C3 constitutes 70% of the total protein in the complement system and is essential to the activation of both classic and alternative pathways. Along with the other components of the complement system, C3 may be used up in reactions that occur in some antigen–antibody reactions. C3 is synthesized in the liver, macrophages, fibroblasts, lymphoid cells, and skin.

This test is done when it is suspected that individual complement component concentrations are abnormally reduced. This test and the C1q and C4 tests are the most frequently ordered complement measurements. There is a correlation between most forms of nephritis, the degree of nephritis severity, and C3 levels. C3 is useful for assessing disease activity in SLE.

• 75–175 mg/dL or 0.75–1.75 g/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 test(s) ordered and place in a biohazard bag for transport to the laboratory.

1. Decreased C3 levels are associated with most active diseases with immune complex formation:

   1. Severe recurrent bacterial infections due to C3 homozygous deficiency

   2. Absence of C3b inactivator factor

   3. Acute poststreptococcal glomerulonephritis

   4. Immune complex disease

   5. Active SLE

   6. Membranoproliferative glomerulonephritis

   7. Nephritis

   8. End-stage liver disease

2. Increased levels are found in numerous inflammatory states.

See Antinuclear Antibody (ANA) Test.