Perhaps the most useful assays for complement are the immunochemical assays of C3 and C4. As noted, which outlines the alternative and classical modes of complement breakdown, a low C3 and C4 but normal factor B suggest that activation of the classical pathways has occured. Examples would be patients with systemic lupus erythemathosus or vasculitis. In contrast, if C3, C4, and factor B are all low, the alternative pathway is also activated via either feedback loops or simultaneous activation. This would point to a gram negative bacteremia. Normal C4 levels with low C3 and factor B levels suggests acute or chronic infection. Acute rheumatic fever is such an example.