C4 is another of the components of the complement system and is synthesized in bone and lung tissue. C4 may be bypassed in the alternative complement pathway when immune complexes are not involved, or it may be used up in the very complicated series of reactions that follow many antigen–antibody reactions.

This is a follow-up test done when total complement levels are abnormally decreased. It can also be ordered to confirm HAE if the C1 inhibitor result is decreased.

• 14–40 mg/dL or 140–400 mg/L by rate nephelometry

1. Collect a 7-mL blood serum sample in a red-topped tube. Observe standard precautions. This amount is sufficient for both C3 and C4 testing.

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

1. Decreased C4 levels are associated with the following conditions:

   1. Acute SLE

   2. Early glomerulonephritis

   3. Immune complex disease

   4. Cryoglobulinemia

   5. Inborn C4 deficiency

   6. Hereditary angioneurotic edema

2. Increased C4 levels are associated with malignancies.

See Antinuclear Antibody (ANA) Test.