What is the C1 Esterase Inhibitor Test?

The C1 Esterase Inhibitor (C1-INH) test measures the level of a protein that controls activation of the classical complement pathway. C1-INH prevents excessive complement activation and helps regulate inflammatory and immune responses.

When C1-INH levels are low or dysfunctional, uncontrolled complement activation can occur, leading to increased vascular permeability and swelling. This mechanism underlies hereditary and acquired forms of angioedema, which present with recurrent episodes of non-allergic swelling.

This test is commonly performed alongside complement components such as C4 to provide a clearer assessment of complement regulation.

Why do I need a test for C1 Esterase Inhibitor?

You may need this test if you experience recurrent episodes of unexplained swelling, particularly involving the face, lips, tongue, throat, abdomen or extremities. Unlike allergic angioedema, C1-INH–related angioedema does not respond to antihistamines or steroids.

Testing helps distinguish hereditary angioedema from allergic causes and guides appropriate treatment and long-term management. It is also used to investigate acquired angioedema, which may be associated with autoimmune or lymphoproliferative disorders.

Symptoms and scenarios where testing is recommended:

• Recurrent facial or lip swelling
• Throat or tongue swelling
• Abdominal pain with swelling
• Swelling without itching or hives
• Poor response to antihistamines
• Family history of angioedema
• Suspected hereditary angioedema
• Investigation of acquired angioedema

Benefits of the C1 Esterase Inhibitor Test

·      Key diagnostic marker for angioedema
Identifies complement-related causes of swelling.

·      Differentiates allergic from non-allergic angioedema
Prevents misdiagnosis and inappropriate treatment.

·      Supports diagnosis of hereditary angioedema
Essential for confirming inherited complement disorders.

·      Assesses acquired angioedema
Useful when symptoms begin later in life.

·      Guides appropriate treatment strategies
Helps determine need for targeted therapies.

·      Reduces risk of life-threatening complications
Early diagnosis helps prevent airway compromise.

·      Supports family screening where appropriate
Identifies inherited risk in relatives.

·      Simple and informative blood test
Requires only a standard blood sample.

Step-by-Step: How Testing Works

Choose your preferred testing method

You can attend one of our UK partner clinics or arrange a home or hotel visit with a mobile phlebotomist.

Option 1 – In-Clinic Blood Draw

1. Select your nearest UK clinic and book an appointment.
2. A clinician or phlebotomist collects a venous blood sample.
3. The sample is analysed for C1 esterase inhibitor levels.
4. Results are delivered securely online.

Option 2 – Home or Hotel Phlebotomy (+£60)

1. Book a UK-wide home or hotel visit at a convenient time.
2. A trained phlebotomist collects your blood sample on-site.
3. The sample is packaged and transported to the laboratory.
4. Results are uploaded digitally without the need to travel.

Access your report

You can download and share your results with your GP, immunologist or specialist.

Your Results Explained

Low Levels

Low C1 esterase inhibitor levels suggest deficiency or increased consumption.
This is commonly seen in hereditary or acquired angioedema.
Typical UK reference range: approximately 0.21–0.39 g/L (ranges may vary by laboratory).

Normal Levels

Normal levels indicate adequate complement regulation.
This makes C1-INH–related angioedema less likely.

High Levels

Elevated levels are uncommon and usually not clinically significant.
Results are interpreted alongside C4 levels and clinical history.