Laboratory Methods of Testing for the Complement Factors C3 & C4

The C3 and C4 complement factors are two of the over 20 molecular proteins contained in the complement system of the body. The complement system is activated by an immune response that causes a waterfall effect as proteins are triggered and work to eliminate pathogens and infections. Abnormal or low levels of these proteins due to genetic or physical risk factors can cause the complement system to malfunction. Laboratory technicians can use several tests to monitor levels and behavior of the C3 and C4 proteins.

Why Test?

• Tests examining the levels of complement proteins can identify a variety of disorders, including autoimmune illnesses such as lupus nephritis, systemic lupus erythematosis, and hereditary angioedema. All three disorders are identified through decreased function of the C3 and C4 complement factors. Diseases such as cirrhosis, septicemia, malaria and hepatitis are also known to decrease function for the two, while cancer and ulcerative colitis are indicated in an increase in C3 and C4 function. Testing C3 and C4 complement factors can help identify these illnesses and, in some cases, monitor treatments.

Testing the Entire Complement System

• Initial problems with the level and functioning of C3 and C4 proteins can be identified with a screening of the overall complement system, known as the CH50 assay, total hemolytic complement, or whole complement titer. Such a test can identify if the problem causing the patient's symptoms is in the complement system. If the test shows abnormal activity levels, further tests of the individual complements can be tested.

C3 Testing

• A simple blood test is performed to determine the function and level of C3 in the body. Depending on the illness, the laboratory technician may wish to test other fluids such as joint fluid which can be a more accurate indicator of the complement function in specific parts of the body. The normal level for C3 is 75 to 135 mg per deciliter.

C4 Testing

• The C4 protein is closely related to the C3 protein, acting as the next step in the sequence of reactions that characterizes the complement system function. Extraction of the protein from the blood or joint fluid enables technicians to measure the C4 level. C4 levels should range from 12 mg per deciliter to 72 mg per deciliter in men, and 13 mg per deciliter to 75 milligrams per deciliter in women.