Disease-specific, patient-assessed measures of health outcome in ankylosing spondylitis: reliability, validity and responsiveness

Objective. To assess the acceptability and measurement properties of four ankylosing spondylitis (AS)-specific, patient-assessed measures of health outcome: AS Quality of Life Questionnaire (ASQoL), Bath AS Disease Activity Index (BASDAI), the Body Chart and the Revised Leeds Disability Questionnaire (RLDQ). Methods. Instruments were administered by means of a self-completed questionnaire to patients recruited from across the United Kingdom (UK). Instruments were assessed for data quality and scaling assumptions. Where appropriate, dimensionality was assessed using principle component analysis (PCA). Internal consistency reliability was tested using Cronbach's alpha. Test-retest reliability was assessed in those patients reporting no change in AS-specific health at 2 weeks. The convergent validity of the instruments was assessed and scores were correlated with responses to the health transition questions. Responsiveness was assessed for patients reporting change in health at 6 months. Results. The BASDAI and Body Chart have low self-completion rates. Item responses for the RLDQ were skewed towards higher levels of functional ability. PCA supported instrument unidimensionality. Cronbach's alpha ranged from 0.87 (BASDAI) to 0.93 (RLDQ). Test-retest reliability estimates support the use of the ASQoL and RLDQ in individual evaluation (>0.90). Correlations between instruments were in the hypothesized direction; the largest was between the ASQoL and BASDAI (0.79). The BASDAI had the strongest linear relationship, with responses to both specific and general health transition questions (P<0.01). With the exception of the Body Chart, instruments had a stronger relationship with general health transition. The BASDAI was the most responsive instrument. The Body Chart and RLDQ had low levels of responsiveness. Conclusion. The instruments have undergone a comprehensive comparative evaluation to assess the measurement properties required for patient-assessed measures of health outcome. Adequate levels of reliability and validity were found for all instruments. The BASDAI and the ASQoL were the most responsive to self-perceived change in health, but the BASDAI had low levels of self-completion.