Complement C4 Blood Test

This test measures the concentration of complement component C4 in serum, in mg/dL, using an immunoturbidimetric method. For adults aged 15 to 80, Quest's reference range is 15-53 mg/dL for males and 15-57 mg/dL for females. Reference ranges are not established under age 1 or above age 80, and ranges vary between laboratories.

People with a positive ANA or anti-dsDNA result who are being evaluated for lupus; people with diagnosed lupus or another autoimmune disease whose clinician uses complement levels to track activity and treatment response; and people whose clinician is investigating immune-complex kidney disease, unexplained inflammation or recurrent bacterial infections.

When lupus or another immune-complex disease is being evaluated, usually at the same draw as C3, anti-dsDNA and an ANA. In diagnosed lupus it is often repeated at intervals your rheumatologist sets, since a falling C4 can accompany a flare. No fasting or timing around meals is needed.

A normal C4 level does not rule out autoimmune disease, but makes heavy complement consumption less likely at the time of the draw. A low C4 suggests complement is being used up, as can happen in active lupus, lupus nephritis and other forms of glomerulonephritis, or it may reflect reduced production, for example in cirrhosis or malnutrition, or rare inherited complement deficiencies. Because C4 is consumed early in the classical complement pathway, a low C4 with a normal C3 can also point to hereditary angioedema, cryoglobulinemia or an inherited partial C4 deficiency. A high C4 is less specific; complement proteins can rise with inflammation, and MedlinePlus lists some cancers and ulcerative colitis among causes of increased complement. Interpretation depends on C3, anti-dsDNA, kidney tests and symptoms, so review your result with a clinician. JustLabs results are informational and are not a diagnosis.