Complement C3 cleavage product in synovial fluids detected by immunofixation

54 synovial fluids (SFs), 46 of them derived from various inflammatory diseases (30 rheumatoid arthritis (RA) SFs, 8 undefined arthritis (UA) SFs, 8 psoriatic arthritis (PSA) SFs) and 8 SFs from degenerative joint diseases (OA) were tested for C3c split product, using the immunofixation method. There were significant differences in the C3c product between the four groups investigated. In the OA group in the mean the percentage of C3c was low in comparison to the native C3 (C3c = 2.95%). RA SFs and UA SFs showed considerably higher values (20.1% for RA and 23.2% for UA) which were statistically significant in comparison to the OA SFs. With the exception of one SF the PSA SFs exhibited a relatively low percentage of the cleavage product. Despite the one high value the average C3c content of the PSA SFs was not statistically different from the OA SFs. In contrast to this low percentage of the C3c split product the PSA SFs showed the highest C3 concentration of all groups (87.0 +/- 36.5 mg/100 ml). Immunofixation is a simple and effective tool to determine the C3c split product in SFs. It might also be helpful for establishing the differential diagnosis of PSA vs RA on the basis of the C3 level of the SF in those patients where an elevated level of C3 is present.