Responsiveness of general health status in chronic low back pain: a comparison of the COOP Charts and the SF-36

The objective of this study was to compare the responsiveness and assess the concurrent validity of two functional health status instruments, the Dartmouth COOP charts and the SF-36 in chronic low-back pain (CLBP) patients. The data came from 129 of 174 patients who participated in a randomized clinical trial of the therapeutic management of CLBP. Reliable and valid disease-specific outcomes, patient-rated low-back pain and disability, were used as external criteria (EC) to identify improved and non-improved patients. Unpaired t-statistics and receiver operating characteristic (ROC) curve calculations were used to quantify responsiveness. The two instruments had sufficient and very similar responsiveness using both EC. Comparisons between improved and non-improved patients for the COOP charts and SF-36, respectively, using pain as EC, yielded differences which translated into large effect sizes (0.8 and 0.7) (P = 0.0008 and 0.003). Using disability as EC, differences of moderate effect size were found (0.5 and 0.6) (P = 0.02 and 0.002). The ROC curve calculations using pain as EC resulted in areas under the curve of 0.76 (95% CI: 0.64, 0.88) for the COOP charts, and 0.74 (95% CI: 0.60, 0.88) for the SF-36. The corresponding areas using disability as EC were 0.67 (95% CI: 0.55, 0.79) and 0.72 (95% CI: 0.60, 0.84). The best cut-off point in both instruments for differentiating between improved and non-improved patients was approximately six percentage points. The constructs of functional health status, as reflected in the global scores of the two instruments, are highly correlated (r = 0.82). Six of the instruments' nine dimensions are moderately to highly correlated (r = 0.52 to 0.86), and the overall canonical correlation was high (R = 0.9). In conclusion, both instruments seem equally suitable for use as outcome measures in clinical trials on CLBP. The COOP charts are faster to fill out and score. (C) 1999 international Association for the Study of Pain. Published by Elsevier Science B.V.