Complement Test

Written by Ann Pietrangelo | Published on June 4, 2012
Medically Reviewed by Brenda B. Spriggs, MD, MPH, FACP

What Is a Complement Test?

Complement tests are blood tests that measure the activity of a group of proteins in the blood (serum). These proteins make up the complement system, which helps the immune system fight infections and destroy substances that are foreign to the body. While the purpose of this complement system is to help the immune system remove harmful pathogens (viruses, bacteria, and other germs) from the body, it is also activated when the body makes antibodies against its own tissues that it views as foreign. This happens in autoimmune diseases.

A complement test can be used to monitor the progress of patients undergoing treatment for autoimmune diseases, such as lupus and rheumatoid arthritis. The test can measure how advanced these diseases are based on the activity of the complement protein in the blood. It can also be used to gauge the effectiveness of ongoing treatments for autoimmune disorders and to diagnose some cancers and infectious diseases.

There are nine major complement proteins, which are labeled C1 through C9. A total complement measurement is used to check the function of your complement system by gauging the total amount of complement protein in your blood.

The complement test involves a simple blood draw. It requires no preparation and carries few risks. The blood sample is sent to a laboratory for analysis and the results will be forwarded to the doctor.

Types of Complement Tests

A total complement measurement (also known as a total hemolytic complement or a CH50

measurement) checks how well the complement system is functioning.

Total complement tests are usually ordered for patients with a family history of complement deficiency and those who have symptoms of:

  • rheumatoid arthritis
  • kidney disease
  • lupus (a multisystem autoimmune disease)
  • an infectious disease (such as meningitis)
  • cryoglobulinemia (the presence of abnormal proteins in the blood)

The test can be normal, however, while several components of the complement system may be low. Other complement tests, such as C3 and C4 tests, may ordered to evaluate the course of systemic lupus erythematosus and its variants. Depending on the presenting symptoms, a doctor will order either a total complement measurement, one of the more targeted tests, or all three. A simple blood test is all that is required in either case.

How the Complement Test Is Performed

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

A nurse will disinfect an area of skin on the arm or hand. He or she will wrap an elastic band around the upper arm to allow the vein to fill with blood.

A small needle will be inserted into the vein and the blood drawn into a small vial. A sensation of a prick from the needle or a stinging sensation can be felt. When the vial is filled, the nurse will 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. Some mild bruising or throbbing after the blood draw may also be experienced.

Rare complications from a blood draw include excessive bleeding, lightheadedness, fainting, and infection (this can happen anytime the skin is broken). If a patient experiences any of these symptoms, the doctor should be notified right away.

Interpreting Your Test Results

Complement activity in the blood (serum) is characteristically low in active autoimmune diseases such as lupus. However, in other diseases like rheumatoid arthritis, the serum complement may be normal or high, but low levels are detected in the joint fluid. The joint fluid is the most active site of inflammation in rheumatoid arthritis.

People with active lupus may have low levels of the complement proteins C3 and C4. Low C3 levels can also indicate an infection in the bloodstream, shock, a fungal infection, and some parasitic infections, such as malaria.

Total blood complement (total hemolytic complement or CH50) is reported in hemolytic units because of the type of testing used. This method involves the lysis or destruction of red blood cells (hemolysis) used in test, therefore, hemolytic units. C3 and C4 are measured by other methods including immunodiffusion techniques, quantified and reported in mg/dL.

Below are some baseline complement readings.

Normal Results

According to the National Institutes of Health (NIH), normal results are as follows:

  • Total blood complement level: 41 to 90 hemolytic units
  • C1 level: 16 to 33 milligrams per deciliter (mg/dL)
  • C3 levels:
    Males: 88 to 252 mg/dL
    Females: 88 to 206 mg/dL
  • C4 levels:
    Males: 12 to 72 mg/dL
    Females: 13 to 75 mg/dL (NIH, 2010)

Abnormal Results

Higher than normal values may be an indication of:

Lower than normal values may mean:

  • cirrhosis (severe liver damage)
  • glomerulonephritis (a kidney disease)
  • hereditary angioedema (episodic swelling of face, hands, feet and some internal organs)
  • hepatitis
  • rejection of a transplanted kidney
  • lupus
  • malnutrition
  • infection
  • a flare-up of an autoimmune disease

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

People who lack early complement proteins (C1 through C4) are more susceptible to infections. Complement deficiency may also be a factor in the development of autoimmune diseases. Patients with low levels of late complement proteins (C5 through C9) may be at a higher risk of infections caused by a type of bacteria called Neisseria.

After the test, your doctor may recommend additional testing to make a final diagnosis.

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