Measuring function in rheumatoid arthritis: Identifying reversible and irreversible components

Measurement of physical function at one point in time cannot distinguish impairment caused by the active disease process from chronic irreversible impairment. We aimed to dissect these two components of functional limitation in rheumatoid arthritis (RA) by using the disability index of the Health Assessment Questionnaire (HAQ) as the measure of function.

We performed a secondary analysis of data from 6 contemporary clinical trials of RA (2,763 patients). Patients in whom remission was achieved in the trials, based on a simplified disease activity index, were identified. In an individual patient, HAQ scores at trial entry represented both reversible and irreversible impairments, while HAQ scores at the time of RA remission represented the mostly irreversible component, and the difference between these corresponded to the component related to disease activity. We tested the concept that the HAQ has a reversible and an irreversible component by associating the HAQ score during remission with 2 measures associated with the degree of accrued damage: duration of RA and radiographic severity.

Among patients in whom clinical remission was achieved (n = 295), average HAQ scores despite clinical remission increased progressively with the duration of RA, from 0.19 (<2 years of RA) to 0.36 (2-<5 years) to 0.38 (5-<10 years) to 0.55 (>/=10 years) (P < 0.001). The reversibility of HAQ scores decreased with the duration of RA (median 100%, 83.3%, 81.9%, and 66.7%, respectively; P < 0.001). Findings were similar in patients subgrouped by quartile of radiographic scores.

Differences in the sources of functional limitations should be considered in the interpretation of functional measures, and in their use for prediction and in cost analyses.