Diagnosis of C1 Esterase Inhibitor Deficiency

Diagnostic Algorithm

The American Academy of Allergy, Asthma, and Immunology recommends measuring complement C4 levels as the initial screening test for C1 inhibitor deficiency, followed by C1 inhibitor antigen and functional levels, with specific patterns distinguishing between hereditary and acquired forms of the condition 1, 2, 3
A low C4 level is generally present in patients with untreated C1 inhibitor deficiency, but C4 levels can be normal if the patient is already receiving treatment; in such cases, testing should be repeated during an angioedema attack 3, 4
If C4 is low, measure both C1 inhibitor antigen (protein) level and C1 inhibitor functional activity, as functional level should be less than 50-60% of the lower limit of normal to be compatible with hereditary angioedema (HAE) 2, 3, 4

Type I HAE is characterized by low C4 + low C1 inhibitor antigen levels + low C1 inhibitor functional levels, accounting for approximately 85% of cases, according to the American Academy of Allergy, Asthma, and Immunology 4, 5
Type II HAE is characterized by low C4 + normal C1 inhibitor antigen levels + low C1 inhibitor functional levels, as recommended by the American Academy of Allergy, Asthma, and Immunology 4, 5
Acquired C1 inhibitor deficiency is characterized by low C4 + low C1 inhibitor antigen/function + low C1q levels, as stated by the American Academy of Allergy, Asthma, and Immunology 6

Chromogenic functional C1 inhibitor assays are superior to ELISA-based functional assays, with a strength of evidence level of high, according to the American Academy of Allergy, Asthma, and Immunology 3
Hemolytic complement assays for C1 inhibitor function are most accurate but technically difficult and not readily available, with a strength of evidence level of moderate, as stated by the American Academy of Allergy, Asthma, and Immunology 3
Positive screening test results should be repeated once to exclude ex vivo degradation of the sample or laboratory error, with a strength of evidence level of high, as recommended by the American Academy of Allergy, Asthma, and Immunology 4

In pregnancy, plasma C1 inhibitor levels decrease due to increased plasma volume, so results should be interpreted with caution, with a strength of evidence level of moderate, according to the American College of Obstetricians and Gynecologists 8, 9
C1 inhibitor and C4 levels reach mature adult levels between 6-36 months and 2-3 years of age, respectively, with a strength of evidence level of high, as stated by the American Academy of Pediatrics 9