Your complement system is made up of a group of protein molecules in your bloodstream that helps your immune system remove harmful pathogens (viruses, bacteria, and other germs) and other foreign substances from your body.
There are nine major complement proteins, which are labeled C1 through C9. C4 plays an important role in eliminating certain infections. The complement component 4 (C4) test is a simple blood test that measures your level of C4. A low level of C4 is associated with autoimmune and collagen vascular diseases, such as lupus and rheumatoid arthritis.
The complement component 4 test requires no preparation and carries few risks. Samples from your blood test will be sent to a laboratory for analysis, and the results will be forwarded to your doctor.
Your doctor may order a complement test if you’re experiencing symptoms that could indicate an autoimmune disease, such as:
- extreme fatigue
- muscle or joint pain
- weight loss
- muscle weakness or paralysis
The most common complement component tests are C3 and C4. The C4 test is sometimes used to monitor protein levels in patients who have already been diagnosed with an autoimmune disease. Complement tests can provide doctors with valuable information about the effectiveness of your current treatment for an autoimmune condition.
A C4 measurement involves a routine blood test. No preparation or fasting is required.
A small needle will be inserted into your vein and blood will be drawn into a small vial. You will likely feel a prick from the needle or a slight stinging sensation. When the vial is full, the nurse will remove the elastic band and needle and place a small bandage over the puncture site.
Your arm may be sore where the needle entered your skin. You may also have some mild bruising or throbbing after the blood draw.
Most patients experience no serious side effects. However, rare complications from a blood test may include:
- excessive bleeding
- lightheadedness or fainting
- infection at the puncture site (a small risk whenever your skin is broken).
If you experience any of these symptoms, contact your doctor immediately.
Your doctor will explain the results of your test and what they mean for you.
The normal C4 complement measurement for men is between 12 and 72 milligrams of complement per deciliter of blood (mg/dL). For women, the range is usually between 13 and 75 mg/dL.
Lower-than-normal values could indicate:
- cirrhosis (significant liver damage)
- glomerulonephritis (a type of kidney disease)
- rejection of a kidney transplant
Component activity varies throughout your body. Patients with rheumatoid arthritis, for example, may have high complement levels in their blood, but low complement levels in their joint fluid.
Your doctor will recommend follow-up testing and/or treatment based on your results.