Complement

Why is the Test 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 number of diseases. Complement activity is also measured to monitor how severe a disease is or to determine if treatment is working.

For example, patients with active lupus erythematosus may have lower-than-normal levels of C3 and C4, and the levels of these complement components may be monitored to gauge disease activity.

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

Patients with gram negative septicemia and shock often have very low C3 and components of what's known as the alternative pathway. C3 is often also low in fungal infections and some parasitic infections such as malaria.