FUNCTIONAL ABILITY has been identified as a crucial

Transcription

1 1141 ORIGINAL ARTICLE Factor Structure of the Pain Disability Index in Workers Compensation Claimants With Low Back Injuries Raymond C. Tait, PhD, John T. Chibnall, PhD ABSTRACT. Tait RC, Chibnall JT. Factor structure of the Pain Disability Index in workers compensation claimants with low back injuries. Arch Phys Med Rehabil 2005;86: Objective: To examine the factor structure of a telephoneadministered Pain Disability Index (PDI) and the effects of race and sex on the PDI. Design: Computer-assisted telephone interviews of a cohort with occupational low back injuries. Setting: General community. Participants: Missouri workers compensation claimants (N 1329) with low back injuries. Interventions: Not applicable. Main Outcome Measures: PDI, levels of pain severity, Social Security Disability Insurance status, and the Fear- Avoidance Behavior Questionnaire. Results: Results for the total sample and by race/sex group indicated support for a 2-factor model of the PDI corresponding to voluntary activities (eg, social, occupational, recreational) and obligatory activities (eg, activities of daily living, eating, sleeping). Additional psychometric analyses of the voluntary and obligatory subscales indicated adequate reliability and construct validity overall and in each of the race/sex groups. African Americans reported more pain-related disability on both subscales than whites. Women reported more disability on the voluntary subscale than men. Conclusions: The results support use of the PDI as a bidimensional measure of pain-related disability, with strong psychometric properties. They also support its administration by telephone. Key Words: Disability evaluation; Low back pain; Occupational medicine; Rehabilitation; Workers compensation by American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation FUNCTIONAL ABILITY has been identified as a crucial component of the assessment of any chronic pain condition. Several converging lines of evidence have led to this recognition: (1) perhaps more than any other dimension of the pain experience, interference with activity is a crucial predictor of future adjustment among patients with chronic pain 1-3 ; (2) return of function drives a wide range of favorable treatment outcomes, so that functional restoration has become a linchpin of treatment 4-8 ; and (3) insurance carriers, especially workers From the Department of Psychiatry, Saint Louis University School of Medicine, St. Louis, MO. Supported by the Agency for Healthcare Research and Quality (grant no. R01 HS ). No commercial party having a direct financial interest in the results of the research supporting this article has or will confer a benefit upon the authors or upon any organization with which the authors are associated. Reprint requests to Raymond C. Tait, PhD, Dept of Psychiatry, Saint Louis University School of Medicine, 1221 S Grand Blvd, St. Louis, MO 63104, taitrc@slu.edu /05/ $30.00/0 doi: /j.apmr compensation carriers, have prioritized return of function as an outcome to which they attach value relative to other possible outcomes, such as improved quality of life. 9,10 Despite this evidence, clinical assessment of pain-related disability often does not occur in a medical encounter, 11 putatively because of time pressures. Hence, considerable attention has been given to the development of instruments that facilitate the assessment of pain-related disability. The clinical context often drives decisions regarding which aspects of disability should be assessed. In the context of workers compensation, the criterion standard for evaluating return of function involves return to work. In the context of clinical treatment, return to work represents 1 metric against which to measure function, but it represents only 1 dimension of disability. For many patients, return to work is neither realistic nor relevant. Hence, pain treatment programs have considered disability more broadly, in terms consistent with those proposed by the Institute of Medicine:... a disadvantage for a given individual (resulting from an impairment or a functional limitation) that limits or prevents the fulfillment of a role that is normal... for that individual. 12(p17) Numerous self-report measures have been developed that measure painrelated disability, including the Sickness Impact Profile, 13 the Multi-Dimensional Pain Inventory, 14 the Roland scale, 15 the Oswestry Disability Questionnaire (ODQ), 16 the Neck Pain and Disability Scale (NPDS), 17 and the Pain Disability Index (PDI). 18 The PDI has had widespread use since its introduction because it is brief and has strong psychometric properties, including evidence for its validity, reliability, 23 and sensitivity to change. 24 Further, unlike some inventories that are condition specific (eg, the NPDS is designed for neck pain, the ODQ for low back pain [LBP]), the PDI was constructed for use with multiple types of pain conditions. Consequently, it has been included in studies of mixed chronic pain conditions, 25,26 mixed acute pain conditions, 27 chronic LBP, 28,29 painful diabetic neuropathy, 30 postherpetic neuralgia, 31 osteoporosis, 32 traumatic extremity injuries, 33 and breast cancer. 34 Despite its widespread use, questions remain about the factor structure of the PDI. Early research using exploratory principal components analysis indicated that it was composed of 2 factors, one reflecting voluntary activities (eg, social, occupational, recreational) and the other reflecting obligatory activities (eg, activities of daily living [ADLs], sleeping, eating). 19,20 Later research (also using exploratory factor analytic methods), based on much larger samples of patients presenting for multidisciplinary pain management, indicated a 1-factor solution. 22 Research using the PDI reflects this confusion: in some articles the PDI is treated as 1-dimensional, 35 whereas in others it is discussed in terms of its voluntary and obligatory subscales. 36 This study was conducted for 4 primary reasons: (1) to examine the factor structure of the PDI using confirmatory factor analytic techniques rather than exploratory analysis; (2) to perform this analysis on a large, nonclinical but disabilityrelevant sample of people with chronic pain workers compensation claimants; (3) to ascertain whether the structure of the PDI varies as a function of race (African American vs

2 1142 PAIN DISABILITY INDEX, Tait white) and sex; and (4) to evaluate the reliability and construct validity of the scores indicated by the confirmatory factor structure. A secondary purpose was to examine the psychometric properties of the PDI when administered by telephone. METHODS Sampling and Participants Data for the study were extracted from a larger study of racial and socioeconomic disparities in workers compensation treatment and outcomes. 37 All descriptions of the study population and measures are taken from the larger study. The study population was entirely first-incident workers compensation claimants with low back injuries whose claims were settled in Missouri over a 17-month period (January 1, 2001 June 1, 2002) in Jackson County (Kansas City metropolitan area), St. Louis County, and the city of St. Louis (N 3181). These regions account for 82.7% of all adult African Americans in Missouri and 30.1% of the adult whites in Missouri. 38 Across all claimants, 1942 (61.0%) were injured in St. Louis County, 622 (19.6%) in St. Louis, and 617 (19.4%) in Jackson County. The eligible population (excluding from the original total of 3181 those who did not meet the race, injury, and language criteria and those who were either deceased, institutionalized, or incarcerated) totaled 2934 claimants. Of this group, 1475 (50.3%) completed the study and 432 (14.7%) refused to participate. An additional 1027 (35.0%) could not be located, despite intensive tracing efforts. Of those who completed the study, 1073 (72.8%) were located with contact information supplied by the State of Missouri or with simple tracing to update their current telephone numbers. This group did not differ significantly from the 432 people who refused to participate on any of the workers compensation treatment and settlement variables available from the State of Missouri for the whole claimant population. These variables included the duration of the workers compensation claim date of settlement, legal representation, treatment dollars spent, temporary total disability costs, settlement award, and residual disability rating. Intensive tracing of the remaining 402 people (27.3%) who completed the study was necessary to locate current telephone numbers. This group did not differ significantly on any of the workers compensation variables from the 1027 claimants who could not be located. Geographically, the percentages of study participants (n 1475) and nonparticipants (n 1459) represented in the 3 study regions did not differ significantly. Hence, study respondents appeared to be generally representative of the total workers compensation LBP population. Procedures and Measures The institutional review board (IRB) of Saint Louis University approved the study, as did the IRB of Battelle Centers for Public Health Research and Evaluation (BC- PHRE) in St. Louis. After receiving IRB approval, attempts were made to contact the original pool of 3181 claimants by telephone using contact information provided by the State of Missouri. Claimants were eligible if they were self-identified as either African American or non-hispanic white, were fluent in English, and gave informed consent. Using computer-assisted telephone interviewing, BCPHRE personnel conducted interviews of 15 to 20 minutes duration. Participants were paid $25. As part of the telephone interview for the larger study, participants completed the PDI ,22 The PDI measures pain-related interference with role functioning in 7 areas (occupational, home/family, recreational, social, sexual, ADLs, life support), all rated on 11-point Likert-type scales (0, no disability; 10, complete disability). Other measures used to evaluate the construct validity of the PDI included pain intensity, which was assessed on 11-point scales that ranged from 0 (no pain) to 10 (pain as bad as it could be). Participants rated their worst, least, and usual pain levels over the previous 7 days 39 ; a mean of the 3 ratings was calculated. Internal consistency reliability of the 3 pain intensity indicators was.85. Participants also completed the Medical Outcomes Study 12-Item Short-Form Health Survey (SF-12), a short version of the SF-36 that yields a standardized scale score for general physical health that has established reliability and validity in patients with painrelated disorders. 40 The Fear-Avoidance Beliefs Questionnaire (FABQ) 41 measures activity fear-avoidance beliefs about the effect of physical and work activities on pain. It comprises 16 items rated on 7-point Likert-type scales (0, agree; 6, disagree). The mean of the 16 items was calculated for the total score, with higher scores indicating stronger belief that activity should be avoided. Internal consistency reliability of the FABQ was.88. A final variable was the status of the participant regarding Social Security Disability Insurance (SSDI). Participants were dichotomized into 2 groups: those who were (n 116, 8.7%) and were not (n 1211, 91.3%) receiving SSDI. Statistical Analysis One- and 2-factor confirmatory factor models of the PDI were tested using Lisrel 8.5 a for the total sample and for African Americans, whites, men, and women separately. For the 2-factor model, PDI items were divided into a voluntary subscale (items 1 5: family/home, social, recreational, occupational, sexual activities) and an obligatory subscale (items 6 and 7: independent daily activities such as hygiene, driving, dressing; basic life-support behaviors such as eating, sleeping, breathing). Maximum likelihood estimation was used. Model fit was evaluated using the chi-square goodness-of-fit index (minimum fit function), the ratio of the chi-square value to the degrees of freedom, the root mean square error of approximation (RMSEA), and the cumulative fit index (CFI). For confirmatory models, chi-square values (and the ratio of this value to the degrees of freedom) that are closer to zero indicate better fit. RMSEA values of.08 or less are considered evidence of adequate fit, 42 and a CFI greater than.90 is desirable. Differences in fit between the 2 models were estimated using a chi-square difference test. PDI scores were compared as a function of race and sex using analysis of variance (ANOVA). Effect size was estimated using the Cohen d statistic (ratio of mean difference to pooled standard deviation [SD]). Conventionally, a Cohen d of 0.2 indicates a small effect, 0.5 indicates a moderate effect, and 0.8 indicates a large effect. Multiple linear regression with simultaneous entry of predictors was used to evaluate the construct validity of the PDI. RESULTS Sample Description Complete PDI data were available for 1329 (90.1%) of the 1475 respondents. Of the sample, 797 (60.0%) were white, 532 (40.0%) were African American, 826 (62.2%) were male, and 503 (37.8%) were female. Participants averaged years of education. As a result of the low back injury, 277 (20.8%) of the participants had undergone at least 1 low back

3 PAIN DISABILITY INDEX, Tait 1143 Table 1: Confirmatory Factor Analysis Results: 1- and 2-Factor Models of the PDI Internal Consistency Reliability Sample 2 (df) 2 /df RMSEA CFI 2 Diff (df) Factor 1 Factor 2 Total (N 1329) 1-factor model 177 (14) factor model 71 (13) (1)* White (n 797) 1-factor model 137 (14) factor model 62 (13) (1)* African American (n 532) 1-factor model 61 (14) factor model 31 (13) (1)* Male (n 826) 1-factor model 146 (14) factor model 57 (13) (1)* Female (n 503) 1-factor model 57 (14) factor model 35 (13) (1)* Abbreviation: Diff, difference. *P.001. surgery. At the time of data collection, 839 participants (63.1%) were working full time, 136 (10.2%) were working part time, and 354 (26.6%) were unemployed. In age, 304 (22.9%) were between 18 and 34 years, 828 (62.4%) were between 35 and 54, and 195 (14.7%) were 55 or older. Annual household income was less than $20,000 for 548 participants (35.0%), between $20,000 and $49,999 for 544 (41.6%), and $50,000 or more for 307 (23.5%). Participants were, on average, months postsettlement of their workers compensation claim at the time of data collection. Confirmatory Factor Analyses As shown in table 1, the 2-factor models of the PDI (voluntary vs obligatory) had a better fit to the data than the 1-factor models for the total sample and as a function of race and sex. The increase in fit from 1 factor to 2 factors was especially pronounced in the total sample, in whites, and in men. Internal consistency reliabilities were adequate for the 2-factor models, ranging from.78 to.93. Descriptive Data Summary scores for the voluntary and obligatory items, consistent with the confirmatory factor models, were calculated. The voluntary score consisted of PDI items 1 to 5 (range, 0 50); the mean of this subscale in the total sample was , with a mean item score of 4.8. The obligatory score consisted of PDI items 6 and 7 (range, 0 20); the mean in the total sample was , with a mean item score of 3.5. The correlation of the voluntary and obligatory subscales was.78 in the total sample,.75 in whites,.79 in African Americans,.77 in men, and.79 in women (all P.001). As shown in table 2, race by sex ANOVAs indicated significant main effects for race for the voluntary subscale (F 1, , P.001) and for the obligatory subscale (F 1, , P.001), with African Americans reporting more disability than whites. The Cohen d for the race effect was in the small-to-moderate range:.36 for the voluntary subscale and.43 for the obligatory subscale. A main effect for sex emerged only for the voluntary subscale Table 2: PDI Descriptive Data by Race and Sex Variables Men (n 826) Women (n 503) Race Main Effect* PDI voluntary subscale White (n 797) African American (n 532) Sex main effect PDI obligatory subscale White (n 797) African American (n 532) Sex main effect NOTE. Values are mean SD. *P.001, voluntary and obligatory subscales. Subscale score is the sum of scores for items 1 to 5. P.05, voluntary subscale. Subscale score is the sum of scores for items 6 and 7.

4 1144 PAIN DISABILITY INDEX, Tait *P.001. P.01. P.05. Table 3: Multiple Regression Equations: Construct Validity of PDI Voluntary Subscale Standardized Regression Coefficients by Sample Predictors Total White African American Men Women SF-12 physical health.28*.32*.26*.23*.36* SSDI (0 no; 1 yes).08* FABQ (fear avoidance).23*.24*.23*.22*.24* Pain intensity.43*.39*.42*.47*.36* Total equation R 2.62*.63*.58*.62*.62* (F 1, , P.05), with women reporting more disability than men. The Cohen d was.23 for this effect. There were no significant interactions. Construct Validity Using multiple linear regression with simultaneous entry of predictors, the construct validity of the PDI voluntary and obligatory subscales was evaluated by predicting these scores from measures of general physical health (SF-12 physical), disability (SSDI), fear-avoidance beliefs (FABQ), and pain intensity. As shown in table 3, from 58% to 62% of the variance in the PDI voluntary subscale could be explained by the 4 predictors, with all predictors contributing significantly to the equation. This pattern was consistent across race and sex groups. As shown in table 4, the 4 predictors explained from 44% to 48% of the variance in the PDI obligatory subscale. All 4 predictors were significant for the total sample and for men. For whites, African Americans, and women, only SSDI failed to significantly predict the obligatory subscale. DISCUSSION Our results support a 2-factor structure for the PDI in people with chronic pain related to occupational low back injury. Not only did the pattern obtain to the total sample, but it also applied across race and sex subgroups. The factors representing voluntary and obligatory activities also demonstrated levels of internal consistency reliability that ranged from acceptable to excellent, which is particularly notable in light of the brevity of the obligatory subscale (2 items). Finally, associations between the subscales and measures of pain, disability, activity avoidance, and general physical health supported the construct validity of the subscales. A secondary consequence of these results is that the methods of administering the PDI can be expanded to include telephone administration, a potential advantage in prospective studies using telephone follow-up or community-based studies of pain-related disability. These results provide strong support, together with the literature on the PDI, for the usefulness of the PDI in outcomes research for pain. Given the strong psychometric properties of the PDI, its bidimensional measurement of disability, its brevity, and its ease of administration, it is also useful in clinical settings. As a screening tool, the PDI can be used to identify patients reporting moderate to high levels of pain-related disability, who could then be targeted for more detailed clinical assessment to identify disability-augmenting factors, including psychologic and psychosocial issues As a tracking measure, the PDI could be used to facilitate evidence-based medical decision making. If administered routinely (either in the office or by telephone) to patients recovering from a painful injury, deviations in progress (eg, a slower-thananticipated recovery) could be readily identified. Patients with self-limiting injuries could be easily discriminated from those with problematic recoveries, based on changes in PDI levels. Patients exhibiting elevated PDI levels after a reasonable recovery time could be flagged to receive intensive, functionally oriented treatment. This balanced approach could avoid costly overtreatment while delivering more aggressive treatment without delays that could impede good outcomes. 46,47 The primary limitation of the current findings is generalizability. Our respondents were workers compensation claimants who had settled their claims for low back injury, so the results may not generalize to clinical pain populations or patients with other types of pain. Further, the study was conducted in Missouri, where workers compensation claimants cannot choose their initial treatment providers. The extent to which the results generalize to other states where claimants can do this cannot be assumed. Respondents were also either African American or white, so our results may not apply to other racial or ethnic groups, including Asians and Hispanics. Finally, the study respondents could have P.05. *P.001. Table 4: Multiple Regression Equations: Construct Validity of PDI Obligatory Subscale Standardized Regression Coefficients by Sample Predictors Total White African American Men Women SF-12 physical health.24*.24*.26*.18*.31* SSDI (0 no; 1 yes) FABQ (fear avoidance).19*.22*.18*.18*.21* Pain intensity.40*.33*.39*.44*.32* Total equation R 2.47*.44*.48*.47*.47*

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