Use of a numerical rating scale as an answer modality in ankylosing spondylitis–specific questionnaires

OBJECTIVES: To determine the agreement of scores on the original visual analog scale (VAS) or Likert scale of the Bath Ankylosing Spondylitis Disease Activity Index (BASDAI), Bath Ankylosing Spondylitis Functional Index (BASFI), and Dougados Functional Index (DFI) with scores on a numerical rating scale (NRS). To assess the reproducibility and responsiveness of the instruments with the original scale and NRS. METHODS: Five hundred thirty-six patients with ankylosing spondylitis from the Netherlands, Mexico, and Switzerland completed a questionnaire in which all questions from the BASDAI, BASFI, and DFI were presented twice in random order with an 11-point NRS and either a 10-cm VAS (BASDAI and BASFI) or a 5-point Likert scale (DFI). Agreement of scores using Bland-Altman plots and intraclass correlation coefficients (ICCs), reproducibility using ICCs, and responsiveness were assessed. RESULTS: Large variability between the scores on the original scales and the NRS was found in individual questions of all 3 questionnaires, although total scores showed ICCs of at least 0.88. Reproducibility of all answer modalities showed low ICCs in individual questions, but moderate to good ICCs in total scores (Dutch group 0.62-0.89; Mexican group 0.53-0.72). Moderate to large effects (0.48-1.04) were found in responsiveness scores in the 3 questionnaires. No major differences in reproducibility and responsiveness between the answer modalities were found. CONCLUSION: Although large variability between the scores on the original answer scales and the NRS was observed, the BASDAI, BASFI, and DFI can be administered with an NRS, which does not show important differences compared with the original scales.