Studies of the third component of complement in synovial fluid from arthritic patients. I. Immunochemical quantitation and relation to total complement

In a study of forty-one patients, most of whom had longstanding rheumatoid arthritis, a comparison was made in serum and synovial fluid between total complement activity (CH₅₀) and C3, measured immunochemically. Nineteen out of twenty-seven patients with rheumatoid arthritis and four out of four cases of systemic lupus erythematosus (SLE) or SLE-like syndromes had a depressed CH₅₀ value in the synovial fluid, while ten cases with a non-rheumatoid form of arthritis all had normal activity.

The total C activity in serum was normal or increased in most cases. While no correlation was found between CH₅₀ and C3 in serum, there was a statistically highly significant correlation in synovial fluid, indicating that C3 determinations can be used to reveal alterations in C activity of synovial fluid.