Sorted By Test Name - Mayo Medical Laboratories

C4
8171
AH50
88676
Clinical Information: The complement system is an integral part of the immune defenses. The
primary complement pathway consists of recognition (Clq, Clr, Cls), activation (C4, C2, C3), and attack
(C5, C6, C7, C8, C9) mechanisms with respect to their role in antibody-mediated cytolysis. The
complement system can be activated via immune complexes, and the alternative pathway (properdin
pathway), which is activated primarily by foreign bodies such as microorganisms. C3 activation involves
cleavage by C3 convertase into C3a and C3b. When immune complexes are not involved, the alternate
method of complement activation initiates the reactant sequence at C3, bypassing C1, C4, and C2. Severe
recurrent bacterial infections occur in patients with homozygous C3 deficiency and in those patients with
low levels of C3 secondary to the absence of C3b activator. Decreased C3 may be associated with acute
glomerulonephritis, membranoproliferative glomerulonephritis, immune complex disease, active systemic
lupus erythematosus, septic shock, and end-stage liver disease.
Useful For: Assessing disease activity in systemic lupus erythematosus Investigating an undetectable
total complement (CH[50]) level
Interpretation: A decrease in C3 levels to the abnormal range is consistent with disease activation in
systemic lupus erythematosus.
Reference Values:
75-175 mg/dL
Clinical References: 1. Ross SC, Densen P: Complement deficiency states and infection:
epidemiology, pathogenesis, and consequences of neisserial and other infections in an immune deficiency.
Medicine 1984;63:243-273 2. Frank MM: Complement in the pathophysiology of human disease. N Engl
J Med 1987;316:1525-1530
Complement C4, Serum
Clinical Information: The complement system is an integral part of the immune defenses. It can be
activated via immune complexes (classic pathway) or by bacterial polysaccharides (alternative pathway).
The classic complement pathway consists of recognition, (C1q, C1r, C1s), activation (C2, C3, C4), and
attack (C5, C6, C7, C8, C9) mechanisms with respect to their role in antibody-mediated cytolysis. C4 is 1
of the activation proteins of the classic pathway. In the absence of C4, immune complexes will not be
cleared by C3 activation peptides, but bacterial infections can still be defended via the alternative
pathway. C4 may be decreased in systemic lupus erythematosus, early glomerulonephritis, immune
complex disease, cryoglobulinemia, hereditary angioedema, and congenital C4 deficiency.
Useful For: Investigating an undetectable total complement (CH[50]) Confirming hereditary
angioedema (with low C1 inhibitor) Assessing disease activity in systemic lupus erythematosus,
proliferative glomerulonephritis, rheumatoid arthritis, and autoimmune hemolytic anemia
Interpretation: Decreased in acquired autoimmune disorders, in active phase of lupus erythematosus,
and in rheumatoid arthritis An undetectable C4 level (with normal C3) suggests a congenital C4
deficiency Increased in patients with autoimmune hemolytic anemia
Reference Values:
14-40 mg/dL
Clinical References: 1. Ross SC, Densen P: Complement deficiency states and infection:
epidemiology, pathogenesis, and consequences of neisserial and other infections in an immune deficiency.
Medicine 1984; 63:243-273 2. Frank MM: Complement in the pathophysiology of human disease. N Engl
J Med 1987;316:1525-1530 3. Tiffany TO: Fluorometry, nephelometry, and turbidimetry. In Textbook of
Clinical Chemistry. Edited by NW Tietz. Philadelphia, WB Saunders Company, 1986, pp 79-97
Complement, Alternate Pathway (AH50), Functional, Serum
Clinical Information: Complement proteins are components of the innate immune system. There are
3 pathways to complement activation: the classic pathway, the alternative (or properdin) pathway, and the
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