Activation of the classical pathway complement system has long been implicated in stimulating immune complex mediated tissue destruction in systemic lupus erythematosus (SLE). C3 and C4 complement levels are utilized as part of SLE diagnosis and monitoring criteria. Recently, cell bound complement activation products (CBCAPs) have shown increased sensitivity in diagnosing and monitoring lupus activity, compared to traditional markers. CBCAPs are increasingly utilized in rheumatology practice as additional serological markers in evaluating SLE patients. We report a case of a patient diagnosed with SLE that had chronically low C3 and C4, along with negative CBCAPs. We surmise that the patient has an inherited complement deficiency as the etiology of her SLE and that CBCAPs could be used to predict such deficiency.
