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Complement C1 esterase inhibitor levels and function (C1 inhibitor)

Category Immunology
Test background

C1 esterase inhibitor regulates the classical complement pathway, the coagulation and the kinin systems. Genetic mutations in the SERPING1 gene leading to C1 inhibitor deficiency cause hereditary angioedema (HAE). Patients present with bradykinin mediated acute deep tissue oedema, typically of the face, larynx or abdomen that can be life threatening if respiratory obstruction occurs. Urticaria is not a feature.

85% of HAE patients have low antigenic C1 esterase inhibitor levels (type I HAE), a smaller proportion have normal levels but a non-functioning protein (HAE 2). During acute attacks the complement C4 level is low and C3 level is normal.

Acquired C1 inhibitor deficiency (AAE) caused by complement activation or autoantibodies to C1 inhibitor is clinically indistinguishable from HAE but presents in older patients and is usually associated with lymphoproliferative or autoimmune disease. Statins, ACE inhibitors and some antipsychotic drugs can also cause angioedema without urticaria. Patients with AAE can be distinguished from HAE by measuring C1q levels.

Rarer forms of HAE can be caused by mutations in other members of the kallikrein/kinin pathway (FactorXII, plasminogen, angiopoietin 1, kininogen 1, myoferlin), these patients have normal C1 inhibitor levels.

C1 inhibitor levels are measured by turbidimetry and C1 inhibitor function is measured by chromogenic assay.

Clinical Indications

Recurrent angioedema in the absence of urticaria
Family history of HAE

Reference range

C1 inhibitor level = 0.22-0.38 g/L
C1 inhibitor function = 0.7-1.3 IU/mL (70-130% activity compared to the reference standard)
Reference ranges are manufacturer derived and were verified in house

Sample & container required Serum (rust top RST tube)
Sample volume 5-10 mL blood (1 mL serum)
Transport storage For external users, separate and freeze immediately. Transport frozen. For internal users, transport to Charing Cross Hospital Immunology ASAP
Turnaround time 21 days