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Unit Code 81493:
C1 Esterase Inhibitor, Functional Assay, Serum

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Useful For

Diagnosing hereditary angioedema and for monitoring response to therapy

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.

Reference Values

>67% normal (normal)

41-67% normal (equivocal)

<41% normal (abnormal)

Interpretation

HAE can be definitely diagnosed by laboratory tests demonstrating

a marked reduction in 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.

Cautions

No significant cautionary statements

Clinical Reference

1.   Stoppa-Lyonnet D, Tosi M, Laurent J, et al:  Altered C1 inhibitor

      genes in type I hereditary angioedema. N Engl J Med 1987;317:1-6

 

2.   Frigas E:  Angioedema with acquired deficiency of the C1 inhibitor:

      a constellation of syndromes.  Mayo Clin Proc 1989;64:1269-1275

Key