C1 esterase inhibitor (C1-INH) is a glycoprotein. It acts as a regulatory brake on the complement activation process. Decreased production of this glycoprotein results in HAE.

This determination is an important tool for diagnosing HAE, a disorder caused by a low concentration of C1-INH or by an abnormal structure of the protein. Affected persons are apparently heterozygous for the condition. It is also used in the differential diagnosis of the more prevalent but less serious allergic and nonfamilial angioedema.

• 18–40 mg/dL or 180–400 mg/L by an immunoturbidimetric assay

1. Collect a 7-mL serum specimen in a red-topped tube. Observe standard precautions. Label the specimen with the patient’s name, date, and test(s) ordered and place in a biohazard bag for transport to the laboratory.

2. Spin down, separate from clot, and freeze 1.0 mL of serum at −70 °C until testing is performed.

1. Decreased values are associated with HAE, a genetic disease characterized by acute edema of subcutaneous tissue, gastrointestinal tract, or upper airway tract.

2. During acute attacks of the disease, C4 and C2 components can be markedly reduced.

See Antinuclear Antibody (ANA) Test (next section).