What Is a Complement C4 Test?

A complement C4 test measures the level of complement C4 in your blood. Complement C4 is a protein that plays an important role in the complement system**, a complex network of proteins that helps your immune system fight off infection.

When you're exposed to an infection, your immune system activates the complement system. The complement system produces a series of proteins that work together to destroy bacteria and viruses. Complement C4 is one of these proteins.

A complement C4 test may be ordered if you have symptoms of a complement deficiency, such as:

Frequent infections

Severe infections

Autoimmune diseases

A complement C4 test may also be ordered if you're taking a medication that can affect the complement system, such as corticosteroids or chemotherapy drugs.

The complement C4 test is a blood test. A blood sample will be taken from a vein in your arm and sent to a laboratory for analysis.

The normal range for complement C4 is 10-40 mg/dL. A low complement C4 level may indicate a complement deficiency. A high complement C4 level may be seen in some autoimmune diseases.

If your complement C4 level is abnormal, your doctor may order additional tests to diagnose the cause.

Treatment for a complement deficiency may include:

Replacing the missing complement protein

Taking medications to boost the immune system

Avoiding infections

If you're taking a medication that can affect the complement system, your doctor may adjust your dose or prescribe a different medication.

The complement C4 test is a simple and accurate way to measure the level of complement C4 in your blood. This information can help your doctor diagnose and treat a variety of health problems.