Complement Component C4
Complement component C4 is another protein in the complement system that supports your immune response. This test measures the amount of C4 in your blood.
Because C4, measured together with C3, gives a fuller picture of how the complement system is behaving in autoimmune and inflammatory conditions. It is commonly used when evaluating or monitoring lupus and related disorders. Changes in C4 can help a clinician judge whether disease is active or improving.
A higher-than-normal C4 is generally less concerning and can reflect ordinary inflammation, as complement proteins may rise during the body's acute-phase response. In autoimmune evaluation, attention usually centers on low rather than high C4 values.
A low C4 can indicate complement being consumed in active autoimmune disease, such as a lupus flare, and it is sometimes used to help track disease activity. Low C4 can also reflect inherited complement deficiencies or certain other conditions, so your clinician interprets it alongside C3 and your overall picture.
A result within your lab's reference range is typical. Because low levels can indicate complement consumption in active autoimmune disease, any low or shifting result should be reviewed by your clinician.
Sources
Educational context only, not medical advice or a diagnosis. Reference ranges vary by lab, age and sex; always discuss your results with a clinician.