Complement Assays

Complement is a collective term for a system of at least 15 serum proteins designed to destroy foreign cells and help remove foreign materials. Complement deficiency can increase susceptibility to infection and predispose to other diseases. Complement assays are indicated in patients with known or suspected immunomediated disease or repeatedly abnormal response to infection. Various laboratory methods are used to evaluate and measure total complement and its components; hemolytic assay, laser nephelometry, and radial immunodiffusion are the most common.

Although complement assays provide valuable information about the patient's immune system, the results must be considered in light of serum immunoglobulin and autoantibody tests for a definitive diagnosis of immunomediated disease or abnormal response to infection.

Purpose

• To help detect immunomediated disease and genetic complement deficiency.
• To monitor effectiveness of therapy

Patient preparation

• Explain to the patient that this test measures a group of proteins that fight infection.
• Advise him that he needn't restrict food or fluids.
• Tell him that the test requires a blood sample and who will perform the venipuncture and when.
• Reassure him that although he may experience transient discomfort from the needle puncture and the tourniquet, collecting the sample usually takes less than 3 minutes.
• If the patient is scheduled for C1q assay, check his history for recent heparin therapy. Report such therapy to the laboratory.

Procedure and posttest care

• Perform a venipuncture, and collect the sample in a 7-ml red-top tube.
• Because many patients with complement defects have a compromised immune system, keep the venipuncture site clean and dry.
• If a hematoma develops at the venipuncture site, apply warm soaks.

Precautions

• Handle the sample gently to prevent hemolysis.
• Send it to the laboratory immediately because complement is heat labile and deteriorates rapidly.

Reference values

Normal values for complement range as follows:

• total complement: 25 to 110 units
• CI esterase inhibitor: 8 to 24 mg/dl
• C3: 70 to 150 mg/dl
• C4: 14 to 40 mg/dl

Abnormal findings

Complement abnormalities may be genetic or acquired; acquired abnormalities are most common. Depressed total complement levels (which are clinically more significant than elevations) may result from excessive formation of antigen-antibody complexes, insufficient synthesis of complement, inhibitor formation, or increased complement catabolism and are characteristicin such conditions as systemic lupus erythematosus (SLE), acute poststreptococcal glomerulonephritis, and acute serum sickness. Low levels may also occur in some patients with advanced cirrhosis of the liver, multiple myeloma, hypogammaglobulinemia, or rapidly rejecting allografts.

Elevated total complement may occur in obstructive jaundice, thyroiditis, acute rheumatic fever, rheumatoid arthritis, acute myocardial infarction, ulcerative colitis, and diabetes.

C1 esterase inhibitor deficiency is characteristic in hereditary angioedema, the most common genetic abnormality associated with complement;

C3 deficiency is characteristic in recurrent pyogenic infection and disease activation in SLE; C4 deficiency is characteristic in SLE and rheumatoid arthritis. C4 is increased in autoimmune hemolytic anemia.

Interfering factors

• Hemolysis due to rough handling of the sample
• Failure to send the sample to the laboratory immediately
• Recent heparin therapy