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C1 Inhibitor, Protein and Functional Tests

Test code(s) 298, 297

The C1 Inhibitor, Protein test (test code 298) measures the concentration of the C1 esterase inhibitor protein using a fixed rate time nephelometry method. The C1 Inhibitor, Functional test (test code 297) measures the relative functionality of an individual’s C1 esterase inhibitor protein using an enzyme immunoassay. Both tests may be helpful in the diagnosis of hereditary angioedema (HAE). HAE is an autosomal dominant genetic disorder that manifests in 3 ways:

  • Type I is the most common type of HAE, affecting about 85% of patients.1 It is caused by mutations in the SERPING1 gene and is characterized by low levels (<11 ng/mL) of the protein and normal (>68%) protein function.
  • Type II affects about 15% of HAE patients1 and is also caused by SERPING1 mutations. Concentrations of the C1 inhibitor protein are normal (11-26 ng/mL), but the protein is dysfunctional (functional activity <40%).
  • Type III is a rare type of HAE2 that is associated with mutations in the F12 gene, which encodes the coagulation protein factor XII. HAE type III symptoms are exacerbated by pregnancy and oral contraceptives. The C1 esterase inhibitor protein concentration is typically normal, as is the protein function.


  1. Kerr, MA and AAC Yeung-Laiwah. C1-Inhibitor deficiency and angioedema. In: K. Whaley, ed. Complement in Health and Disease. MTP Press Limited; 1987;p 53.
  2. Zuraw BL. Clinical practice. Hereditary angioedema. N Engl J Med. 2008;359:1027–1036.


This FAQ is provided for informational purposes only and is not intended as medical advice. A clinician’s test selection and interpretation, diagnosis, and patient management decisions should be based on his/her education, clinical expertise, and assessment of the patient.

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