Complement component 4 is a blood test that measures the activity of a certain protein that is part of the complement system. The complement system is a group of proteins that move freely through your bloodstream. The proteins work with your immune system and play a role in the development of inflammation.

There are nine major complement proteins. They are labeled C1 through C9. This test measures C4.

See also:

• Complement
• Complement component 3 (C3)
• CH50
• CH100

Blood is drawn from a vein, usually from the inside of the elbow or the back of the hand. The site is cleaned with germ-killing medicine (antiseptic). The health care provider wraps an elastic band around the upper arm to apply pressure to the area and make the vein swell with blood.

Next, the health care provider gently inserts a needle into the vein. The blood collects into an airtight vial or tube attached to the needle. The elastic band is removed from your arm.

Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

In infants or young children, a sharp tool called a lancet may be used to puncture the skin and make it bleed. The blood collects into a small glass tube called a pipette, or onto a slide or test strip. A bandage may be placed over the area if there is any bleeding.

The sample is then sent to the laboratory where the amount of C4 is measured.

No special preparation is needed.

When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.

If your child is to have this test performed it may be helpful to explain how the test will feel, and even practice or demonstrate on a doll. The more familiar your child is with what will happen to them, and the purpose for the procedure, the less anxiety he or she will feel.

C3 and C4 are the most commonly measured complement components. Complement activity may be measured to determine how severe a disease is or if treatment is working.

A complement test may be used to monitor patients with an autoimmune disorder. For example, patients with active lupus erythematosus may have lower-than-normal levels of the complement proteins C3 and C4.

Complement activity varies throughout the body. For example, in patients with rheumatoid arthritis, complement activity may be normal or higher-than-normal in the blood, but much lower-than-normal in the joint fluid.

• Males: 12 to 72 mg/dL
• Females: 13 to 75 mg/dL

Note: mg/dL = milligrams per deciliter

Normal value ranges may vary slightly among different laboratories. Talk to your doctor about what your specific test results mean.

Increased complement activity may be seen in:

• Cancer
• Ulcerative colitis

Decreased complement activity may be seen in:

• Bacterial infections (especially Neisseria)
• Cirrhosis
• Glomerulonephritis
• Hepatitis
• Hereditary angioedema
• Kidney transplant rejection
• Lupus nephritis
• Malnutrition
• Systemic lupus erythematosus

There is very little risk involved with having your blood taken. Veins and arteries vary in size from one patient to another and from one side of the body to the other. Taking blood from some people may be more difficult than from others.

Other risks associated with having blood drawn are slight but may include:

• Excessive bleeding
• Fainting or feeling light-headed
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken)