C1 Inhibitor - plasma and serum

Last updated: Thursday, 08, April, 2004

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Item Process

Immunochemical assay: 5 mL blood in plain tube.
Functional assay: 5 mL blood in EDTA tube.

The specimens should be placed on melting ice and transported immediately to the laboratory.


Functional and immunochemical assay.

Reference Interval

Immunochemical: 0.25-0.42 g/L
Functional: consult pathologist.


Levels of C3 and C4 should be determined before this test is requested; see Complement components C3 and C4 - serum.

Investigation of angioedema: C1 inhibitor levels are unlikely to be reduced if C4 is normal.

Diagnosis of hereditary angioedema due to C1 inhibitor deficiency, acquired C1 inhibitor deficiency in malignancy and SLE.

Genetic and acquired forms of C1 inhibitor deficiency are distinguished by family history and on clinical grounds.


In genetic deficiency, 85% of kindreds have low functional and immunochemical levels; 15% of kindreds have low functional levels with normal or high immunochemical levels. Levels may rise with danazol treatment.


Walport  MJ. N Engl J Med 2001; 344(14): 1058-1066.