Objective: Various rheumatic diseases are often complicated by nephropathy and cases combined with nephropathy show a poorer prognosis. Traditional diagnostic tools for nephropathy are complement activity (CH₅₀) or serum levels of C3 and C4. These tests are neither sensitive nor precise for clinical use because they have a wide reference range and a number of reagents that are difficult to standardize. Polymerized C9 testing is a novel approach for measurement of total classical complement activity. After in vitro activation of complement in a well of a microtiter plate, the neoantigen of terminal polymerized C9 is quantified by using an enzyme-conjugated monoclonal antibody. We evaluate clinical significance of polymerized C9 as a predictor and differential marker for nephropathy in various renal and rheumatic diseases. Methods: Polymerized C9 testing (CAE, INCSTAR-DiaSorin, Italy) on 69 patients with various rheumatic or renal diseases and 13 normal controls was undertaken. According to the polymerized C9 levels, we grouped each disease into five categories and compared means between groups. Results: The increased group included pyelonephritis and the normal group included CRF, normal control and nephrotic syndrome. The slightly decreased group included glomerulonephritis and SLE without nephritis. The moderately decreased group included RA with nephritis and the markedly decreased group included lupus nephritis. There were significant mean differences between each group (p＜0.05). Conclusion: We conclude that polymerized C9 can be a useful reference in the initial differential diagnosis of nephropathy and in the appropriate approach for each rheumatic disease.

Keywords: Complement, Glomerulonephritis, Systemic lupus erythematosus, Rheumatoid arthritis