorder pain. 39 A subsequent study provided further validation of the three-item pain interference (Disability) scale for use in persons with back pain. 40 The GCP Disability scale assesses the extent to which pain interferes with daily activities and has changed one s ability to take part in recreational, social, and family activities, as well as one s ability to work. The scale has been found to have adequate reliability and validity in a sample of persons with pain secondary to spinal cord injury, 36 but not in a sample of persons with cerebral palsy and pain. 37 To our knowledge, however, the psychometric properties of the GCP Disability scale have not been examined in persons with MS-related pain. Examining the reliability and validity of this scale in a sample of persons with MS and pain may help clarify whether it is suitable for use in persons with pain secondary to a disability. If shown to be valid for use in persons with MS, it could provide an alternative and briefer measure of pain-related interference with functioning than the BPI Interference scale. The purpose of this study was to examine the psychometric properties of the BPI Interference scale and the GCP Disability scale in a sample of persons with MS and pain. The primary aim was to determine whether these measures are appropriate for use in this population. A secondary aim was to replicate the findings from previous studies indicating that the BPI Interference scale is valid for use in persons with pain secondary to a disability and to clarify the discrepant findings related to the validity of the GCP Disability scale. The final aim was to compare the reliability and validity of these measures in a sample of persons with MS and pain. Materials and Methods Participants Participants were drawn from a sample of persons with MS who completed a survey described as examining quality of life in persons with disabilities, which included a number of pain-related assessment measures. Individuals were recruited for the survey study in two primary ways. The majority of potential participants were randomly selected from a group of persons who had previously been contacted through the Multiple Sclerosis Association (MSA) of King County, WA as part of a survey that assessed multiple dimensions of MS 2,41,42 and indicated that they would be willing to be contacted about future research opportunities. Potential participants also contacted our research office (in response to fliers or referrals from health care providers) to inquire about research opportunities related to quality of life in persons with disabilities. The current survey was mailed to 300 individuals (287 surveys were sent to individuals randomly selected from the prior MSA and an additional 13 surveys were sent to individuals who contacted our research office directly). A consent form and a cover letter inviting potential study participants to participate in the study accompanied each survey. Respondents were paid $25 for completing and returning the consent forms and survey. The University of Washington Human Subjects Review Committee approved all study procedures. Of the 300 surveys sent, 13 were returned due to incorrect addresses, 4 individuals were deceased, and another 8 individuals were not eligible to participate in the study because they indicated that they did not have MS. Of the 275 remaining possible participants, 187 returned completed surveys, for a response rate of 68.0%. As the current study sought to evaluate the psychometric properties of a measure of pain interference, only participants who indicated a current or recent pain problem were included in the study s analyses (n ¼ 125). (It should be noted that of the 187 participants in the total sample, 180 individuals were among those randomly selected from the prior MSA sample. Demographic data and detailed information about the prevalence and nature of pain problems reported by these individuals, and which are beyond the scope of the current study, were recently reported. 14 The sample [n ¼ 180] was larger than the sample in the current study because it included survey respondents with and without pain.) The current sample (n ¼ 125) was predominantly female (75.2%), with a mean age of years (SD ¼ years). The majority of participants reported their ethnicity as Caucasian (96.8%), with the remaining participants reporting their ethnicity as Asian-American (1.6%), Native American (1.6%), African- American (0.8%), Hispanic/Chicano (0.8%),

4 220 Osborne et al. Vol. 32 No. 3 September 2006 and Pacific Islander (0.8%). Most respondents were either married or living with a significant other (68.0%). Only a quarter of the sample was employed full or part time (25.6%). Regarding the education level of the sample, relatively few participants had only a high school education or General Education Development equivalent (10.4%), or did not graduate from high school (1.6%). The majority of participants were either college graduates (29.6%) or had attended graduate school (24.0%), with the remaining participants having completed some college (28.8%) or vocational/technical training (5.6%). With regard to MS-related characteristics, participants reported a range of MS disease courses: 53.3% relapsing/remitting, 23.8% secondary progressive, 13.1% primary progressive, and 9.8% progressive relapsing. Although the majority of participants reported mild (Expanded Disability Status Scale [EDSS] score of 0e4.0) or moderate (EDSS score of 4.5e6.0) disease severity (27.9% and 22.1%, respectively), half of the sample (50.0%) reported severe disease progression (EDSS score of 6.5e9.5). The mean number of years since MS diagnosis in the sample was (SD ¼ 10.04). Participants reported having pain symptoms for an average of 8.75 years (SD ¼ 8.88). Regarding average pain severity in the past week, 1.6% reported no pain (0 on 0e10 rating scale), 46.4% reported mild pain (1e4), 27.2% reported moderate pain (5e6), and 24.8% reported severe pain (8e10). Measures Demographics. Several questions assessed demographic characteristics, including gender, age, race/ethnicity, marital status, education level, and employment status. MS-Related Factors. The MS disease-related characteristics assessed included MS disease course, MS disease progression, and disease duration. MS disease course was assessed by asking participants to select a pictoral graph that most closely corresponded to their disease course over time. 43 These graphs were accompanied by written descriptions of the four clinical courses of MS that were recognized at the time of the study: relapsing/remitting, secondary progressive, primary progressive, and progressive relapsing. 43 The self-report version 44 of the EDSS 45 was used to assess disease progression and degree of neurological impairment. The self-report version of the EDSS is highly correlated (r ¼ 0.89) with the physicianadministered EDSS. 44 In the current study, EDSS scores were categorized as mild (0e4.0), moderate (4.5e6.0), and severe (6.5e9.5) to reflect milestones in progressive loss of functioning. Finally, the survey included a question assessing the date of MS diagnosis to determine disease duration. Pain. Survey respondents were asked to indicate the presence or absence of any recent pain problem(s) by answering the following question: Are you currently experiencing, or have you in the past three months experienced, any pain (other than occasional headaches or menstrual cramps)? Participants who responded yes to this question were then asked to make several pain intensity ratingsdcurrent pain, average pain in the past week, and worst pain in the past weekdeach on a 0e10 Numeric Rating Scale (NRS), with 0 ¼ No pain and 10 ¼ Pain as bad as could be. Such 0e10 scales have demonstrated their validity as measures of pain by their strong association with other measures of pain intensity, as well as by their responsivity to pain treatments. 46 These items were adapted from pain intensity items on the GCP. 39 We changed the time frame for the ratings of average and worst pain intensity from the past 6 months to the past week to be consistent with the other pain measures (described below). Pain-related interference with various domains of functioning was assessed using a modified version of the BPI Interference scale. 23,24 The original version of this scale asks respondents to rate the degree to which they perceive that pain interferes with seven activities of daily life, including general activity, mood, walking ability, normal work, relations with other people, sleep, and enjoyment of life. We modified Item 3 ( Walking ability ) to read Mobility (ability to get around), given that many participants in our sample cannot walk. The original 7-item BPI Interference scale has demonstrated excellent internal consistency and validity in samples of persons with cancer pain 23 and chronic nonmalignant pain, 33 as well as arthritis and low back pain. 32

5 Vol. 32 No. 3 September 2006 Pain Interference Measures in Persons with MS 221 In our prior research in persons with disabilities and pain, 37,47,48 we have added three items to the BPI Interference scale (yielding a 10-item scale) to assess perceived pain interference with additional areas of functioning: self-care, recreational activities, and social activities. These items were added to the scale to assess a wider range of functional domains important to persons with disabilities that can be affected by pain. The modified 10- item version of the scale has demonstrated excellent internal consistency (Cronbach a ranging from 0.89 to 0.95) and validity through its strong associations with pain intensity in samples of persons with cerebral palsy, 37 limb loss, 47 and spinal cord injury. 48 More recently, we have added two more items to the BPI Interference scale to further broaden the assessment of pain interference, thus, creating a 12-item version of the scale. These additional items assess pain-related interference with 1) communication with others, and 2) learning new information and skills. Preliminary data from a sample of persons with spinal cord injury and pain appear to provide sound psychometric support for this version of the scale. 36 The five items in total that we have added to the BPI Interference scale extend the functional domains assessed by the measure to those defined as relevant and unique by the World Health Organization s (WHO) International Classification of Functioning (ICF), Disability, and Health. 49 The current study examined the psychometric properties of the modified original 7-item version of the BPI Interference scale, as well as the expanded 10- and 12-item modified versions of the scale, as none of the versions of the scale have previously been evaluated in a sample of persons with MS and pain. For all three versions, the total interference score is calculated by averaging the ratings for the items that comprise each scale. The possible range for all three scales is 0e10, with higher scores indicating a greater degree of pain-related interference with functioning. A modified version of the three-item Disability scale from the GCP 39 was also used to assess pain-related interference with functioning. The first item asks respondents to rate the degree to which pain interferes with daily activities on a 0e10 scale (0 ¼ No interference, 10 ¼ Unable to carry on activities ). The other two items assess change in two specific areas of functioning due to pain: 1) recreational, social, and family activities, and 2) work (including housework). Responses for both items are made on a 0e10 scale (0 ¼ No change, 10 ¼ Extreme change ). We modified the three GCP Disability items in two ways. First, we changed the item instructions by asking that respondents make ratings based on the past week instead of the past 6 months to be consistent with the time frame used for the BPI Interference scale. Second, we calculated a total Disability scale score by averaging the ratings for the three items that comprise the scale. This yielded a possible range for the scale of 0e10 (with higher scores indicating greater pain-related disability) and allowed for direct comparisons with scores from the BPI Interference scale. The GCP Disability scale has demonstrated excellent internal consistency (a ¼ 0.89) and test-retest reliability (0.85 over a 1- to 2-week period) in a sample of persons with chronic low back pain. 40 Prior studies by our research group examining the psychometric properties of the scale in samples of persons with disabilities and pain indicated that the internal consistency and validity of the scale were stronger in a sample of persons with spinal cord injury and pain (a ¼ 0.93) 36 than in a sample of persons with cerebral palsy and pain (a ¼ 0.59). 37 Psychological Functioning. The five-item Mental Health scale from the SF was used to assess global psychological functioning. The Mental Health scale is strongly correlated (r ¼ 0.95) with the longer 38-item Mental Health scale from which it was derived and is a commonly used measure of psychological functioning with established internal consistency (0.81e0.95) and test-retest reliability (0.75e0.80). 50 Scores on the Mental Health scale range from 0 to 100, with higher scores indicating better psychological functioning. Mental Health scale scores have shown a consistent pattern of association with various indices of mental health (e.g., depressive symptoms, suicidal ideation, and inpatient mental health treatment in the past 12 months) in the expected directions. 50

6 222 Osborne et al. Vol. 32 No. 3 September 2006 Data Analysis We conducted a series of analyses to evaluate the reliability and validity of the three versions of the modified BPI Interference scale (7-, 10-, and 12-item versions) and the modified GCP Disability scale in this sample of persons with MS and pain. First, we calculated Cronbach a for each scale to examine internal consistency. We evaluated the internal consistency coefficients based on published standards (e.g., coefficients below 0.60 indicating inadequate reliability and coefficients greater than 0.90 indicating excellent reliability that is appropriate for making treatment decisions). 51,52 Second, to establish construct validity, we conducted exploratory factor analyses, one for each of the three versions of the modified BPI Interference scale and one examining the modified GCP Disability scale; in each analysis, the three pain intensity ratings were also included. The purposes of these analyses were to determine whether 1) the items on each interference scale loaded onto a single factor, and 2) the items on each interference scale could be viewed as assessing a domain that was distinct from the domain assessed by the three pain intensity items. We predicted that each factor analysis would have a two-factor solution and that the pain interference/disability items would load on one factor and the pain intensity items would load on a second factor. Third, to evaluate content validity, we conducted correlation analyses to examine the associations between the pain interference items and scale scores and the measures of pain intensity and psychological functioning. We determined that a valid measure of pain interference should correlate at least moderately (>0.30) with both pain intensity and psychological functioning. Results Descriptive Data for Pain Interference Measures On the modified BPI Interference scale items, participants reported a range of pain-related interference with functioning (Table 1). The mean ratings for individual items on the scale ranged from a low of 2.48 (SD ¼ 2.76) for self-care to a high of 4.10 (SD ¼ 3.21) for sleep. The mean scores for each of the three versions of the modified BPI Interference scale were 7-item version, M ¼ 3.50 (SD ¼ 2.47); 10-item version, M ¼ 3.44 (SD ¼ 2.50); and 12-item version, M ¼ 3.34 (SD ¼ 2.47). Participants reported a more limited range of pain-related interference with functioning on the modified GCP Disability scale items. Means for the items assessing pain-related interference with daily activities, change in recreation and social activities, and change in work were 3.38 (SD ¼ 2.64), 3.18 (SD ¼ 2.98), and 3.31 (SD ¼ 3.01), respectively. The mean score for the GCP Disability scale was 3.29 (SD ¼ 2.71). Internal Consistency Internal consistency estimates were high for the three versions of the modified BPI Interference scale (7-item, a ¼ 0.93; 10-item, a ¼ 0.95; 12-item, a ¼ 0.96), as well as for the modified GCP Disability scale (a ¼ 0.94). The internal consistency estimates for the scales were within a range suggesting that all of the measures may be used for clinical decisionmaking and for detecting changes in individuals ratings of pain-related interference. 51 Factor Analyses The principal factor analysis examining the items from the modified 7-item version of the BPI Interference scale and the three pain intensity items resulted in a two-factor solution that was rotated using Direct Oblimin (Table 2). The eigenvalues for the two factors were 6.40 and 1.10 and decreased to 0.58 for the third factor. The first factor accounted for 64.0% of the variance and the second factor accounted for an additional 11.0% of the variance. Factor loadings greater than 0.40 are done in boldface in all factor analysis tables, consistent with guidelines suggested for meaningful factor loadings. 53 Each of the seven BPI Interference items had factor loadings greater than 0.40 on Factor 1 and loadings of 0.40 or lower on Factor 2. Additionally, the three pain intensity items had factor loadings greater than 0.40 on Factor 2 and weak loadings on Factor 1. As predicted, the pattern of factor loadings that emerged showed that, overall, the pain interference and pain intensity items loaded on separate factors. The principal factor analysis examining the items from the modified 10-item version of the BPI Interference scale and the three pain intensity items also resulted in a two-factor solution that was rotated with Direct Oblimin

7 Vol. 32 No. 3 September 2006 Pain Interference Measures in Persons with MS 223 Variable Table 1 Descriptive Information for Pain Interference Scales and Correlations with Measures of Pain Intensity and Psychological Functioning (n ¼ 125) Mean (SD) Correlation With Pain Intensity a Psychological Functioning a BPI Interference items General activity 3.20 (2.69) Mood 3.55 (2.84) Mobility 3.62 (3.00) b Normal work 3.65 (3.14) Relations with other people 2.74 (2.89) Sleep 4.10 (3.21) b Enjoyment of life 3.66 (2.96) Self-care 2.48 (2.76) Recreational activities 4.02 (3.29) Social activities 3.38 (3.18) Communication with others 2.50 (2.88) Learning new skills or information 3.18 (3.34) BPI Interference scale scores BPI Interference 7-item scale score 3.50 (2.47) BPI Interference 10-item scale score 3.44 (2.50) BPI Interference 12-item scale score 3.34 (2.47) GCP Disability items Interference with daily activities 3.38 (2.64) Change in recreation/social activities 3.18 (2.98) Change in ability to work 3.31 (3.01) b GCP Disability scale score 3.29 (2.71) Pain Intensity ¼ average pain intensity in the past week, 0e10 NRS; Psychological Functioning¼ SF-36 Mental Health scale, with higher scores indicating better mental health. Possible range for all BPI and GCP individual items and scale scores is 0e10. a P < except where indicated. b P < (Table 3). The eigenvalues for the two factors were 8.40 and 1.20 and decreased to 0.61 for the third factor. The first factor accounted for 64.6% of the variance and the second factor accounted for an additional 9.2% of the variance. All 10 interference items had high factor loadings on Factor 1 (factor loadings ranging from 0.59 to 0.94) and the three pain intensity items had high factor loadings on Factor 2 (factor loadings between 0.60 and 0.92). Again, as predicted, the pattern of factor loadings supported the interpretation of Factor 1 as pain interference and Factor 2 as pain intensity. The principal factor analysis examining the items from the modified 12-item version of the BPI Interference scale and the three pain intensity items resulted in a two-factor solution that was rotated using Direct Oblimin (Table 4). The eigenvalues for the two factors were 9.45 and 1.49 and decreased to 0.69 for the third factor. The first factor accounted for 63.0% of the variance, and the second factor accounted for an additional 10.0% of the variance. The overall pattern of factor loadings for this analysis was less clear than that of the loadings from the factor analyses of the 7- and 10-item versions of the modified BPI Interference scale. Eleven of the 12 interference items had factor loadings greater than 0.40 on Factor 1, but 2 of these 11 items also had factor loadings greater than 0.40 on Factor 2. For both of these items, however, the loadings on Factor 1 were slighter higher than those for Factor 2. One of the original BPI Interference items, general activity, loaded more strongly on Factor 2 (factor loading of 0.58) than Factor 1 (factor loading of 0.39). All three of the pain intensity items loaded strongly on Factor 2 (factor loadings ranging from 0.68 to 0.88) and weakly on Factor 1 (factor loadings ranging from 0.06 to 0.17). In sum, although a two-factor solution emerged as predicted, the pain interference and pain intensity factors were not as distinct from one another than was observed in the factor analyses examining the 7- and 10-item versions of modified BPI Interference scale.

8 224 Osborne et al. Vol. 32 No. 3 September 2006 Table 2 Factor Analysis Results for the Modified 7-Item Version of the BPI Interference Scale and Three Pain Intensity Items Factor 1 Factor 2 BPI Interference items General activity Mood Mobility Normal work Relations with other people Sleep Enjoyment of life Pain intensity items Current pain Average pain Worst pain Principal Axis Factoring was conducted with a Direct Oblimin rotation. Eigenvalues for Factors 1 and 2 were 6.40 and 1.10, respectively, decreasing to 0.58 for the third factor. Factor loadings greater than 0.40 are in bold, consistent with guidelines suggested for meaningful factor loadings. The principal factor analysis examining the three modified GCP Disability items and the three pain intensity items resulted in a one-factor solution (Table 5). The first eigenvalue from this analysis was 4.49 and the second was This first factor accounted for 74.9% of the total variance. The loadings on this factor for the three pain-related disability items ranged from 0.85 to 0.88 and the loadings for the three pain intensity items ranged from 0.78 to The results of this factor analysis Table 3 Factor Analysis Results for the Modified 10-Item Version of the BPI Interference Scale and Three Pain Intensity Items Factor 1 Factor 2 BPI Interference items General activity Mood Mobility Normal work Relations with other people Sleep Enjoyment of life Self-care Recreational activities Social activities Pain intensity items Current pain Average pain Worst pain Principal Axis Factoring was conducted with a Direct Oblimin rotation. Eigenvalues for factors 1 and 2 were 8.40 and 1.20, respectively, decreasing to 0.61 for the third factor. Factor loadings greater than 0.40 are in bold, consistent with guidelines suggested for meaningful factor loadings. Table 4 Factor Analysis Results for the Modified 12-Item Version of the BPI Interference Scale and Three Pain Intensity Items Factor 1 Factor 2 BPI Interference items General activity Mood Mobility Normal work Relations with other people Sleep Enjoyment of life Self-care Recreational activities Social activities Communication with others Learning new skills or information Pain intensity items Current pain Average pain Worst pain Principal Axis Factoring was conducted with a Direct Oblimin rotation. Eigenvalues for factors 1 and 2 were 9.45 and 1.49, respectively, decreasing to 0.69 for the third factor. Factor loadings greater than 0.40 are in bold, consistent with guidelines suggested for meaningful factor loadings. did not support our hypothesis that the GCP pain-related disability items and the pain intensity items would load on separate factors. Associations with Pain Intensity Pearson correlation coefficients between the pain interference items and scale scores and ratings of average pain intensity during the past week are presented in Table 1. These analyses indicate that the first seven items from the BPI Interference scale were each significantly associated with average pain intensity, with correlations ranging from 0.42 to Additionally, each of the five items that we added to this scale was also significantly associated with average pain intensity. However, the correlation coefficients for the two items that were added for the 12-item version of the scale (communication with others and learning new information and skills) demonstrated the weakest associations with pain intensity of all the modified BPI Interference items (rs ¼ 0.38 and 0.35, respectively). The scale scores for each of the three versions of the modified BPI Interference scale were all significantly related to average pain intensity (coefficients ranging from 0.61 to 0.63). Regarding the modified GCP Disability scale, all three items, as well as the total scale score,

9 Vol. 32 No. 3 September 2006 Pain Interference Measures in Persons with MS 225 Table 5 Factor Analysis Results for the Modified GCP Disability Scale Items and Three Pain Intensity Items Factor 1 GCP Disability items Interference with daily activities 0.85 Change in recreation/social activities 0.88 Change in ability to work 0.88 Pain intensity items Current pain 0.78 Average pain 0.81 Worst pain 0.82 Principal Axis Factoring was conducted. The eigenvalue for Factor 1 was 4.49, decreasing to 0.68 for the second factor. Factor loadings greater than 0.40 are in bold, consistent with guidelines suggested for meaningful factor loadings. were significantly and strongly associated with average pain intensity in the past week (correlations ranging from 0.62 to 0.67). A series of t-test analyses 54 indicated no significant differences in the strength of the correlation between average pain intensity and the modified GCP Disability scale score and correlations between average pain intensity and the scale scores for each of the three versions of the modified BPI Interference scale. Associations with Psychological Functioning Correlations between the pain interference items and scale scores and global psychological functioning, as measured by the SF-36 Mental Health scale, are presented in Table 1. The first seven items of the BPI Interference scale were all significantly and negatively associated with mental health, as were the five items that we added to the scale. In all cases, greater pain-related interference with functioning was associated with poorer psychological functioning. The item assessing pain-related interference with mood demonstrated the strongest association with psychological functioning (r ¼ 0.59), whereas the item assessing painrelated interference with mobility demonstrated the weakest association with psychological functioning (r ¼ 0.24). Moreover, the scale scores for each of the three versions of the modified BPI Interference scale were also significantly and negatively related to global psychological functioning (correlations ranging from 0.47 to 0.51). Similarly, the individual items and the scale score for the modified GCP Disability scale were all significantly and negatively related to global psychological functioning. However, the degree of association between pain interference and psychological functioning was weaker overall for the modified GCP Disability scale than for the modified BPI Interference scales. The results from a series of t-tests 54 indicated that the strength of the correlation between psychological functioning and the modified GCP Disability scale score (r ¼ 0.31) was significantly weaker than the strength of the correlations between psychological functioning and the scale scores for the 7-item (r ¼ 0.47; t (122) ¼ 3.60, P < 0.01), 10- item (r ¼ 0.48; t (122) ¼ 4.16, P < 0.01), and 12-item (r ¼ 0.51; t (122) ¼ 4.69, P < 0.01) versions of the modified BPI Interference scale. Discussion The results of this study suggest that the BPI Interference scale may be a more valid measure than the GCP Disability scale for assessing pain-related interference with functioning in persons with MS. Specifically, despite comparable reliability, findings concerning the construct and concurrent validity of the modified BPI Interference scales and GCP Disability scale indicate that the former was more psychometrically sound in the current sample. All three versions of the modified BPI Interference scale, as well as the modified version of the GCP Disability scale, demonstrated excellent internal consistency in the current sample. In all cases, Cronbach alphas were greater than 0.90, suggesting that the scales are appropriate for use in research 52 and for making clinical treatment decisions. 51 Regarding construct validity, factor analyses that included pain interference and intensity items yielded a two-factor solution for all three versions of the modified BPI Interference scale (7-item, 10-item, and 12-item versions). In each case, pain interference items loaded predominantly on one factor and pain intensity items loaded strongly on a second factor. These findings are consistent with those of the numerous studies that have examined the factor structure of the BPI and found that the scale s pain intensity and interference items load on two distinct factors. 26,29,30,32,33,55

10 226 Osborne et al. Vol. 32 No. 3 September 2006 In contrast, the three pain-related disability items from the modified GCP Disability scale loaded on the same factor as the pain intensity items. Interestingly, this single-factor solution is consistent with the findings of a study that examined the validity of the GCP in postal research, indicating that the items from the measure s pain intensity and disability scales all loaded strongly on one factor. 56 Those findings concerning the GCP, coupled with the current results, suggest that the GCP scale (which includes both pain intensity and disability items) measures one underlying construct and that the Disability scale items do not necessarily measure a construct that is distinct from pain intensity. With regard to concurrent validity, we examined the associations between the three modified BPI Interference scales (7-item, 10- item, and 12-item versions) and the modified GCP Disability scale and measures of pain intensity and psychological functioning. We determined that the pain interference scales should correlated at least moderately (>0.30) with these measures. All three versions of the modified BPI Interference scale, as well as the modified GCP Disability scale, correlated strongly with average pain intensity ratings (correlation coefficients ranging from 0.61 to 0.67). In addition, the three versions of the modified BPI Interference scale all correlated at least moderately with global psychological functioning scores (correlation coefficients ranging from 0.47 to 0.51). Although the correlation between the modified GCP Disability scale and psychological functioning met our minimum criteria (r ¼ 0.31), the strength of this relationship was significantly weaker than the strength of the association between psychological functioning and all three of the modified BPI Interference scales. Based on the current findings, it appears that the modified version of the GCP Disability scale may be less adequate than the modified BPI Interference scales for assessing pain-related interference with functioning in persons with MS. Although the GCP Disability scale did demonstrate excellent reliability and was strongly related to average pain intensity, the scale items were not distinct from several items assessing pain intensity, suggesting that the GCP Disability scale items may lack discriminative validity. To the extent that assessing pain interference as a distinct domain is important, then the GCP Disability scale items appear to have limited utility for this purpose. In contrast, this study provides strong preliminary support for the use of the modified versions of the BPI Interference scale for assessing pain interference in persons with MS. These findings are generally consistent with those of studies examining the validity of the BPI Interference scale in persons with spinal cord injury and pain 36 and cerebral palsy and pain. 37 All three versions of the modified BPI Interference scale (7-item, 10- item, and 12-item versions) demonstrated excellent internal consistency, showed moderate to strong associations with average pain intensity and global psychological functioning, and the items from the scale were, in general, found to be distinct from the pain intensity items. Although many of the psychometric properties were similar across the three versions of the modified BPI Interference scale, factor analytic findings differed somewhat across the scales. Specifically, the analysis examining the modified 12-item version of the scale indicated that several items (general activity, mobility, normal work) cross-loaded on the pain intensity factor. Although the general activity item had somewhat high factor loadings on the pain intensity factor in all three of the BPI-related factor analyses, the pattern of factor loadings indicated that overall, the pain interference items were less distinct from the pain intensity items when examining the modified 12-item version of the scale. Although it would be necessary to determine if this finding replicates in other samples of persons with MS and chronic pain, a conservative recommendation would be to use the modified 7-item or 10-item versions of the scale in this population. One potential advantage to using the 10-item modified version is that it assesses more of the domains of functioning deemed as relevant according to the WHO ICF recommendations 49 than the original 7-item scale, and may, therefore, be particularly helpful when assessing pain-related interference with functioning in persons with disabilities and pain. There are several limitations of the current study. First, only cross-sectional data were available for the present analyses, so we were unable to evaluate the test-retest reliability of

11 Vol. 32 No. 3 September 2006 Pain Interference Measures in Persons with MS 227 the measures examined or determine whether the measures are sensitive to changes in pain over time or in response to treatment. Longitudinal research is needed in persons with MS and pain to address these important issues. Given the lack of longitudinal research in the broader MS and pain literature, such studies are particularly important for advancing our understanding of pain and pain-related interference with functioning in persons with MS. Second, only self-report measures were included in the study. It is possible that some of the significant associations observed between variables are a function of self-report assessment bias. However, the similarities between the current findings and those from other studies examining the psychometric properties of the BPI Interference scale suggest that our findings are not spurious. Nevertheless, additional research is needed that includes data from other informants, including health care providers, significant others, and/or caregivers, to further establish the concurrent validity of the measures evaluated. Despite these limitations, the current study addresses a significant gap in the MS and pain literature by providing evidence for the validity of a widely used measure of pain interference, the BPI Interference scale, in persons with MS. Additionally, the findings provide further support that this measure is appropriate for use in persons with disabilities and pain. Clinically, this measure may provide important information about the impact of pain on functioning to health care providers working with persons with MS. Moreover, well-validated measures of pain interference are integral to research on pain treatment outcomes. Given the substantial need for such research regarding MS-related pain, validating pain interference measures in this population is an important part of establishing an adequate foundation for future treatment studies. Acknowledgments The authors gratefully acknowledge the contributions of Amy Hoffman, Lindsay Washington, Emily Phelps, Laura Nishimura, Kristin McArthur, Kevin Gertz, Noel Pereyra-Johnston, Silvia Amtmann, Joe Skala, and Kerry Madrone, University of Washington Department of Rehabilitation Medicine, in data collection and database management. References 1. Solaro C, Brichetto G, Amato MP, et al. The prevalence of pain in multiple sclerosis: a multicenter cross-sectional study. Neurology 2004;63(5): 919e Ehde DM, Gibbons LE, Chwastiak L, et al. Chronic pain in a large community sample of persons with multiple sclerosis. Mult Scler 2003; 9(6):605e Stenager E, Knudsen L, Jensen K. Acute and chronic pain syndromes in multiple sclerosis. Acta Neurol Scand 1991;84(3):197e Archibald CJ, McGrath PJ, Ritvo PG, et al. Pain prevalence, severity and impact in a clinic sample of multiple sclerosis patients. Pain 1994;58(1): 89e Moulin DE. Pain in multiple sclerosis. Neurol Clin 1989;7(2):321e Indaco A, Iachetta C, Nappi C, Socci L, Carrieri PB. Chronic and acute pain syndromes in patients with multiple sclerosis. Acta Neurol (Napoli) 1994;16(3):97e Goodin DS. Survey of multiple sclerosis in northern California. Northern California MS Study Group. Mult Scler 1999;5(2):78e Beiske AG, Pedersen ED, Czujko B, Myhr KM. Pain and sensory complaints in multiple sclerosis. Eur J Neurol 2004;11(7):479e Rae-Grant AD, Eckert NJ, Bartz S, Reed JF. Sensory symptoms of multiple sclerosis: a hidden reservoir of morbidity. Mult Scler 1999;5(3): 179e Svendsen KB, Jensen TS, Overvad K, et al. Pain in patients with multiple sclerosis: a population-- based study. Arch Neurol 2003;60(8):1089e Kassirer M. Multiple sclerosis and pain. Int J MS Care 2000;2(3):30e Ehde DM, Osborne TL, Jensen MP. Chronic pain in persons with multiple sclerosis. Phys Med Rehabil Clin N Am 2005;16(2):503e Heckman-Stone C, Stone C. Pain management techniques used by patients with multiple sclerosis. J Pain 2001;2(4):205e Ehde DM, Osborne TL, Hanley MA, Jensen MP, Kraft GH. The scope and nature of pain in persons with multiple sclerosis. Mult Scler, in press. 15. Aronson KJ. Quality of life among persons with multiple sclerosis and their caregivers. Neurology 1997;48(1):74e Osborne TL, Turner AP, Williams RM, et al. Correlates of pain interference in multiple sclerosis. Rehabil Psychol, in press.

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