Generic measures of health-related quality of life in ankylosing spondylitis: reliability, validity and responsiveness

OBJECTIVE: To assess the acceptability and measurement properties of two generic measures of health-related quality of life (HRQL): the EuroQol and the Short Form 12-item Health Survey Questionnaire (SF-12) in ankylosing spondylitis (AS). METHODS: Instruments were administered by means of a self-completed questionnaire to AS patients recruited from across the United Kingdom. Instruments were assessed for data quality and scaling assumptions. Test-retest reliability was assessed in those patients reporting no change in general health at 2 weeks. The convergent validity of both instruments was assessed and scores were correlated with responses to health transition questions. Responsiveness was assessed for patients reporting change in health at 6 months. RESULTS: The instruments had high completion rates. Although slightly skewed towards better levels of health, scores covered the available range for both sections of the EuroQol [EQ-5D and EQ-visual analogue scale (EQ-VAS)]. Score distributions approximated normality for the SF-12. Test-retest reliability estimates support the use of both instruments in group evaluation and the SF-12 Physical Component Summary score (PCS) in individual evaluation (>0.90). Correlations between instruments were in the hypothesized direction and were of a moderate level. The EQ-VAS had the strongest linear relationship, with responses to both specific and general health transition questions (P<0.01). The EQ-VAS and SF-12 PCS were the most responsive instruments. The EQ-5D was the least responsive instrument. CONCLUSION: The instruments have undergone a comprehensive comparative evaluation to assess the measurement properties required for patient-assessed measures of health outcome in AS. Adequate levels of acceptability, reliability and validity were found for both instruments. Although evidence supporting instrument responsiveness was strong for the EQ-VAS and SF-12 PCS, it was very weak for the EQ-5D and SF-12 Mental Component Summary Scale (MCS). The EQ-VAS and SF-12 PCS can both be recommended for use in group evaluation, and the SF-12 PCS is recommended in routine practice or research. However, the lower reliability of the SF-12 MCS and the limited ability of both the EQ-5D and SF-12 MCS to detect change in health may restrict these roles.