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Blood Test Tracker

C1 Esterase Inhibitor, antigenic (protein level)

The amount of C1 inhibitor protein in your blood. A low level is one clue in the testing for hereditary angioedema.

AI-verified against cited sources. Awaiting expert reviewAlso called C1-INH Ag, C1INH Ag, C1INHqLOINC 4477-6

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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 complement system and the pathway that produces bradykinin. Bradykinin makes blood vessels leaky, which causes swelling. This test measures how much C1 inhibitor protein is there. A separate functional test shows how well it works.

Why it matters

Too little working C1 inhibitor can allow bradykinin to build up and cause repeated episodes of swelling of the skin, belly or airway. This is called angioedema. Doctors measure the protein level, the function and C4 together to sort out the cause. The protein level alone can look normal in one form of hereditary angioedema, so it is never read by itself.

If it’s low

A low protein level means there is less C1 inhibitor than expected. It is linked to hereditary angioedema type 1 and to acquired C1 inhibitor deficiency. A single low result does not confirm either one. Guidelines suggest repeating the tests to confirm.

Common causes

  • Lab or sample variation: false low results can happen, which is one reason repeat testing is advised
  • Samples from newborns, whose levels are lower than adult levels
  • Pregnancy, when levels fall and then return to usual after delivery
  • Hereditary angioedema type 1, an inherited C1 inhibitor deficiency
  • Acquired C1 inhibitor deficiency, which can appear later in life, sometimes alongside another condition

What can help

  • Ask your clinician about repeating the test to confirm it, ideally at a lab that runs it often
  • Ask about the functional test, 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

A high C1 inhibitor protein level is usually not a concern on its own. In hereditary angioedema type 2, the protein level can be normal or high even though the protein does not work well. That is why the functional test is done too.

Common causes

  • Normal variation between people
  • Hereditary angioedema type 2, where the protein is present but works poorly
  • Recent treatment with C1 inhibitor replacement

What can help

  • Ask your clinician to read the protein level together with the functional test and C4
  • 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: 21–39 mg/dL. Source: Parkview Health Laboratory, Cureus, Frontiers in Allergy, Mayo Clinic Laboratories (Extended Catalog), UW Medicine Laboratory Medicine and Pathology, Frontiers in Immunology, World Allergy Organization Journal.

What can affect your result

  • C1 inhibitor replacement treatment, which raises levels
  • Method and lab: ranges differ between assay methods
  • Age: newborn and infant levels are lower than adult levels, and children lack well-established reference values
  • Pregnancy: levels fall during pregnancy and return to usual after delivery
  • Sample handling and the lab's experience with this test
  • Treatment with plasma-derived or recombinant C1 inhibitor raises blood levels, so timing of the draw matters for people on replacement treatment.
  • Different methods (nephelometry, turbidimetry, radial immunodiffusion) have different reference ranges, so compare results only with the range printed for that method.
  • Cord blood and infant samples run lower than adult samples.

Inherited low levels stay low for life. Levels rise after C1 inhibitor replacement treatment and fall 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 we also look at the functional test and C4?
  • Could treatment I'm getting affect this level?
  • Should we repeat this test to confirm it?
  • Should we check the functional test, C4 and C1q as well?

Questions people ask

Can my protein level be normal and I still have a C1 inhibitor problem?

Yes. In hereditary angioedema type 2, the protein level is normal or even high but the protein does not work well. That is why labs also measure function.

Why do reference ranges differ between labs?

Labs measure the protein with different methods. Many print 21 to 39 mg/dL (0.21 to 0.39 g/L), Mayo Clinic Laboratories prints 19 to 37 mg/dL, and one published radial immunodiffusion kit used 19.5 to 34.5 mg/dL. Always compare your result with the range on your own report.

Show the science and sources

Units

  • 1 mg/dL = 1 mg/dL
  • 1 mg/dL = 0.01 g/L
  • 1 mg/dL = 10 mg/L
  • 1 mg/dL = 10 µg/mL

Open the C1-INH (protein) unit converter

For clinicians and researchers

C1-INH is a serine protease inhibitor (SERPING1) that regulates C1r/C1s, kallikrein and factor XIIa, among others. In HAE-1 both antigenic and functional levels are low (protein below 50% of the normal mean). In HAE-2 the antigenic level is normal or elevated and function is low (WAO/EAACI 2021). Antigenic assays are immunoturbidimetry/nephelometry or radial immunodiffusion, and their ranges are not interchangeable (for example 0.21 to 0.39 g/L by one nephelometry method, 19 to 37 mg/dL at Mayo, and 19.5 to 34.5 mg/dL by a radial immunodiffusion kit in one study). The guideline recommends testing function, protein and C4 together, repeating positive results to confirm, and using C1q to look for acquired deficiency (low in about 75% of acquired cases). C4 alone is not recommended for diagnosis. Cord blood antigenic and functional levels are about 70% and 62% of adult values, and children under 1 year with HAE usually have low levels but with exceptions. Early results in offspring of affected parents should be repeated after age 1. C1-INH levels fall during pregnancy and return after delivery, so results from pregnancy are read with caution. Confirmation testing must not delay treatment of an acute attack.

LOINC 4477-6 (primary). 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. Hereditary Angioedema With Normal C1 Esterase Inhibitor: A Case of a Late Diagnosis in a 48-Year-Old Male, Cureus (2025)
  3. 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)
  4. Evaluating functional C1INH with multiple laboratory methods across Hereditary Angioedema types, Frontiers in Immunology (2025)
  5. C1 Esterase Inhibitor Protein (Quest send-out test 298), Parkview Health Laboratory
  6. C1 Esterase Inhibitor Antigen, Serum (test ID C1ES), Mayo Clinic Laboratories (Extended Catalog)
  7. 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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