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Evaluate and follow up SLE (systemic lupus erythematosus) patient's response to therapy; may predict disease flare in SLE; evaluate for complement component deficiency; evaluate complement activity in cases of immune complex disease, glomerulonephritis, rheumatoid arthritis, SBE, cryoglobulinemia. The CH50 assay mainly evaluates the classical pathway.
CH50 can be used to assess the integrity of the classical pathway (ie, the presence of complement components C1-C9), but cannot be used as a sensitive test for in vivo fixation of complement. A normal CH50 level indicates that all the components of the C1 through C9 are present; however, even in the presence of a normal CH50, the absolute levels of some complement components (eg, C3 or C4) can be significantly (50% to 80%) lower than normal without affecting CH50 activity. This is because normal serum contains C3 and C4 in substantial excess of that required to yield a normal result for CH50. Depletion of alternative factors is not detected. For this reason, it may be necessary to measure individual complement components.
Complement is a system of 25 cell membrane associated and plasma proteins, which when activated produce multiple inflammatory mediators, opsonins, lysins, and down regulators vital to the normal function of the immune system. Complement components belong to a “classical” and “alternative” pathway whose activation steps differ. Complement proteins can be increased as part of the acute-phase response to inflammation or infection, and they can be decreased or absent due to hypercatabolism, expenditure in immune complexes, or hereditary deficiency. Patients with hereditary absence of a complement protein may have decreased total complement and recurrent bacterial infections or a rheumatic illness. Conversely, patients with rheumatic diseases, particularly with active illness and activation of complement and formation of immune complexes, may have low total complement. Falling complement levels may presage clinical flares, particularly of lupus nephritis.
