Complement, Total (CH₅₀)

Serum, frozen

1 mL

0.5 mL (Note: This volume does not allow for repeat testing.)

Red-top tube or gel-barrier tube

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.

Quantitative liposome lysis by spectrophotometry

See table.

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.