Introduction Five instruments, including an individualised measure of AS-related quality of life, the Patient-Generated Index (PGI-AS), have been assessed for measurement properties of reliability, validity and responsiveness in a large group of AS patients. Responsiveness refers to the ability to detect clinically important change and is an essential requirement for evaluative instruments.
Objective To establish comparative evidence in support of instrument longitudinal validity and responsiveness.
Methodology A questionnaire containing all instruments was mailed to over 450 patients randomly selected from six AS databases. Patients completed the questionnaire at baseline (response rate 76.9%; males 74.2%; mean age 46.0 yrs (SD 12.6), range 18–75 yrs; mean symptom duration 19.8 yrs) and 6 months. Self-reported AS and general health transition at 6 months were the criteria used to assess validity and responsiveness. The relationship between changes in instrument scores and responses to transition items was assessed for a linear trend.
Results Significant correlations between mean change in instrument scores and both forms of transition question supports the longitudinal validity of all instruments (p < 0.001): the largest levels of change were found for the PGI-AS and the Bath AS Disease Activity Index (BASDAI), both demonstrating a stronger relationship with general transition. The modified standardised response mean (MSRM) was used to assess responsiveness (Table 1). The Body Chart, a measure of AS specific pain, had a moderate relationship with AS transition only (better –0.44, worse 0.51), which may support the importance of specific pain evaluation.
Conclusion Although certain domains may not be expected to change over 6-months, this period reflects normal practice in AS evaluation in many rheumatology centres. The responsiveness of the PGI-AS and BASDAI may make them suitable for routine monitoring where management may result in small but important changes in health related quality of life.