Spinal mobility in ankylosing spondylitis: reliability, validity and responsiveness

To evaluate the measurement properties of an evidence-based selection of measures of spinal mobility in patients with ankylosing spondylitis (AS).

Measurements of spinal mobility were taken by trained observers within a UK rheumatology centre. Inter-observer reliability was assessed. Intra-observer reliability was assessed in patients reporting no change in AS-specific health at 2 weeks. Validity was assessed and scores were correlated with responses to health transition questions. Responsiveness was evaluated for patients reporting change in health at 6 months.

Reliability estimates support the use of all measures in individual evaluation (intraclass correlation>0.90). Correlations between measures of spinal mobility were in the hypothesized direction; the largest was between the modified Schober index (15 cm) (MSI) and the other measures. As hypothesized, small to moderate levels of correlation were found between mobility measures and patient-assessed health status. There was no significant linear relationship between mobility measures and self-reported health transition. Fingertip-to-floor distance following trunk forward flexion (FFD) was the most responsive mobility measure but was not as responsive as two AS-specific patient-assessed instruments, the Ankylosing Spondylitis Quality of Life Questionnaire (ASQoL) and the Bath Ankylosing Spondylitis Disease Activity Index (BASDAI). The MSI and cervical rotation (Crot) also had evidence of responsiveness. Low levels of responsiveness were found for the remaining measures.

All mobility measures had adequate levels of reliability and validity. The MSI had a strong relationship with all mobility measures, and the FFD and Crot were the most responsive to self-perceived changes in health at 6 months. The MSI, FFD and Crot are recommended for clinical practice and research.