Complement levels are a measure of the individual or total level of nine proteins involved in the function of the immune system. They are measured from a blood test. They are usually measured after other tests have failed to establish a diagnosis in an attempt to identify disorders of the immune system. For example, measuring complement component 3 used to be the standard test for lupus and is also still measured to confirm the diagnosis after an ANA test.

Increased complement levels can indicate cancer, infection or ulcerative colitis. Decreased levels can indicate cirrhosis, glomerulonephritis, angioedema, hepatitis or malnutrition in addition to lupus. In a patient with a kidney transplant, it can indicate transplant rejection.