Sorted By Test Name - Mayo Medical Laboratories

FC1EQ
81493
FCQBA
functional measurement of C1 esterase inhibitor; FC1EQ/81493 C1 Esterase Inhibitor, Functional Assay,
Serum. Measurement of C1q antigen levels; C1Q/8851 Complement C1q, Serum, is key to the differential
diagnoses of acquired or hereditary angioedema. Those patients with the hereditary form of the disease
will have normal levels of C1q, while those with the acquired form of the disease will have low levels.
Studies in children show that adult levels of C1 inhibitor are reached by 6 months of age.
Reference Values:
19-37 mg/dL
Clinical References: 1. Frank MM: Complement deficiencies. Pediatr Clin North Am
2000;47(6):1339-1354 2. Gelfand JA, Boss GR, Conley CL, et al: Acquired C1 esterase inhibitor
deficiency and angioedema: a review. Medicine 1979;58(4):321-328 3. Rosen FS, Alper CA, Pensky J, et
al: Genetically determined heterogeneity of the C1 esterase inhibitor in patients with hereditary
angioneurotic edema. J Clin Invest 1971;50(10):2143-2149 4. Frigas E: Angioedema with acquired
deficiency of the C1 inhibitor: a constellation of syndromes. Mayo Clin Proc 1989;64:1269-1275 5.
Soldin SJ, Hicks JM, Bailey J, et al: Pediatric reference ranges for estradiol and C1 esterase inhibitor. Clin
Chem 1998;44(6s):A17
C1 Esterase Inhibitor, Functional Assay, Serum
Clinical Information: C1 inhibitor (C1-INH) is a multispecific, protease inhibitor that is present in
normal human plasma and serum, and which regulates enzymes of the complement, coagulation,
fibrinolytic, and kinin-forming systems. The enzymes (proteases) regulated by this protein include the C1r
and C1s subunits of the activated first component of complement, activated Hageman factor (factor XIa),
kallikrein (Fletcher factor), and plasmin. A deficiency of functionally active C1-INH may lead to
life-threatening angioedema. Two major forms of C1-INH deficiency have been reported: the congenital
form, termed hereditary angioedema (HAE), and the acquired form that is associated with a variety of
diseases, including lymphoid malignancies. HAE is characterized by transient but recurrent attacks of
nonpruritic swelling of various tissues throughout the body. The symptomatology depends upon the
organs involved. Intestinal attacks lead to a diversity of symptoms including pain, cramps, vomiting, and
diarrhea. The most frequent cause of death in this disease is airway obstruction secondary to laryngeal
edema occurring during an attack. There are 2 types of HAE that can be distinguished biochemically.
Patients with the more common type (85% of HAE patients) have low levels of functional C1-INH and
C1-INH antigen. Patients with the second form (15% of HAE patients) have low levels of functional
C1-INH but normal or increased levels of C1-INH antigen that is dysfunctional. The variable nature of the
symptoms at different time periods during the course of the disease makes it difficult to make a definitive
diagnosis based solely on clinical observation.
Useful For: Diagnosing hereditary angioedema and for monitoring response to therapy
Interpretation: Hereditary angioedema (HAE) can be definitely diagnosed by laboratory tests
demonstrating a marked reduction in C1 inhibitor (C1-INH) antigen or abnormally low functional
C1-INH levels in a patient's plasma or serum that has normal or elevated antigen. Nonfunctional results
are consistent with HAE. Patients with current attacks will also have low C2 and C4 levels due to C1
activation and complement consumption. Patients with acquired C1-INH deficiency have a low C1q in
addition to low C1-INH.
Reference Values:
>67% normal (normal)
41-67% normal (equivocal)