Other names: C1 Esterase Inh. Ag
C1 Esterase Inhibitor Antigen, often abbreviated as C1-INH Ag, is a blood test that measures the amount of C1 esterase inhibitor protein in your blood. This protein plays a crucial role in regulating parts of your immune system and the complement system, which helps fight infections and control inflammation.
C1 esterase inhibitor helps keep your immune system in balance by preventing excessive activation of the complement pathway, a part of your immune defense. If you don’t have enough of this protein, or if it’s not working properly, it can lead to uncontrolled inflammation and swelling in the body.
This test is most commonly used to help diagnose Hereditary Angioedema (HAE) Type I — a rare but serious genetic condition that causes sudden, recurrent episodes of swelling in the face, throat, abdomen, limbs, or airway.
It may also be used to evaluate:
Acquired angioedema (AAE), which can occur in adults due to other conditions like lymphomas or autoimmune disorders.
Unexplained recurrent swelling or abdominal pain without allergic triggers.
| C1-INH Antigen Level | Possible Interpretation |
|---|---|
| Low | Suggests Type I Hereditary Angioedema (HAE) or Acquired Angioedema (AAE) |
| Normal or High | If symptoms persist, a functional C1-INH test may be needed to check whether the protein is working properly (to rule out Type II HAE) |
It’s important to interpret this result alongside C4 levels and possibly a C1-INH functional assay for a full diagnostic picture.
Swelling of the face, lips, tongue, throat, or extremities
Abdominal pain, cramping, or bloating
Difficulty breathing during swelling episodes
No response to standard allergy medications (like antihistamines or steroids)
A blood sample is drawn, usually from a vein in your arm. No special preparation is needed in most cases.
Doctors often order additional tests alongside C1-INH Ag for a more complete diagnosis:
C1 Esterase Inhibitor, Functional – Measures whether the protein works properly
Complement C4 levels – Often low during HAE attacks
C1q level – May help differentiate hereditary vs. acquired angioedema
The C1 Esterase Inh. Ag (Antigen) test measures how much of this critical immune-regulating protein is in your blood. A low result can point to Type I Hereditary Angioedema, a condition that causes episodes of swelling that can be life-threatening if untreated. If your antigen level is normal but symptoms continue, your doctor may order further tests to check how well the protein is functioning.
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Elevated levels of C1 esterase inhibitor antigen (C1-INH Ag) are uncommon and, by themselves, are usually not clinically significant. This test is most often used to detect low levels of the protein in conditions such as Hereditary Angioedema (HAE) Type I or Acquired Angioedema (AAE). However, if your C1-INH Ag levels are above the reference range, here’s what it may mean:
C1-INH is a serine protease inhibitor and can act as an acute-phase protein. Its levels may rise modestly in response to:
Inflammation
Infection
Tissue injury
Autoimmune activity
However, this increase is generally less pronounced and less consistent than with other acute-phase proteins like C-reactive protein (CRP) or serum amyloid A (SAA). In these cases, elevated C1-INH Ag reflects a nonspecific immune response.
Rarely, elevated C1-INH Ag may reflect a compensatory increase in production due to chronic activation of the complement or contact systems. In Hereditary Angioedema Type II (HAE-II), for example:
C1-INH Ag levels may be normal or even elevated (sometimes significantly above the reference range)
The protein is dysfunctional and does not work properly
This paradoxical elevation can serve as a clue for HAE-II, especially when accompanied by low C4 levels.
Some individuals may naturally have higher C1-INH Ag levels due to genetic variation or minor laboratory fluctuation, without any clinical consequence.
As with any laboratory test, spurious elevation can occur due to:
Sample handling or processing issues
Concurrent medications
Technical variability in assay methods
An elevated C1-INH antigen level does not guarantee that the protein is functioning correctly. For example, in HAE-II:
C1-INH Ag may be normal or high
The protein is dysfunctional
Functional testing is essential to determine whether the C1-INH protein is working as it should.
If your C1-INH Ag is elevated, your clinician may consider:
Ordering a C1-INH functional assay to assess protein activity
Testing C4 and C1q levels to evaluate complement system activity
Low C4 with normal C1q: Suggests HAE-II
Low C4 with low C1q: Suggests AAE
Reviewing symptoms, medical history, and other inflammatory markers
Elevated C1 esterase inhibitor antigen levels are usually not a cause for concern on their own and are most often seen in the context of inflammation, protein dysfunction (as in HAE-II), or laboratory variation. They are not associated with angioedema unless accompanied by symptoms and other abnormal complement markers. If elevated C1-INH Ag is found, further functional and complement testing is recommended to guide diagnosis and management.
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I have been using Healthmatters.io since 2021. I travel all over the world and use different doctors and health facilities. This site has allowed me to consolidate all my various test results over 14 years in one place. And every doctor that I show this to has been impressed. Because with any health professional I talk to, I can pull up historical results in seconds. It is invaluable. Even going back to the same doctor, they usually do not have the historical results from their facility in a graph format. That has been very helpful.
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Advanced Plan Member since 2020
What fantastic service and great, easy-to-follow layouts! I love your website; it makes it so helpful to see patterns in my health data. It's truly a pleasure to use. I only wish the NHS was as organized and quick as Healthmatters.io. You've set a new standard for health tracking!
Paul
Healthmatters Pro Member since 2024
As a PRO member and medical practitioner, Healthmatters.io has been an invaluable tool for tracking my clients' data. The layout is intuitive, making it easy to monitor trends and spot patterns over time. The ability to customize reports and charts helps me present information clearly to my clients, improving communication and outcomes. It's streamlined my workflow, saving me time and providing insights at a glance. Highly recommended for any practitioner looking for a comprehensive and user-friendly solution to track patient labs!
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