Complement tests, most commonly C3 and C4, are used to determine whether deficiencies or abnormalities in the complement system are causing, or contributing to, a person's disease or condition. Total complement activity (CH50 or CH100) may be ordered to look at the integrity of the entire classical complement pathway. Other complement components are ordered as needed to look for deficiencies. Complement levels may be decreased due to increased consumption or, more rarely, a hereditary deficiency. Hereditary deficiency in one of the complement proteins will usually lead to a high frequency of recurrent microbial infections. Decreased complement levels also are associated with an increased risk of developing an autoimmune disease. Both C3 and C4 levels are typically depressed inSLE while C3 alone is low in septicemia and infections caused by fungi or parasites such as malaria. If the deficiency is due to an underlying acute or chronic condition, complement levels will usually return to normal if the underlying condition can be resolved. Decreased complement activity may be seen with: Recurrent microbial infections (usually bacterial) Autoimmune diseases, including SLE and rheumatoid arthritis Hereditary angioedema Acquired angioedema Various types of kidney disease, including glomerulonephritis, lupus nephritis, membranous nephritis, and IgA nephropathy as well as kidney transplant rejection Cirrhosis Hepatitis Malnutrition Septicemia, shock Serum sickness (immune complex disease) Complement protein levels are usually increased, along with other unrelated proteins called acute phase reactants, during acute or chronic inflammation. These all usually return to normal when the underlying condition is resolved. However, complement proteins are rarely measured in these conditions, compared to the widely ordered C-reactive protein (CRP),and the relevance of their measurement in these situations is not reviewed here. Increased complement activity may also be seen with: Cancer (leukemia, Hodgkin lymphoma, sarcoma) Ulcerative colitis Thyroiditis Acute myocardial infarction Sarcoidosis