Hereditary angioedema (HAE) is diagnosed through a combination of blood tests that measure complement system proteins, sometimes followed by genetic testing. The standard workup involves three blood tests done together: C1-inhibitor protein level, C1-inhibitor functional activity, and C4 level. No single test is sufficient on its own, and positive results need to be confirmed with repeat testing one to three months later.
Who Should Get Tested
Testing is appropriate if you experience recurrent episodes of swelling in the limbs, face, genitals, or airway that last roughly three to four days per episode. Abdominal attacks with severe cramping, nausea, and vomiting are another hallmark. About one-third of people develop a flat, non-itchy rash during episodes. Symptoms typically begin in childhood and worsen around puberty, with untreated individuals averaging an episode every one to two weeks.
A family history of similar swelling raises suspicion, but about 25% of people with HAE have no known family history, so the absence of relatives with the condition does not rule it out. One of the strongest clues is swelling that fails to respond to antihistamines, corticosteroids, or epinephrine. If standard allergy treatments have never helped your swelling episodes, that pattern alone warrants testing.
The Three Core Blood Tests
International guidelines from the World Allergy Organization and the European Academy of Allergy and Clinical Immunology recommend ordering all three of the following tests together. Using all three gives much higher diagnostic accuracy than any single test alone.
- C4 level. This complement protein is low in about 90% of HAE patients even between attacks, and virtually always low during an active swelling episode. Normal reference range is 20 to 40 mg/dL. A low C4 is a useful red flag, but a normal C4 does not rule out HAE. Up to 10% of confirmed HAE patients have normal C4 levels when they are not having an attack.
- C1-inhibitor protein level (antigenic). This measures how much C1-inhibitor protein is circulating in your blood. The normal reference range is roughly 19.5 to 34.5 mg/dL. A low level points toward Type I HAE, the most common form.
- C1-inhibitor functional activity. This measures whether the C1-inhibitor protein actually works properly. Values above 50% are considered normal. This test is critical because in Type II HAE, the body produces normal or even elevated amounts of C1-inhibitor protein, but the protein is dysfunctional. Without the functional assay, Type II HAE would be missed entirely.
How Results Map to HAE Types
The pattern of results across these three tests tells your doctor which type of HAE you have. In Type I HAE, which accounts for the majority of cases, both the C1-inhibitor protein level and functional activity are low, and C4 is typically low as well. In Type II HAE, the C1-inhibitor protein level comes back normal or high, but the functional activity is low. This is why the functional assay is essential and cannot be skipped.
If all three tests come back normal but clinical suspicion remains strong, a third category exists: HAE with normal C1-inhibitor. This form can only be confirmed through genetic testing.
Sample Handling Matters
The C1-inhibitor functional assay is notoriously sensitive to how the blood sample is processed. The sample must be placed on wet ice immediately after collection, centrifuged, and frozen within 30 minutes. Samples that have been thawed or show signs of breakdown are rejected outright. If your functional assay comes back abnormally low but you have no symptoms, improper handling could be the reason. This is one of the reasons guidelines call for repeat testing to confirm any positive result.
Confirming the Diagnosis
A single round of abnormal results is not enough for a definitive diagnosis. Guidelines recommend repeating the full panel one to three months after the initial tests. HAE is confirmed when both rounds show the same pattern of low values. This repeat testing catches lab errors, sample handling problems, and transient fluctuations that could mimic HAE.
Distinguishing Hereditary From Acquired Forms
Some people develop angioedema later in life due to an acquired deficiency of C1-inhibitor rather than an inherited one. The lab profiles can look similar, but one additional test helps separate them: the C1q level. In hereditary angioedema, C1q is normal. In acquired angioedema, C1q is low. If your symptoms started in adulthood and you have no family history, your doctor may add C1q to the panel to distinguish between the two.
Genetic Testing
Genetic testing plays two distinct roles in HAE diagnosis. First, it is the only way to diagnose HAE with normal C1-inhibitor. In this form, all complement blood tests come back normal, and diagnosis depends on finding a mutation in one of at least six genes linked to the condition: F12, PLG, ANGPT1, KNG1, MYOF, and HS3ST6. These mutations are the only diagnostic biomarkers available for this subtype. Even so, many patients with this form of HAE still have no identifiable mutation, which makes diagnosis challenging.
Second, genetic testing is valuable for screening family members, especially children, before symptoms develop. It provides a definitive answer regardless of age and avoids the pitfalls of complement testing in very young children.
Testing Children and Infants
If a parent has HAE, their children have a 50% chance of inheriting it. Testing these children early allows treatment plans to be in place before a dangerous episode occurs, but timing matters. The complement system is immature at birth, with C1-inhibitor and C4 levels running at 60 to 100% of adult values. These levels generally normalize around one year of age. French consensus guidelines recommend blood testing from six months onward, with a repeat test if results are low.
The problem is that normal results in infancy can be falsely reassuring. In one documented case, a child tested at eight months showed normal complement levels, but retesting at age four revealed low C1-inhibitor activity. For this reason, many experts now recommend prioritizing genetic testing in infants born to affected parents, since a genetic result is not influenced by the child’s age or complement maturity.
Testing During an Active Attack
If you are currently experiencing a swelling episode, C4 levels are almost universally low in HAE patients during an attack, making it a reliable screening marker in that moment. However, testing performed solely during an attack still needs to be confirmed with repeat testing during a symptom-free period. Imaging studies taken during an attack can show intestinal wall swelling or fluid accumulation, but these findings are only visible during a narrow window and are not used as primary diagnostic tools.