Your complement system consists of a group of important proteins in your bloodstream. These proteins help your immune system fight off harmful invaders, such as bacteria, and other foreign substances.

There are nine major complement proteins, which are labeled C1 through C9. Complement C4 plays an important role in eliminating certain infections.

The complement component 4 (C4) test is a simple blood test that measures the amount of complement C4 circulating in your bloodstream. A low level of C4 is associated with autoimmune diseases, such as lupus and rheumatoid arthritis.

The complement C4 test only requires basic preparation and carries few risks. The sample of your blood will be sent to a laboratory for analysis. The results will then be sent to your doctor.

The complement C4 test is one of the most frequently used complement component tests. Your doctor may order a complement C4 test if you’re experiencing symptoms that indicate an autoimmune disease. These symptoms may include:

  • extreme fatigue
  • muscle pain
  • joint pain
  • sudden, unexpected weight loss
  • muscle weakness
  • muscle paralysis

The complement C4 test is also used to monitor protein levels in people who have already been diagnosed with an autoimmune disease.

Complement tests can provide doctors with valuable information about the effectiveness of current treatment for an autoimmune condition.

No special preparation is needed before undergoing a complement C4 test. However, you should tell your doctor about all the medications, vitamins, herbs, and other supplements you’re taking.

Aside from prescription medications, you should also make sure to mention any over-the-counter medications you’re taking.

A complement C4 test involves taking a small sample of blood. A blood draw involves the following steps:

  1. A healthcare provider will first disinfect the area of skin where the blood will be drawn.
  2. In most cases, they’ll draw blood from the inside of your elbow or from the top of your hand. If they’re using a vein located on the inside of your elbow, they’ll tightly wrap an elastic band around your upper arm to make the veins swell with blood.
  3. They’ll then insert a small needle into your vein and draw blood into a tube attached to the needle. If a young child or infant is being tested, the healthcare provider may use a sharp tool called a lancet instead of a needle to prick the skin. When the needle is inserted, you’ll likely feel a prick or a slight stinging sensation. Once enough blood has been drawn, they’ll remove the needle and place a small bandage over the puncture site.
  4. After the skin begins to bleed at the puncture site, the healthcare provider will collect the blood onto a slide or testing strip. It may also be placed into a small tube called a test tube.
  5. They’ll then apply a bandage over the puncture site to stop any bleeding.

Your arm may be sore in the area where the needle entered your skin. You may also have some mild bruising or throbbing after the blood draw.

Most people don’t experience any serious side effects. However, rare complications from a blood test include:

  • excessive bleeding (hematoma)
  • lightheadedness
  • fainting
  • infection at the puncture site

Contact your doctor right away if you experience any of these symptoms.

The normal complement C4 measurement for men is typically between 12 and 72 milligrams (mg) of complement per deciliter (dL) of blood. The normal range for women is usually between 13 and 75 mg of complement per dL of blood.

However, normal values may vary among different testing laboratories, so it’s important to talk to your doctor about what the results mean for you.

Higher-than-normal complement C4 values may be a sign of cancer, ulcerative colitis, and inflammation in general.

Lower-than-normal complement C4 values could indicate:

  • bacterial infections
  • hepatitis
  • malnutrition
  • rejection of a kidney transplant
  • systemic lupus erythematosus, which is an autoimmune disease affecting the skin, joints, kidneys, and other organs
  • lupus nephritis, which is a kidney disorder that often occurs as a result of systemic lupus erythematosus
  • cirrhosis, which indicates significant liver damage
  • glomerulonephritis, which is a type of kidney disease
  • hereditary angioedema, which is a rare but serious condition that causes swelling in various parts of the body

Complement component activity varies throughout the body. People 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 or treatment based on your results.