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C1 Esterase Inhibitor, functional

How well your C1 inhibitor protein works, shown as a percent of normal. A low result is a key clue in testing for hereditary angioedema.

AI-verified against cited sources. Awaiting expert reviewAlso called C1-INH Fx, fC1-INH, C1INH FunctionLOINC 48494-9

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What it is

C1 inhibitor is a protein made by the liver. It acts as a brake on several systems in the blood, including the pathway that makes bradykinin, a chemical that makes blood vessels leaky and causes swelling. This test measures how well the protein works. It is reported as a percent of normal activity.

Why it matters

Too little working C1 inhibitor can let bradykinin build up and cause repeated swelling of the skin, belly or airway, called angioedema. The functional test is the most direct lab measure of this problem. It is low in hereditary angioedema types 1 and 2. Doctors read it together with the protein level and C4, and results that point to hereditary angioedema are repeated to confirm.

If it’s low

A low functional result means the C1 inhibitor is not working as well as it should. It is the key lab finding in hereditary angioedema types 1 and 2, and it also occurs in acquired C1 inhibitor deficiency. It does not confirm either one on its own. It is read with the protein level and C4, and guidelines advise repeating the tests to confirm.

Common causes

  • Sample handling problems, because this test needs the blood kept cold and frozen quickly
  • Test variation: many labs run this test only occasionally, so false results happen and repeat testing is advised
  • Pregnancy, when C1 inhibitor levels fall and then return to usual after delivery
  • Hereditary angioedema from C1 inhibitor deficiency or poor function (types 1 and 2)
  • Acquired C1 inhibitor deficiency, which tends to appear later in life and can go along with another condition or with antibodies against C1 inhibitor

What can help

  • Ask your clinician about repeating the test to confirm it, ideally at a lab that runs it often
  • Ask about the protein level, C4 and, if swelling started later in life, C1q
  • Ask whether any of your medicines, such as some blood pressure medicines or estrogen-containing products, can make swelling more likely

If it’s high

Labs print only a lower limit for this test, so a result above it is the usual finding. A high number on its own is not usually a concern.

Common causes

  • Normal variation between people
  • Recent treatment with C1 inhibitor replacement

What can help

  • Your clinician can put a high result in context with your other tests
  • If you get C1 inhibitor treatment, ask how the timing of the blood draw affects the result

Usual ranges

What labs print as the reference range, by who was tested. Ranges differ a little between labs, so the range on your own report is the one your result is judged against.

For adults: 67 and above % of normal. Source: Mayo Clinic Laboratories (Extended Catalog), UW Medicine Laboratory Medicine and Pathology, Cureus, Frontiers in Allergy.

Mayo Clinic Laboratories interpretation (enzyme immunoassay; exactly 67% is equivocal at Mayo)

  • Low function (flagged abnormal by Mayo Clinic Laboratories)below 41 % of normal
  • Equivocal (Mayo Clinic Laboratories)41 to under 67 % of normal
  • Normal (Mayo Clinic Laboratories)67 and above % of normal

WAO/EAACI 2021 description of hereditary angioedema types 1 and 2

  • Below 50% of normal (the level described in HAE types 1 and 2)below 50 % of normal
  • 50% of normal or higher50 and above % of normal

What can affect your result

  • How the blood sample was handled and how fast it was frozen
  • C1 inhibitor replacement treatment, which raises function
  • Pregnancy: C1 inhibitor levels fall during pregnancy and return to usual after delivery
  • Assay method and lab: cutoffs differ
  • Age: newborn and infant levels are lower than adult levels, and children lack well-established reference values
  • The sample needs careful handling. Mayo Clinic Laboratories asks for the tube to go on wet ice, be spun cold and the serum frozen within 30 minutes of spinning. Samples that are not handled this way may not give reliable results.
  • Treatment with plasma-derived or recombinant C1 inhibitor raises blood function, so timing of the draw matters for people on replacement treatment.
  • Assay method matters: chromogenic and immunoassay-based tests give results that are not interchangeable between labs.
  • Cord blood and infant samples run lower than adult samples.

Inherited low function stays low for life. Function rises after C1 inhibitor replacement treatment and falls again as it clears.

When to talk to a clinician

Call 911 or your local emergency number right away for swelling of the tongue, throat or face that makes it hard to swallow or breathe, or a sense that your throat is closing. See a clinician soon for repeated swelling of the skin or belly pain attacks with no clear cause, especially if a family member has had similar swelling.

Questions you could ask

  • Why was this test ordered for me, and what are you looking for?
  • Could any of my medicines make swelling more likely?
  • Should I still have the protein level or C4 checked?
  • Could treatment I'm getting affect this result?
  • Should we repeat this test to confirm it, and where?
  • Should we check the protein level, C4 and C1q as well?

Questions people ask

What does "equivocal" mean on my report?

Mayo Clinic Laboratories uses it for results between 41% and 67% of normal. They are not clearly normal and not clearly low. Clinicians often repeat the test or look at C4 and the C1 inhibitor protein level.

Why do I need both the function test and the protein test?

The protein test counts how much C1 inhibitor there is. The function test shows how well it works. In hereditary angioedema type 2 the amount can look normal while the function is low, so labs measure both.

Why was my blood drawn on ice?

This test is sensitive to how the sample is handled. Some labs, such as Mayo Clinic Laboratories, ask for the tube to go on ice and for the serum to be frozen quickly so the result is reliable. The collection staff follow the lab's instructions.

Show the science and sources

For clinicians and researchers

Functional C1-INH is measured with a chromogenic substrate assay, an ELISA-type assay that detects C1-INH complexed with C1s, or (at Mayo Clinic Laboratories) an enzyme immunoassay; the read-out affects apparent function and results are not interchangeable across kits (WAO/EAACI 2021). Mayo reports above 67% of normal as normal, 41 to 67% equivocal and below 41% abnormal. UW Medicine uses above 68% as normal, and published hospital reports use 67% or 68% as the lower limit. C1-INH levels fall during pregnancy and return after delivery (WAO/EAACI 2021). In HAE-1 and HAE-2, function is below 50% of normal; protein is also low in HAE-1 and normal or elevated in HAE-2. C4 is usually low but is not recommended as the only test. Acquired C1-INH deficiency looks like HAE-1 in the lab but often has low C1q (about 75% of cases) and may involve anti-C1-INH autoantibodies. Positive results should be repeated, ideally in an experienced lab, without delaying treatment. Cord blood functional levels are about 62% of adult values, so cord blood can give false low results.

LOINC 48494-9 (primary), 4476-8, 82876-4. Specimen: serum.

Sources

  1. The international WAO/EAACI guideline for the management of hereditary angioedema - The 2021 revision and update, World Allergy Organization Journal (2022)
  2. C1 Esterase Inhibitor, Functional, Serum (test ID C1INF), Mayo Clinic Laboratories (Extended Catalog)
  3. Hereditary Angioedema With Normal C1 Esterase Inhibitor: A Case of a Late Diagnosis in a 48-Year-Old Male, Cureus (2025)
  4. Diagnostic delay, SERPING1 allelic heterogeneity, and real-world lanadelumab prophylaxis in Chinese patients with hereditary angioedema due to C1 inhibitor deficiency, Frontiers in Allergy (2026)
  5. C1 Esterase Inhibitor Panel (lab code C1EP), UW Medicine Laboratory Medicine and Pathology

AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.

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