Total Hemolytic Complement (CH₅₀)

Complement (C) is a complex sequential cascade system in which inactive proteins become active and interact much like the clotting system. The complement system is important as part of the body’s defense mechanism against infection (Figure 8.1). Activation of complement results in cell lysis, release of histamine from mast cells and platelets, increased vascular permeability, contraction of smooth muscle, and chemotaxis of leukocytes and neutralization of certain viruses. These inactive proteins constitute about 10% of the globulins in normal blood serum. The complement system is also interrelated with the coagulation, fibrinolytic, and kinin systems. The action of complement, however, is not always beneficial. The potent reactions mediated by this complex system are not always contained. In the presence of Gram-negative bacteremia, the complement can escape its built-in control mechanisms, causing severe damage to the body. It is not clear how this happens, but it is known that complement abnormalities develop before shock occurs.

This test screens for certain autoimmune diseases, estimates the extent of immune complex formation, and detects all inherited and most acquired immune deficiencies. Serial measurements monitor disease course and treatment in SLE, RA, and glomerulonephritis. It is a useful adjunct for RF and antinuclear antibody (ANA) testing when immune complexes appear to be the primary mediators of tissue injury.

The CH₅₀ measures the classical complement pathway (C1–C9). If there is a deficiency of one of the components (C1–C8), then the CH₅₀ value will be zero. Deficiency in C9, however, will result in a low CH₅₀. The CH₁₀₀ is an alternative test and is used as a rapid screening for complement deficiency.

• 60–144 complement activity enzyme units by ELISA

• CH₅₀: 30–60 U/mL or 30–60 kU/L

• CH₁₀₀: >70 U/mL or >70 kU/L

1. Collect a 7-mL blood serum sample in a red-topped tube. Observe standard precautions. A joint fluid specimen of at least 1 mL can also be used and should be collected in a tube that does not contain additives.

2. Label the specimen with the patient’s name, date, and test(s) ordered and place in a biohazard bag for transport to the laboratory.

1. Increased total complement values are associated with most inflammatory responses; these acquired elevations are usually transient, and concentrations return to normal when the situation is resolved.

2. Decreased total complement values are associated with hereditary defects of specific complement components. With C2 deficiency, autoimmune disorders occur as SLE, and C1q deficiency may cause hereditary angioedema (HAE).

   1. Complement consumption by activation of the alternative pathway, an amplification of the classic pathway not requiring an “immunologic” stimulus, can be seen with the following conditions:

      1. Gram-negative septicemia

      2. Subacute bacterial endocarditis

      3. Acute poststreptococcal glomerulonephritis

      4. Membranoproliferative glomerulonephritis

   2. Complement consumption due to activation of the classic pathway by immune complex formation occurs with the following conditions:

      1. SLE

      2. Serum sickness

      3. Acute vasculitis

      4. Severe RA

      5. Hepatitis

      6. Cryoglobulinemia

See Antinuclear Antibody (ANA) Test.