C1 Esterase Inhibitor Test

Written by Darla Burke
Medically Reviewed by George Krucik, MD

What Is C1 Esterase Inhibitor?

Your immune system protects the body from bacteria and viruses. One of the ways it does this is by producing antibodies. Another way is through the innate immune system.

The innate immune system can respond to threats before antibodies are formed. The complement system is part of the innate immune system. It consists of a set of nine proteins numbered C1 through C9. They help your body recognize foreign cells that may cause disease.

Blood tests can be used to check your complement protein levels. One such test is the C1 esterase inhibitor or C1-INH test. The C1-INH test can determine if you don’t have enough C1-INH. Deficiencies may indicate a health problem.

Why Is the Test Ordered?

Your doctor may order the C1-INH test if you have unexplained inflammation or swelling (edema). It may also be ordered for symptoms of systemic lupus erythematosus (SLE), such as:

  • fatigue
  • fever
  • joint swelling, pain, or stiffness
  • a rash over the bridge of the nose that looks like a butterfly
  • skin lesions
  • Raynaud’s phenomenon (toes or fingers become blue when you are stressed or cold)
  • chest pain
  • dry eyes
  • headaches
  • changes in mental status, including memory loss
  • shortness of breath

The C1-INH test can also monitor treatment of autoimmune diseases, such as SLE or hereditary angioedema.

How Is the Test Administered?

There is no special preparation needed for the C1-INH test. The test is performed on a blood sample. A nurse or technician will take your blood using a needle. Your blood will be collected in a tube and sent to a lab for analysis. Once the results are available, your doctor will explain them to you.

What Are the Risks of the Test?

Risks of the C1-INH test are minimal. There may be some discomfort when the blood sample is drawn. There could also be pain at the puncture site during or after the test.

Other potential risks from a blood draw include:

  • difficulty obtaining a sample, resulting in multiple needle sticks
  • excessive bleeding at the needle site
  • fainting as a result of blood loss
  • accumulation of blood under the skin, known as a hematoma
  • infection at the puncture site

Understanding the Results

The results of your test may vary depending on the laboratory used. Talk to your doctor about your specific results. In general, normal levels of C1-INH range from 16 to 33 milligrams per deciliter.

If your C1-INH levels are lower than normal this may indicate:

  • recurring bacterial infections
  • SLE
  • hereditary or acquired angioedema
  • kidney diseases, such as lupus nephritis, glomerulonephritis, or membranous nephritis
  • septicemia (blood infection)
  • malnutrition

Low levels of C1-INH may indicate the presence of a health problem. However, this test alone will not provide a specific diagnosis. The C1-INH test must be used in conjunction with other blood tests.

C1-INH levels may be increased if you have an ongoing infection. Treatment of the infection should bring your C1-INH levels back to normal.

The C1-INH test is not commonly used to identify infections. Other tests, such as C-reactive protein, are generally used instead.

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