What is a complement test?

A complement test is a blood test that measures the activity of a group of proteins in the bloodstream. These proteins make up the complement system, which is one part of the immune system.

The complement system helps antibodies fight off infections and destroy substances that are foreign to the body. These foreign substances may include viruses, bacteria, and other germs.

The complement system is also involved in how autoimmune disease and other inflammatory conditions work. When a person has an autoimmune disease, the body views its own tissues as foreign and makes antibodies against them.

There are nine major complement proteins, labeled C1 through C9. However, this system is very complex. Currently, over 60 known substances in the immune system combine with complement proteins when activated.

A total complement measurement checks the activity of the main complement components by gauging the total amount of complement protein in your blood. One of the more common tests is known as a total hemolytic complement, or a CH50 measurement.

Complement levels that are too low or too high can cause problems.

A common use for a complement test is to diagnose autoimmune diseases or other immune function conditions. Certain diseases can have abnormal levels of a particular complement.

A doctor can use a complement test to monitor the progress of a person undergoing treatment for an autoimmune disease such as systemic lupus (SLE) or rheumatoid arthritis (RA). It can also be used to gauge the effectiveness of ongoing treatments for autoimmune disorders and certain kidney conditions. The test may also be used to identify individuals at high risk of complications in certain diseases.

A total complement measurement checks how well the complement system is functioning.

A doctor often orders total complement tests for people with a family history of complement deficiency and those who have symptoms of:

  • RA
  • hemolytic uremic syndrome (HUS)
  • kidney disease
  • SLE
  • myasthenia gravis, a neuromuscular disorder
  • an infectious disease, such as bacterial meningitis
  • cryoglobulinemia, which is the presence of abnormal proteins in the blood

Specific complement tests, such as C2, C3, and C4 tests, may help evaluate the course of certain diseases. Depending on your symptoms and history, your doctor will order either a total complement measurement, one of the more targeted tests, or all three. A blood draw is all that’s necessary.

A complement test requires a routine blood draw. No preparation or fasting is necessary.

A healthcare provider will follow these steps to perform the blood draw:

  1. They disinfect an area of skin on your arm or hand.
  2. They wrap an elastic band around your upper arm to allow more blood to fill the vein.
  3. They insert a small needle into your vein and draw the blood into a small vial. You may feel a pricking or stinging sensation from the needle.
  4. When the vial is full, they remove the elastic band and needle and place a small bandage over the puncture site.

There may be some soreness of the arm where the needle entered the skin. You may also experience some mild bruising or throbbing after the blood draw.

The blood draw carries few risks. Rare risks from a blood draw include:

Notify your doctor right away if you have any of these symptoms.

Results of a total complement measurement are usually expressed in units per milliliter. Tests that measure specific complement proteins, including C3 and C4, are typically reported in milligrams per deciliter (mg/dL).

The following are typical complement readings for people age 16 and older, according to Mayo Medical Laboratories. Values can vary between laboratories. Sex and age can also affect expected levels.

Higher-than-normal results

Values that are higher than normal may indicate a wide variety of conditions. Often these are related to inflammation. Some conditions associated with elevated complement can include:

Complement activity in the bloodstream is characteristically low in people with active autoimmune diseases such as lupus. However, blood complement levels may be normal or high with RA.

Lower-than-normal results

Certain complement levels that are lower than the normal may occur with:

In some people with infectious and autoimmune diseases, complement levels may be so low that they are undetectable.

People who lack certain complement proteins may be more prone to infections. Complement deficiency may also be a factor in the development of autoimmune diseases.

After the blood draw, your healthcare provider will send the blood sample to a laboratory for analysis. Keep in mind that your total complement test results can be normal even if you’re deficient in several specific complement proteins. Talk to your doctor about how the results apply to you.

Your doctor may recommend more testing to make a final diagnosis.