Eighty percent of patients with chronic back pain (CBP)

SPINE Volume 37, Number 8, pp 711 715 2012, Lippincott Williams & Wilkins HEALTH SERVICES RESEARCH Responsiveness and Minimal Clinically Important Change of the Pain Disability Index in Patients With Chronic Back Pain Remko Soer, PhD, * Michiel F. Reneman, PhD, Patrick C. A. J. Vroomen, MD, PhD, Patrick Stegeman, MPA, * and Maarten H. Coppes, MD, PhD * Study Design. Prospective cohort study. Objective. The objective of this study was to test the responsiveness and minimal clinically important change (MCIC) of the Pain Disability Index (PDI) in patients with chronic back pain (CBP). Summary of Background Data. Treatment of patients with CBP is primarily focused on reduction of disability. For disability measurement, the PDI is a widely used questionnaire. There are, however, no data available on responsiveness and MCIC. Methods. Two hundred forty-two patients with CBP were included in this study. Patients filled in the PDI at baseline and at discharge. The PDI consists of 2 subscales: 1 measuring voluntary activities and 1 measuring obligatory activities. PDI was anchored at 2 selfreported global perceived effect (GPE) scales for complaints and selfcare, respectively. Responsiveness was considered sufficient when Area Under the Receiver Operating Characteristics (ROC) Curve (AUC) was higher than 0.70. To test interpretability, change scores and MCIC were calculated. MCIC was tested by determination of optimal cut-off point of the ROC curve and determination of specificity and sensitivity of the optimal cut-off point. Results. AUCs were 0.76 and 0.77 depending on the external criterion. The subscale obligatory activities did not meet the criteria for responsiveness (AUC: 0.63 0.69). MCIC of the PDI was 9.5 points for GPE complaints and 8.5 for GPE self-care. From the * Groningen Spine Center, University Medical Center Groningen, University of Groningen, the Netherlands ; Department for rehabilitation medicine, University Medical Center Groningen, University of Groningen, the Netherlands ; Department of Rehabilitation Medicine, University Medical Center Groningen, University of Groningen, the Netherlands ; Department of Neurology, University Medical Center Groningen, University of Groningen, the Netherlands ; and Department of Neurosurgery, University Medical Center Groningen, University of Groningen, the Netherlands. Acknowledgment date: February 10, 2011. First revision date: May 13, 2011. Acceptance date: June 25, 2011. The manuscript submitted does not contain information about medical device(s)/drug(s). No funds were received in support of this work. No benefits in any form have been or will be received from a commercial party related directly or indirectly to the subject of this manuscript. Address correspondence and reprint requests to Remko Soer, PhD, Groningen Spine Center, PO Box 30001, 9750 RB Groningen, the Netherlands; E-mail: r.soer@cvr.umcg.nl DOI: 10.1097/BRS.0b013e31822c8a7a Conclusion. The total score of the PDI as well as the subscale of voluntary activities is responsive. Partly because of floor effects, the subscale obligatory activities are not sufficiently responsive in patients with CBP. However, the responsiveness of this subscale in other patient groups should be further tested. In patients with CBP, change can be considered clinically important when PDI score has decreased 8.5 to 9.5 points. Key words: validity, psychometric properties, low back pain, measurement, chronic pain. Spine 2012 ; 37 : 711 715 Eighty percent of patients with chronic back pain (CBP) are considered to experience nonspecific back pain. This large group is characterized by a high health care consumption and substantial work disability. 1 Goals of treatment of patients with CBP vary from treating pain to decreasing disability and consequently improving functioning. Perceived change after treatment is an important measure to evaluate treatment efficacy. One of the most important changes is the change in pain-related disability. To measure disability, different standardized questionnaires can be used including the Roland-Morris Disability Questionnaire (RDQ), 2 the Quebec Back Pain Questionnaire (QBPQ), 3 and the Pain Disability Index (PDI). 4, 5 These questionnaires have demonstrated psychometric qualities and can be considered reliable and valid for self-reported disability. The RDQ and the QBPQ are primarily focused on activities and are region specific (low back). The PDI focuses on participation level and can be used for pain in all body regions. Consequently, the PDI may be better suited for evaluation of treatment based on functional goals reflecting patients values and can be administered in other patient groups, such as fibromyalgia or cancer patients, and therefore enable comparison of patients with CBP with other patients with pain. Utility of the PDI is high, because it is easy to comprehend, it can be administered in a very short time, and it consists of only 7 questions. Responsiveness and interpretation of change scores after treatment, however, are unknown for the PDI, therefore, limiting its use for clinical practice. The objective of this study was to analyze the responsiveness and minimal clinically important change (MCIC) of the PDI in patients with CBP. Spine www.spinejournal.com 711

MATERIALS AND METHODS Procedures Patients who were referred to the Groningen Spine Center of the University Medical Center Groningen in the Netherlands were administered a comprehensive set of questionnaires including the PDI, 2 global perceived effect scales (GPEs), pain measured with a numeric rating scale (NRS), work status, medication, and history. Patients gave signed informed consent for use of their anonymized data for study purposes. Patients filled in the PDI at baseline (first consult; T0) and at discharge of treatment (T1) including 2 GPEs measuring selfcare and complaints. Patients All CBP patients (pain > 3 months), referred to the Groningen Spine Center between January 2009 and May 2010, were included. The Groningen Spine Center is a university-based tertiary care center in the north of the Netherlands. Patients were excluded if they did not complete the PDI, stopped treatment before discharge, or when Dutch reading skills were insufficient to fill in the questionnaire by themselves. Dependent on which treatment was provided, subjects time to discharge varied. After T0, patients can be treated by 1 or more of the following departments: rehabilitation, neurosurgery, or anesthesiology. Interventions were chosen on the basis of indication and patient preference. Patients with nonspecific low back pain were offered outpatient multidisciplinary rehabilitation including psychology, physiotherapy, and occupational therapy. Surgery was offered to patients with specific complaints including herniated disks or stenosis. Anesthesiology was provided to those patients who had sacroiliac-blocks or clear sensitization in well-described dermatomes. Measurements Pain Disability Index The PDI is a 7-item questionnaire to investigate the magnitude of the self-reported disability in different situations such as work, leisure time, activities of daily living, and sports. The questionnaire is constructed on a 0 to 10 NRS and can be considered as an interval scale in which 0 means no disability and 10 maximum disability. The PDI measures 2 factors: factor 1 measuring voluntary activities (PDI items 1 5, family/home responsibilities, recreation, social activity, occupation, and sexual behavior) and factor 2 measuring obligatory activities (items 6 and 7; self-care and life support activity). Construct validity was sufficient with significant correlations with the Beck Depression Inventory, pain, and the State-Trait Anxiety inventory. 4, 6, 7 Internal consistency of the 2 subscales was sufficient (for factor 1, Cronbach α = 0.85; for factor 2, Cronbach α = 0.70). 4 One-week test-retest reliability is good (intraclass correlation coefficient = 0.91). 8 Global Perceived Effect Scale Two frequently used global perceived effect (GPE) scales were used as external criteria (anchors). 9 These GPEs were compared with the outcome of the PDI. Two scales were used that reflect patient perception of disability. Both scales consisted of a 7-point Likert scale ranging from 1 to 7 (1, extremely worsened ; 2, much worsened ; 3, little worsened ; 4, unchanged ; 5, little improved ; 6, much improved ; and 7, completely improved ). The perceived change scales were administered at T1. Questions being asked were as follows: How much did your treated complaints change compared with pretreatment level? (GPE complaints) How much did your self-care ability change compared with pretreatment level? (GPE self-care) The scores were clustered in 3 groups (1, worsened [extremely worsened and much worsened]; 2, unchanged [little worsened, unchanged, and little improved]; and 3, improved [much improved and completely improved]). Reliability of 11-item GPEs has been found to be excellent (intraclass correlation coefficient = 0.90). 10 GPEs significantly correlate with change on Roland Morris Disability Questionnaire (RMDQ), Oswestry Disability Questionnaire, and pain-rating scale. The RDQ is sufficiently responsive based on GPEs used in this study. Concurrent validity was sufficient. 10 Statistics Both responsiveness and MCIC were calculated according to the COnsensus-based Standards for the selection of health Measurement INstruments criteria. 11, 12 Floor and ceiling effects were considered relevant when they were larger than 15%. Responsiveness in this study was defined as the ability of the PDI to detect clinically relevant changes over time. 12, 13 Different statistics were applied to calculate responsiveness. Mean changes, 95% confidence intervals, and percentage of change were calculated. Sensitivity and specificity for change plotted by receiver operating characteristic (ROC) curve and area under the curves (AUCs) were calculated. The AUC is the probability of correctly discriminating between improved and nonimproved patients. 14 When the AUC was more than 0.70, responsiveness was considered sufficient. MCIC was measured by determination of the optimal cut-off point (OCP). This is the point of the ROC curve where the sum of sensitivity and 1-specificity is maximal. 15 Sensitivity and specificity of the OCP were computed. Sensitivity and specificity range from 0 to 1.00, in which higher numbers reflect higher sensitivity or specificity. The main objective of this study was to differentiate between improved and unchanged patients. Therefore, the group with GPE score improved was compared with the group unchanged. The GPE group worsened was not taken into analyses. RESULTS A total of 242 subjects participated in this study. Eighty-one patients were improved on GPE self-care and 87 were improved on GPE complaints. Mean age was 51 years and 53% were women. Descriptive statistics of the subjects are 712 www.spinejournal.com April 2012

TABLE 1. Characteristics of Patients (N = 242) Variable Age (yr; SD) 51.7 (13.3) Sex Males (%) 114 (47) Females (%) 128 (53) Complaints, n (%) Neck 99 (41%) Upper back 90 (37%) Low back 213 (89%) Pain on NRS (0 10) Neck 4.9 Upper back 5.0 Low back 7.1 Work status N Full-time work 59 Sick leave 24 Presenteeism* 41 No job 88 Other 30 OMPQ score 116.4 (25.0) Minimum maximum 60 173 RMDQ score 13.1 (5.2) Minimum maximum 0 23 Values are mean SD unless indicated otherwise. *Presenteeism indicates that patient is at work but not fully productive ( e.g., working at slower pace). NRS indicates numeric rating scale for pain; OMPQ, Örebro Musculoskeletal Pain Questionnaire; RMDQ, Roland Morris Disability Questionnaire. presented in Table 1. Mean duration of complaints was 10.4 ( ± 9.9) years. Responsiveness Mean changes, 95% confidence intervals, AUCs, OCPs, sensitivity, and specificity are presented in Table 2. Floor effects of the obligatory scale were present in 16.9% and 25.6% at T0 and T1, respectively. No patient reached the ceiling. No relevant ceiling or floor effects were present in other scales ( < 5%) although distribution was slightly skewed to the left. Mean changes in improved patients on GPE complaints were 15.6 ( ± 14.0) and 15.4 ( ± 14.1) on GPE self-care. Mean change of unchanged patients on GPE complaints was 2.8 ( ± 11.9) and for GPE self-care 2.3 ( ± 11.6). PDI scores at T0, T1, and change scores are presented in Table 2. AUCs of the PDI were more than 0.70 for both GPEs including the subscales. AUCs obligatory activities were for both GPEs less than 0.70 (see Table 2 ). Minimal Clinically Important Change OCP for the ROC curve was 9.5 points on the GPE complaints, with sensitivity of 0.68 and specificity of 0.73. OCP was 8.5 points on GPE self-care, with sensitivity and specificity of 0.74 and 0.70, respectively. DISCUSSION The objective of this study was to determine the responsiveness and interpretability of the PDI using 2 GPEs in a sample of patients with CBP. The results show that the PDI was responsive. For interpretation of the PDI, a change of 8.5 to 9.5 points can be considered as an MCIC depending on which GPE was used for an external criterion. Total responsiveness of the PDI was considered good except for the subscale obligatory activities (AUC < 0.70). Responsiveness of obligatory activities was insufficient for both GPEs. Within a group of patients with CBP, it may be hypothesized that many patients perceive their disability in terms of participation in life domains, sports, or occupation, rather than on activities such as standing, bending, or taking care of oneself. This hypothesis can be supported by the observation that the mean T0 score on obligatory activities (mean NRS of 3.0) is far lower than on voluntary activities (mean NRS of 8.0). Analyses showed that the floor effects of the obligatory activities were relevant (T0 16.9% to T1 25.6%); therefore, true perceived decrease in disability could be either underestimated or obligatory activities of CBP patients were mildly affected by the back problem. This hypothesis is currently not validated and it is important to test these findings in other patient groups. Another point for discussion is the use of the GPEs in this and other studies. GPEs consist of 1 scale and may therefore lose valuable individual differences in therapy goals or personal disability perceptions. Using GPEs on the basis of the exact perceived disabilities of individual patients may therefore lead to higher responsiveness ratios and lower MCIC points. Using GPEs as a gold standard, external criterion is disputable, but to our knowledge, no better alternatives exist for disability measurement. The results of this study can be considered in different subgroups of patients with back pain. Comparison of the responsiveness of the PDI to instruments measuring back pain related disability such as the RDQ or the QBPQ shows that AUCs of all 3 instruments are sufficiently responsive (AUCs > 0.70). Responsiveness of the RDQ varies between 0.76 and 0.82 using similar GPEs, which is similar to the AUCs of the PDI observed in this study. 9 For MCIC, a change of 12% to 14% in the PDI score was considered a relevant change compared with 25% change in the RMDQ score. 16 For the Quebec Back Pain Disability Scale, an AUC of 0.85 was found, with a smallest detectable change of 5 points. 17 For this study, a general population with specific and nonspecific back pain was used to test a widespread domain of patients with CBP. Choosing for a generic sample of patients with CBP enables many practitioners to use these data of responsiveness and MCIC because the data are not limited to a very specific subgroup of patients. In addition, it is not suspected that Spine www.spinejournal.com 713

TABLE 2. Responsiveness and Minimal Clinically Important Change of the PDI of Both GPEs Total PDI Voluntary Scale Obligatory Scale PDI score T0 34.6 (13.8) 28.7 (10.9) 6.0 (4.6) mean (SD) Minimummaximum 5 69 4 50 0 19 PDI score T1 27.8 (15.4) 22.6 (11.9) 5.1 (4.7) mean (SD) Minimummaximum 1 65 0 50 0 19 Mean change (SD) 6.8 (14.1)* 6.1 (11.3)* 0.8 (4.9)* 95% CI of mean 5.05 8.63 4.63 7.50 0.15 1.38 change Change (%) 24.5 27.0 17.6 GPE complaints Improved 87 (36) patients, n (%) AUC 0.76 0.75 0.68 OCP 9.5 8.5 0.5 Sensitivity 0.68 0.62 0.73 Specificity 0.73 0.74 0.61 GPE self-care Improved 81 (33) patients, n (%) AUC 0.77 0.79 0.63 OCP 8.5 7.5 0.5 Sensitivity 0.72 0.71 0.66 Specificity 0.71 0.70 0.57 Total indicates total score of the PDI (scale 0 70); voluntary, PDI subscale for voluntary activities (scale 0 50); obligatory, PDI subscale for obligatory activities (scale 0 20); PDI, Pain Disability Index; CI, confidence interval; GPE, global perceived effect; AUC, area under the receiver operating characteristic curve (ROC) curve; OCP, optimal cut-off point of the ROC curve. *Significant change between T0 and T2 ( P < 0.01). estimation processes differ between subgroups of patients with back pain. The most likely bias to be expected is bias because of differences in floor effects. This study was performed in the Netherlands. Consequently, a Dutch language version of the PDI was used. Although a forward-backward translation procedure was applied (Dr A. J. A. Koke, written communication), psychometric data concerning the Dutch language version are lacking in literature. It is unknown whether this has caused any bias and requires further research. The possibility of generalization to other countries is unknown and no literature is available. There are, however, no indications that patients from other countries estimate their disability change systematically different on the PDI or the GPE. Nevertheless, data to support this indication are lacking. A great advantage of using the PDI is the length of the questionnaire. It can be administered in less than 5 minutes and represents a large domain of patients lives. The use of participation domains such as occupation and sport enables therapists to set clear therapy goals. Accordingly, not all PDI questions may contribute to a better therapy outcome because not all questions are necessarily related to perceived disability. If the specific goals of the patient are known, specific questions could be asked as to what can be considered the minimally needed improvement for the patient to change relevantly. Using these questions as a GPE may lead to a better understanding and interpretation of patient-chosen therapy goals and the PDI as one of the disability measurement scales. To our knowledge, research has not been performed using these patient-chosen domains as a GPE. Strength of this study was the total number of patients included (N = 242). Patients were excluded if they did not fill in the PDI at discharge. This might have caused bias, because dissatisfied patients may be less willing to fill in the questionnaire. For responsiveness, however, this bias is negligible in contrast to effect studies. It can be concluded that the PDI is responsive in CBP patients and a change of 8.5 and 9.5 points can be considered clinically important. The obligatory activities scale is not responsive. Further research should focus on more patientchosen therapy goals in subgroups of back patients. Key Points The PDI is responsive for change with areas under the ROC curve of 0.76 and 0.77. A change of 8.5 to 9.5 points on the PDI is a clinically important change in patients with CBP. The subscale obligatory activities of the PDI was found not responsive for reported complaints and self-care in patients with CBP. References 1. Andersson GB. Epidemiological features of chronic low-back pain. Lancet 1999 ; 354 : 581 5. 2. Roland M, Morris R. A study of the natural history of back pain. Part I: development of a reliable and sensitive measure of disability in low-back pain. Spine (Phila Pa 1976) 1983 ; 8 : 141 4. 3. Kopec JA, Esdaile JM, Abrahamowicz M, et al. The Quebec Back Pain Disability Scale. Measurement properties. Spine (Phila Pa 1976) 1995 ; 20 : 341 52. 4. Tait RC, Chibnall JT, Krause S. The Pain Disability Index: psychometric properties. Pain 1990 ; 40 : 171 82. 5. Tait RC, Pollard CA, Margolis RB, et al. The Pain Disability Index: psychometric and validity data. Arch Phys Med Rehabil 1987 ; 68 : 438 41. 6. Jerome A, Gross RT. Pain disability index: construct and discriminant validity. Arch Phys Med Rehabil 1991 ; 72 : 920 2. 7. 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 : 1141 6. 8. Gronblad M, Hupli M, Wennerstrand P, et al. Intercorrelation and test-retest reliability of the Pain Disability Index (PDI) and the 714 www.spinejournal.com April 2012

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