OBJECTIVES: The 1-month Graded Chronic Pain Scale (GCPS) commonly used in clinical studies has never been validated. This study compares the GCPS 1-month with the 6-months version for reliability and validity.

METHODS: The Validation Project included 521 subjects with at least one temporomandibular disorder for cross-sectional data and 74 subjects for test-retest data. Internal reliability, stability, and construct validity were used for testing the 1-month version. Comparisons were made between the two versions for characteristic pain intensity (CPI), interference, and chronic pain grade (CPG).

RESULTS: For GCPS 1-month, internal consistency for pain intensity and interference was high (Cronbach's alpha=0.87 and 0.94, respectively), and temporal stability was high for CPI (intraclass correlation coefficient (ICC)=0.91), interference (ICC=0.85), and CPG (weighted kappa=0.88). ICC or kappa between the two versions was 0.78 (CPI), 0.66 (interference), and 0.69 (CPG); high-impact pain, in contrast, was 0.50. Construct validity exhibited higher correlations with predictor variables for 1-month version attributes of CPI, interference, and CPG. Modified Bland-Altman plots indicated that both versions measure CPI well.

DISCUSSION: Overall, reliability of the 1-month GCPS is equal to or better than the 6-months version for pain intensity, disability days, pain interference, CPG, and high-impact pain. However, consistency between versions is lower for measures of disability days and interference, and for the derived measures of CPG and high-impact pain; highly skewed distributions and increasing disagreement in reported status over the time periods affect the measures of function. Therefore, we recommend that GCPS-1 month only be used to calculate pain intensity and pain interference.