Validity and sensitivity to change of spondylitis-specific measures of functional disability

To compare the construct validity and sensitivity to change of 2 spondylitis-specific measures of functional disability, the Health Assessment Questionnaire Disability Index modified for the spondyloarthropathies (HAQ-S) and the Dougados Functional Index, with 2 more generic instruments, the Health Assessment Questionnaire (HAQ) and the Arthritis Impact Measurement Scales-2 (AIMS2), in patients with ankylosing spondylitis (AS).

Construct validity was assessed in 2 ways: (1) by comparisons of the cross sectional correlations between each functional disability instrument and 6 measures of physical impairment in 216 patients, and (2) by relating changes over time in the HAQ-S and the Functional Index with changes in patient reported pain and stiffness in 153 patients followed for at least 2 years. Sensitivity to change was measured from the responses of 155 patients who reported a qualitative change in the activity of their AS during followup.

Most patients had mild functional disability, with median scores of 0.5 on the HAQ-S (possible range 0-3), 0.375 on the HAQ (possible range 0-3), 11 on the Functional Index (possible range 0-40), and 5 on the AIMS2 (possible range 0-60). Scores on the HAQ-S (R2 = 0.24) and the unmodified HAQ (R2 = 0.18) were more highly correlated with measures of physical impairment than were scores on the AIMS2 (R2 = 0.10) or the Functional Index (R2 = 0.09). Changes over time in the HAQ-S and HAQ were more closely related to changes in pain and stiffness than were changes in the Functional Index. The HAQ-S and HAQ were also more sensitive to change than the Functional Index.

The HAQ-S showed greater construct validity and sensitivity to change than the Functional Index, but performed similarly to the unmodified HAQ.