C4 level

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C4 level

Definition:

C4 level is a test that measures the concentration of a component of the complement system in the blood.

Alternative Names:
C4
How the test is performed:

Blood is drawn from a vein, usually on the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and an elastic band is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the band to fill with blood.

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A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the band is removed to restore blood circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

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

How to prepare for the test:

There are no special preparations.

How the test will feel:

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.

Why the test is performed:

Complement activity (CH50, CH100, terminal complement component, or individual complement proteins) is measured to determine if complement is involved in the development of a disease. Complement activity is also measured to monitor how severe a disease is, or if treatment is working.

For example, patients with active systemic lupus erythematosus may have lower-than-normal levels of C3 and C4, and these component levels may be monitored as a rough index of disease activity.