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Relation between synovial fluid C3 degradation products and local joint inflammation in rheumatoid arthritis, osteoarthritis, and crystal associated arthropathy.

C3 degradation products (C3dg/d) were estimated in 288 synovial fluid (SF) samples (rheumatoid arthritis (RA) 93, osteoarthritis (OA) 68, chronic pyrophosphate arthropathy 80, acute pseudogout 20, others 27) from knees of 138 patients (bilateral 67, serial sampling on two to six occasions 40). At each aspiration knees were defined as 'active' or 'inactive' by single observer global assessment using six clinical parameters of inflammation. Lack of correlation between paired SF and plasma C3dg/d implied local C3 activation within joints. Raised SF C3d levels were found in active compared with inactive RA joints (mean (range) 51 (15-105) and 6 (0-15) units/ml respectively). Low SF C3dg/d levels were found in OA (mean (range) 0.8 (0-7) units/ml) and chronic pyrophosphate arthropathy (mean (range) 4 (0-16) units/ml), irrespective of clinical activity. In contrast, very high levels (mean (range) 61 (16-126) units/ml) were present in all cases of pseudogout. These differences remained after correction for SF C3 or albumin. This study is the first to show a positive correlation between SF C3dg/d and local inflammation in RA joints. It further suggests that C3 activation is a constant feature of pseudogout but not an accompaniment of inflammation associated with chronic crystal associated synovitis or OA.