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Epic Code LAB7690 C1 Esterase Inhibitor, Functional Assay, Serum

Important Note

Fasting specimen required

Additional Codes

Mayo Code: FC1EQ

Epic Code: LAB 7690

Interface Order Alias: 10316

Cerner: 4220

Reporting Name

C1 Esterase Inhib, Functional

Performing Laboratory

Mayo Clinic Laboratories in Rochester

Clinical Information

C1 inhibitor (C1-INH) is a multispecific protease inhibitor that is present in normal human plasma and serum, and which regulates enzymes of the complement, coagulation, fibrinolytic, and kinin-forming systems. The enzymes (proteases) regulated by this protein include the C1r and C1s subunits of the activated first component of complement, activated Hageman factor (factor XIa), kallikrein (Fletcher factor), and plasmin.


A deficiency of functionally active C1-INH may lead to life-threatening angioedema. Two major forms of C1-INH deficiency have been reported: the congenital form, termed hereditary angioedema (HAE), and the acquired form that is associated with a variety of diseases, including lymphoid malignancies.


HAE is characterized by transient but recurrent attacks of nonpruritic swelling of various tissues throughout the body. The symptomatology depends upon the organs involved. Intestinal attacks lead to a diversity of symptoms including pain, cramps, vomiting, and diarrhea. The most frequent cause of death in this disease is airway obstruction secondary to laryngeal edema occurring during an attack. There are 2 types of HAE that can be distinguished biochemically. Patients with the more common type (85% of HAE patients) have low levels of functional C1-INH and C1-INH antigen. Patients with the second form (15% of HAE patients) have low levels of functional C1-INH but normal or increased levels of C1-INH antigen that is dysfunctional.


The variable nature of the symptoms at different time periods during the course of the disease makes it difficult to make a definitive diagnosis based solely on clinical observation.

Useful For

Diagnosing hereditary angioedema and for monitoring response to therapy

Specimen Type

Serum Red

Specimen Required

Patient Preparation: Patient should be fasting.

Supplies: Aliquot Tube, 5 mL (T465)

Collection Container/Tube: Red top

Submission Container/Tube: Plastic vial

Specimen Volume: 1 mL

Collection Instructions:

1. Immediately after specimen collection, place the tube on wet ice.

2. Centrifuge at 4° C and aliquot serum into a 5-mL plastic vial.

3. Freeze specimen within 30 minutes.

Laboratory Test Directory Note:

COLLECTION NOTE: Volumes listed are in serum or plasma, draw approximately 2 1/2 times the requested volume in whole blood.

Specimen Minimum Volume

0.1 mL

Specimen Stability Information

Specimen Type Temperature Time Special Container
Serum Red Frozen 28 days

Reject Due To

Gross hemolysis OK
Gross lipemia OK
Gross icterus Reject
Heat-inactivated samples Reject

Day(s) Performed


Reference Values

>67% normal (normal)

41-67% normal (equivocal)

<41% normal (abnormal)


Hereditary angioedema (HAE) can be definitely diagnosed by laboratory tests demonstrating a marked reduction in C1 inhibitor (C1-INH) antigen or abnormally low functional C1-INH levels in a patient's plasma or serum that has normal or elevated antigen.


Nonfunctional results are consistent with HAE.


Patients with current attacks will also have low C2 and C4 levels due to C1 activation and complement consumption.


Patients with acquired C1-INH deficiency have a low C1q in addition to low C1-INH.


This assay is a functional test and is dependent on correct sampling, storage, and shipping conditions.


Absent (or low) C1 inhibitor (C1-INH) function should be confirmed with a new serum specimen to confirm that inactivation has not occurred during shipping.

Clinical Reference

1. Stoppa-Lyonnet D, Tosi M, Laurent J, Sobel A, Lagrue G, Meo T: Altered C1 inhibitor genes in type I hereditary angioedema. N Engl J Med. 1987;317:1-6. doi: 10.1056/NEJM198707023170101

2. Frigas E: Angioedema with acquired deficiency of the C1 inhibitor: a constellation of syndromes. Mayo Clin Proc. 1989;64:1269-1275. doi: 10.1016/s0025-6196(12)61290-7

3. Frazer-Abel A, Sepiashvili L, Mbughuni MM, Willrich MA: Overview of laboratory testing and clinical presentations of complement deficiencies and dysregulation. Adv Clin Chem. 2016;77:1-75. doi: 10.1016/bs.acc.2016.06.001

Method Name

Enzyme Immunoassay (EIA)

Method Description

Patient serum is mixed with C1 inhibitor (C1-INH) reactant (biotinylated, activated C1s). Functionally active C1-INH will bind to the biotinylated C1-INH reactant to form complexes. An aliquot of the serum mixture containing biotinylated C1-INH reactant is added to microtiter wells precoated with avidin. The C1-INH reactant:C1-INH complexes bind to the wells. C1-INH is detected using a horseradish peroxidase (HRP)-conjugated antibody against C1-INH. Color generation is accomplished by incubation with a chromogenic substrate. After incubation the enzyme reaction is stopped chemically, and the color intensity is measured spectrophotometrically. The color intensity is proportional to the concentration of functional C1-INH present in the serum.(Package insert: C1-Inhibitor Enzyme Immunoassay, Quidel; 12/2015)

Report Available

3 to 5 days

Test Classification

This test has been cleared, approved, or is exempt by the US Food and Drug Administration and is used per manufacturer's instructions. Performance characteristics were verified by Mayo Clinic in a manner consistent with CLIA requirements.

CPT Code Information


LOINC Code Information

Test ID Test Order Name Order LOINC Value
FC1EQ C1 Esterase Inhib, Functional 48494-9


Result ID Test Result Name Result LOINC Value
81493 C1 Esterase Inhib, Functional, QN 48494-9