Type of test Blood

Normal findings

Total complement: 30-75 units/mL

C3: 75-175 mg/dL C4: 22-45 units/mL

 Test explanation and related physiology

Measurements of complement are used primarily to diagnose hereditary and acquired deficiencies of complement peptides and to monitor the activity of infectious or autoimmune diseases (e.g., systemic lupus erythematosus, nephritis, membranoproliferative nephritis, poststreptococcal nephritis).

Serum complement is a group of 31 proteins that act as enzymes, cofactors, inhibitors, and membrane-integrated proteins. These effect a cascade-like series of reactions that lead to the synthesis of a group of proteins that facilitate the immunologic and inflammatory responses. The total complement, sometimes labeled CH 50, is made up of a series of reactions involving proteins C1 through C9 (classic cascade reactions). Besides these major components, subcomponents are involved in the system. When activated, total complement (and some precursor proteins) acts to increase vascular permeability, allowing antibodies and WBCs to be delivered to the area of the immune/antigen complex. Complement also acts to increase chemotaxis (attracting WBCs to the area), phagocytosis, and immune adherence of the antibody to antigen. These processes are vitally important in the normal inflammatory or immunologic response.

Reduced complement levels can be congenital or acquired. As the complement system is activated, the complement components are consumed or used up. If the system is persistently or overly activated, serum levels can fall. The complement system is instigated by the presence of antibody–antigen complexes. As in hereditary angioedema, complement components are used up, and serum levels fall. Diseases associated with these immune complexes include serum sickness, lupus erythematosus, infectious endocarditis, renal transplant rejection, vasculitis, some forms of glomerulonephritis, and infections. As these diseases are successfully treated, complement levels can be expected to return to normal.

The total complement assay should be used as a screen for suspected complement-related diseases before ordering individual complement component assays. A deficiency of an individual component of the complement cascade will result in an undetectable total complement level. For a list of common diseases associated with complement abnormalities, see Table 1. Note, however, that this list is not complete. Complement abnormalities may occur in the face of normal blood levels when particular complement proteins are not functioning properly.

Complement levels can also be measured in other bodily fluids such as pleural, pericardial, and synovial fluids. Low fluid complement levels are characteristic of effusions from patients with rheumatoid arthritis (despite elevated serum levels), systemic lupus erythematosus, and bacterial infections.

Table1. Diseases associated with complement deficiencies

Interfering factors

• C3 is very unstable at room temperature.

 Procedure and patient care

 • See inside front cover for Routine Blood Testing.

 • Fasting: no

 • Blood tube commonly used: red

Abnormal findings

Increased levels

- Rheumatic fever (acute)

- Myocardial infarction (acute)

- Ulcerative colitis

- Cancer

Decreased levels

Cirrhosis

- Autoimmune disease (e.g., systemic lupus erythematosus)

- Serum sickness (immune complex disease)

- Glomerulonephritis

- Lupus nephritis

- Renal transplant rejection (acute)

- Protein malnutrition

- Anemia

- Malnutrition

- Hepatitis

- Rheumatoid arthritis

- Sjögren syndrome

- Severe sepsis