What is the C1 Esterase Inhibitor Test and Why is it Ordered?

C1 esterase inhibitor (C1-INH) is a protein that can be found in the fluid part of your blood. C1-INH is responsible for controlling a protein known as C1, which is part of the body’s complement system, which is a group of around 60 proteins present in the blood plasma or also on the surface of certain cells in the body. The complement system is part of your immune system, and it helps the body recognize any foreign cells that might be responsible for causing disease. There are some types of health conditions that lead to a deficiency in these proteins, and a blood test can help check the complement protein levels. This test is known as the C1 esterase inhibitor test or the C1-INH test. Read on to find out more about what is the C1 esterase inhibitor test and what is it used for.

Your body has a very complex system in place for protecting itself from attack by bacteria, viruses, and any other foreign invaders. One of the ways in which your body protects itself is by the production of antibodies that fight off these foreign invaders. Your immune system is responsible for responding to these threats even before the body starts producing antibodies. For example, there are chemicals present in your bloodstream that help heal the cells that have been injured by infection. Furthermore, your skin also helps keep the germs out of your body.

Your complement system is a part of the body’s immune system, and it is made up of a set of nine different proteins, which are numbered C1 to C9. These proteins help your body in recognizing invading cells that may lead to diseases. There are also many types of health conditions that are caused by a deficiency in these proteins. ⁽¹⁾

If a deficiency in these proteins is suspected, your doctor will prescribe blood tests for checking the levels of these complement proteins. One of these tests is the C1 esterase inhibitor test, commonly abbreviated to only as the C1-INH test. C1 esterase inhibitor test helps your doctor find out if you have sufficient levels of C1-INH proteins.

Your doctor is likely to prescribe a C1 esterase inhibitor or C1-INH test if you have been experiencing unexplained swelling or inflammation, a condition known as edema.

Your doctor may also prescribe C1 esterase inhibitor to determine whether you have hereditary angioedema (HAE). ^{(2) (3)} The common symptoms of HAE include:

• Abdominal pain
• Nausea and vomiting
• Swelling in the face, feet, hands, airways, and gastrointestinal wall

Doctors can also make use of the C1-INH blood test to determine how well you are responding to treatment for autoimmune diseases, including systemic lupus erythematosus (SLE). ^{(4) (5)}

There are no special steps or preparations that need to be done for taking the C1 esterase inhibitor or C1-INH test. It is a simple blood test in which the nurse or technician will just take a sample of your blood using a needle. The blood sample will be collected in a tube, and it will be sent to a laboratory for further analysis.

Once the test results become available, your doctor will explain the results to you.

The C1 esterase inhibitor or C1-INH blood test does not involve any risk. There is only minimal risk involved with this blood test, and you might experience slight discomfort when your blood is drawn. You may also feel some minor pain at the puncture site either during or after your blood is drawn.

Some of the other potential risks associated with having your blood drawn include:

• Excessive bleeding at the puncture site
• Difficult in obtaining a blood sample, which results in multiple needle sticks and jabs
• Fainting due to blood loss
• A buildup of blood under the skin, a condition known as a hematoma
• Infection at the injection site

The results of the esterase inhibitor or C1-INH blood test will vary depending on which laboratory was used. Due to this, it is essential to discuss your individual findings with your doctor.

Normal levels of the C1 esterase inhibitor or C1-INH test typically range from 16 to 33 milligrams per deciliter. C1-INH levels that are lower or higher than this normal range could indicate the following:

• SLE
• Hereditary or acquired angioedema
• Malnutrition
• Recurring bacterial infections
• Septicemia, or blood infection
• Kidney diseases such as membranous nephritis, glomerulonephritis, or lupus nephritis

If you have abnormal C1 esterase inhibitor or C1-INH levels, your doctor is likely to prescribe other tests to determine what is the underlying cause. The final diagnosis from these tests will determine your final treatment plan.

For example, if you have an ongoing infection, then your test might come back with high C1 esterase inhibitor or high C1-INH levels. Accordingly, your doctor will prescribe antibiotics for treating the infection, which should bring your C1-INH levels back to normal.

When you visit your doctor, make sure that you ask about information on your test results, any follow-up steps that have to be taken, as well as the long-term outcome for your condition.

References

1. Cicardi, M., Zingale, L.C., Pappalardo, E., Folcioni, A. and Agostoni, A., 2003. Autoantibodies and lymphoproliferative diseases in acquired C1-inhibitor deficiencies. Medicine, 82(4), pp.274-281.
2. Nielsen, E.W., Johansen, H.T., Holt, J. and Mollnes, T.E., 1994. C1 inhibitor and diagnosis of hereditary angioedema in newborns. Pediatric research, 35(2), p.184.
3. Varga, L., Széplaki, G., Visy, B., Füst, G., Harmat, G., Miklós, K., Németh, J., Cervenak, L., Karádi, I. and Farkas, H., 2007. C1-inhibitor (C1-INH) autoantibodies in hereditary angioedema: strong correlation with the severity of disease in C1-INH concentrate naïve patients. Molecular immunology, 44(6), pp.1454-1460.
4. Meszaros, T., Füst, G., Farkas, H., Jakab, L., Temesszentandrasi, G., Nagy, G., Kiss, E., Gergely, P., Zeher, M., Griger, Z. and Czirják, L., 2010. C1-inhibitor autoantibodies in SLE. Lupus, 19(5), pp.634-638.
5. Jazwinska, E.C., Gatenby, P.A., Dunckley, H. and Serjeantson, S.W., 1993. C1 inhibitor functional deficiency in systemic lupus erythematosus (SLE). Clinical & Experimental Immunology, 92(2), pp.268-273.