Antithrombin Test

Why am I having this test?

An antithrombin test may be ordered for two reasons:

You have blood clots, or have had blood clots in the past, and your health care provider wants to find out the cause.
You are receiving heparin therapy but it is not working properly, and your health care provider wants to find out why.

There are two tests to check your antithrombin:

Antithrombin activity test.
Antithrombin antigen test.

The results of these tests can be combined to check how much healthy antithrombin your body is making. Together, these are sometimes called a functional antithrombin III assay. Both are blood tests. Unusually low levels of functional antithrombin can increase your risk for blood clots (hypercoagulable state).

What is being tested?

This test checks how much antithrombin your body is making and how healthy it is. Your body makes different types of proteins to control how your blood clots. Antithrombin is a protein that prevents too much blood clotting. If your body does not make enough antithrombin, or makes antithrombin that does not work properly, you might be at risk of having your blood clot too much. This increases the risk of having blood clots travel through your blood system to your lungs (pulmonary embolism) or brain (stroke).

What kind of sample is taken?

A blood sample is required for this test. It is usually collected by inserting a needle into a blood vessel.

Tell a health care provider about:

All medicines you are taking, including vitamins, herbs, eye drops, creams, and over-the-counter medicines.
Whether you are pregnant or may be pregnant.
Any blood disorders you have.
Any medical conditions you have.

How are the results reported?

Your test results will be reported as values. Your health care provider will compare your results to normal ranges that were established after testing a large group of people (reference ranges). Reference ranges may vary among labs and hospitals. For this test, common reference ranges are:

Antithrombin activity:
- Newborn: 35–40%.
- Age 6 months to adult: 80–130%.
Antithrombin antigen assay:
- Plasma: Greater than 50% of control value.
- Serum: 15–34% lower than plasma value.
- Immunologic: 17–30 mg/dL.
- Functional: 80–120%.

What do the results mean?

Higher than normal functional antithrombin can be seen in:

Acute hepatitis.
Obstructive jaundice.
Vitamin K deficiency.
A history of kidney transplant.

Lower than normal functional antithrombin can be seen in:

Blood clotting disorders such as:
- Disseminated intravascular coagulation (DIC).
- Hypercoagulable state.
Liver failure or scarring of the liver (cirrhosis).
Kidney disease, such as nephrotic syndrome.
Cancer.
A genetic (hereditary) disease that causes low levels of functional antithrombin.

Talk with your health care provider about what your results mean.

Questions to ask your health care provider

Ask your health care provider, or the department that is doing the test:

When will my results be ready?
How will I get my results?
What are my treatment options?
What other tests do I need?
What are my next steps?

Summary

An antithrombin test may be done if you have blood clots, or have had blood clots in the past, and your health care provider wants to find out the cause.
The test may also be done if you are receiving heparin therapy but it is not working properly, and your health care provider wants to find out why.
Antithrombin is a protein that prevents too much blood clotting. This test checks how much antithrombin your body is making and how healthy it is.
Lower than normal functional antithrombin can increase your risk for blood clots (hypercoagulable state).

This information is not intended to replace advice given to you by your health care provider. Make sure you discuss any questions you have with your health care provider.