What is C1 esterase inhibitor?

One of the ways your body protects itself from bacteria and viruses is by producing antibodies to fight them. Another way is through your immune system.

Your immune system can respond to threats before your body forms antibodies. For example, your skin helps keep germs out of your body. Chemicals in your blood help heal cells that are injured by infection. Special proteins help “tag” pathogens for destruction.

Your complement system is part of your 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. Certain health problems can cause deficiencies in these proteins.

Your doctor can order blood tests to check your complement protein levels. One of these tests is the C1 esterase inhibitor test or C1-INH test. The C1-INH test can help your doctor determine if you have enough C1-INH.

Your doctor may order a C1-INH test if you have unexplained inflammation or swelling, known as edema. Your doctor may also order a C1-INH test to assess you for hereditary angioedema (HAE). Symptoms of HAE are:

  • swelling in feet, face, hands, airway, and gastrointestinal wall
  • abdominal pain
  • nausea and vomiting

Your doctor can use a C1-INH test to learn how you’re responding to treatment for autoimmune diseases, such as systemic lupus erythematosus (SLE), as well.

You don’t need to take special steps to prepare for a C1-INH test. All it requires is a blood sample.

A nurse or technician will take a sample of your blood using a needle. They will collect your blood in a tube. Then they will send it to a lab for analysis. Your doctor will explain your results, once they’re available.

The C1-INH test involves minimal risks. You may experience some discomfort when your blood is drawn. You may also feel some pain at the puncture site during or after your blood draw.

Other potential risks of a blood draw include:

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

The results of your C1-INH test can vary, depending on the laboratory used. Talk to your doctor about your specific results.

Normal levels of C1-INH generally range from 16 to 33 milligrams per deciliter. If your C1-INH levels are lower or higher than normal, it may be a sign of:

  • hereditary or acquired angioedema
  • SLE
  • kidney diseases, such as lupus nephritis, glomerulonephritis, or membranous nephritis
  • septicemia, which is also known as blood infection
  • recurring bacterial infections
  • malnutrition

If your C1-INH levels are abnormal, your doctor may order other tests to determine the underlying cause. Your treatment plan will depend on your final diagnosis.

For example, high C1-INH levels may be caused by an ongoing infection. Your doctor may prescribe antibiotics to treat it. This should help bring your C1-INH levels back to normal.

Ask your doctor for more information about your specific test results, follow-up steps, and long-term outcome.