A complement test is a blood test that measures the activity of a group of proteins in the part of the blood called the serum. These proteins make up the complement system, which is part of the immune system.
The complement system helps antibodies fight off infections and destroy substances that are foreign to the body, such as viruses, bacteria, and other germs. It’s 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 monitor the progress of people undergoing treatment for autoimmune diseases, such as lupus and rheumatoid arthritis (RA). The test can measure how advanced these diseases are based on the activity of the complement protein in the blood. It can also gauge the effectiveness of ongoing treatments for autoimmune disorders and diagnose some cancers and infectious diseases.
There are nine major complement proteins. They’re labeled C1 through C9. A total complement measurement checks 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. Your doctor will send the blood sample to a laboratory for analysis. Your doctor will receive the results.
A total complement measurement, also known as a total hemolytic complement or a CH50 measurement, checks how well the complement system is functioning.
A doctor usually orders total complement tests for people with a family history of complement deficiency and those who have symptoms of:
- kidney disease
- lupus, which is a multisystem autoimmune disease
- myasthenia gravis
- an infectious disease, such as meningitis
- cryoglobulinemia, which is 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, can evaluate the course of lupus and its variants. Depending on your symptoms, your doctor will order either a total complement measurement, one of the more targeted tests, or all three. A blood draw is all that is necessary in either case.
A complement test requires a routine blood draw. There’s no preparation or fasting is necessary. A blood draw involves the following steps:
- A healthcare provider will disinfect an area of skin on your arm or hand.
- They’ll wrap an elastic band around your upper arm to allow the vein to fill with blood.
- They’ll insert small needle into your vein and draw the blood into a small vial. You may feel a pricking or stinging sensation from the needle.
- When the vial is full, they’ll 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.
A complement test requires a routine blood draw. Rare risks from a blood draw include:
- excessive bleeding
- infection, which can happen any time the skin is broken
Notify your doctor right away if you have any of these symptoms.
Complement activity in the blood’s serum is characteristically low in active autoimmune diseases, such as lupus. However, the serum complement may be normal or high and still show low levels in the joint fluid if you have a disease like RA. The joint fluid is the most active site of inflammation in RA.
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
- a fungal infection
- some parasitic infections, such as malaria
The measurement for total blood complement (total hemolytic complement or CH50) is in hemolytic units. This method involves the destruction of red blood cells used in the test. The destruction of red blood cells is called hemolysis. Methods including what are called “immunodiffusion techniques” measure C3 and C4 in milligrams per deciliter (mg/dL)
The following are some baseline complement readings:
Normal results are as follows:
- The total blood complement level is 41 to 90 hemolytic units.
- The C1 levels are 16 to 33 mg/dL.
- The C3 levels are 88 to 252 mg/dL for males and 88 to 206 mg/dL for females.
- The C4 levels are 12 to 72 mg/dL for males and 13 to 75 mg/dL for females.
Higher than normal values may be an indication of:
- certain infections
- ulcerative colitis, which is an inflammatory bowel disease
Lower than normal values may mean:
- cirrhosis or severe liver damage
- glomerulonephritis, which is a kidney disease
- hereditary angioedema, which is episodic swelling of the face, hands, feet, and some internal organs
- rejection of a transplanted kidney
- a flare-up of an autoimmune disease
In some patients with infectious and autoimmune diseases, complement levels may be so low that they’re undetectable.
People who lack early complement proteins, which are C1 through C4, are more prone to infections. Complement deficiency may also be a factor in the development of autoimmune diseases. People with low levels of late complement proteins, which are 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 more testing to make a final diagnosis.