Complement-Related Disorders Workup

Updated: Feb 15, 2022
  • Author: M Michael Glovsky, MD; Chief Editor: Michael A Kaliner, MD  more...
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Laboratory Studies

Candidates for evaluation

Complement function should be evaluated in any patient with collagen-vascular disease, PNH, chronic nephritis, recurrent pyogenic infections, severe recurrent angioedema not responsive to antihistamines, N meningitides or disseminated gonococcal infections, or a second attack of septicemia at any age.

A family history of recurrent systemic infections caused by encapsulated bacteria, especially meningococci, should suggest complement deficiency.

Hemolytic assays

Hemolytic assays were devised as early as the beginning of the 20th century, and they measure the ability of the complements to participate in hemolysis.

CH50 tests the capacity of proteins of the classic pathway and membrane attack complex to lyse antibody-coated sheep erythrocytes. The dilution of the serum that lyses 50% of the cells marks the end point.

The CH50 value is zero in homozygous congenital deficiencies of C1 to C8, and its value is half-normal in C9 deficiency. Also, deficiencies in factors H or I result in a low value due to C3 consumption. The test does not measure deficiencies of the alternative pathway activation proteins.

It is very useful as a screening test for most diseases of the complement system. Because of the unstable nature of several of the complement proteins, the CH50 assay requires appropriate collection, processing, and storage of specimens. Serum samples should be assayed the day of collection or stored frozen. A common cause of a decreased CH50 values is improper specimen handling.

The alternative hemolytic complement activity (AH50), although less commonly used, measures alternative pathway function that requires the presence of adequate factor B, factor D, and properdin.

Selective complement assay

Serum concentrations of C1q, C1r, C1s, C4, C2, C3, C5, C6, C7, C8, C9, and factor B are measured by radial immunodiffusion, and testing is easily available. A decrease in C4 levels with normal C3 levels represents classic pathway activation, a decrease in factor B levels with normal C4 levels signifies alternative pathway activation, and a decrease in C3 levels reflects activation of either pathway. Partial (heterozygous) deficiencies of C4, factor B, or C3 could mimic the above findings in the absence of activation.

In hereditary angioedema, depression of C4 and C2 levels during an attack may or may not reduce the CH50 value. The C4 level is characteristically low with a normal C3 level. Concentration of C1-INH can be determined with an antibody-based assay. A functional test for C1-INH should be performed in patients in whom a high index of suspicion exists but the protein level is normal.

Low titers of both C3 and C4 suggest activation of the classic pathway by immune complexes. On the other hand, low C3 and normal C4 levels suggest alternative pathway activation. This difference may be useful in differentiating nephritis due to immune complex deposition from that due to NeF. Also, factor B levels are reduced in persons with NeF-induced nephritis. However, normal complement levels do not exclude complement activation that is biologically important but not massive enough to lower the serum concentrations.

Tests for MBL levels are fairly widely available and should be considered in any patients with recurrent pyogenic infections in whom tests for antibody deficiency are being considered.

Abnormal elevations of Ba, Bb, C3a, C4a, and iC3b levels have been found to correlate with lupus flares in patients. However, larger prospective studies are required before this type of testing is recommended for routine use. Synovial fluid from patients with rheumatoid arthritis or gout has markedly increased levels of C3a, whereas C5a levels are within the reference range. C3a is a more sensitive marker of in vivo complement activation than C5a, which is rapidly cleared from the circulation. Cerebrospinal levels of C5b-9 are increased in persons with autoimmune neurologic diseases, such as multiple sclerosis and lupus cerebritis.