The concentrations of the fourth component of complement and of the C1 inactivator in synovial fluid from arthritic patients.

The concentration of the fourth component (C4) of complement (C) in synovial fluid was immunochemically determined in forty-nine cases of arthritis. The lowest C4 values were found in the patients with rheumatoid arthritis (RA), in particular those with depressed synovial fluid C values (and positive rheumatoid factor tests), whereas the highest were obtained in cases of non-rheumatoid arthritis (pelvospondylites ossificans, Reiter's disease and psoriasis arthropathica) and in the majority of the sero-negative RA patients. Low C4 values also proved closely associated with low values for the third component (C3), and with pronounced conversion of this component. The C[unk] inactivator (immunochemically determined) of the synovial fluid which was correlated with the albumin concentration, did not vary with total C, C4 or C3.