IgA serum levels and disease activity in ankylosing spondylitis: a prospective study.

We investigated the possible association between serum IgA, IgM, and IgG and disease activity in a longitudinal study of 48 weeks' duration in 38 male patients with active ankylosing spondylitis receiving regular treatment with either phenylbutazone or diflunisal. Throughout the study serum IgA levels correlated most frequently with chest expansion and lumbar flexion index, and patients with extensive radiological changes also had the highest serum IgA levels. Likewise, changes in IgA, but not in IgM and IgG, correlated with changes in a composite index of disease activity (IDA). Changes in erythrocyte sedimentation rate (ESR) showed a similar correlation with changes in IDA, whereas changes in serum IgA and ESR showed no consistent correlation, suggesting that both parameters reflect different aspects of disease. Serum IgA, ESR, and IDA values all decreased during regular drug treatment, suggesting a disease modifying effect of the non-steroidal anti-inflammatory drugs (NSAIDs) studied. Regular measurement of serum IgA may be useful in the assessment of disease activity of ankylosing spondylitis.