Cytidine deaminase (CD), a cytoplasmic enzyme, is thought to leak out of damaged cells and can be measured in fluids by a simple biochemical assay. This study has shown that serum CD activity is raised in rheumatoid arthritis (RA) compared with osteoarthritis (OA). Synovial fluid (SF) CD activity was always less than the corresponding serum activity (mean SF/serum ratio = 0.6) in OA but up to 22 times greater than the corresponding serum activity in RA (mean SF/serum ratio = 13.1), suggesting CD production in inflammatory joints. Evidence to support the SF neutrophil as a cell of CD origin is provided by the CD gradient running from cells to SF to synovium. The close correlation between SF CD activity and neutrophil count (r = 0.93) indicates that SF CD activity is an accurate measure of acute synovial inflammation. Weak correlation of serum CD activity with erythrocyte sedimentation rate (ESR) (r = 0.44) and C-reactive protein (CRP) (r = 0.49) implies that CD estimations supply different though related information about rheumatoid disease activity. We suggest that CD released from damaged neutrophils diffuses from all inflamed joints into the blood, so that serum CD activity may provide an integrated measure of joint inflammation more specific than traditional measures such as the ESR.
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