Antithrombin Test

Why am I having this test?

You may have an antithrombin test if:

You have blood clots or have had blood clots in the past. This test can help find the cause.
You are receiving heparin, a blood thinner, but it is not working like it should. This test can help find out why.

There are two blood tests done. They are the antithrombin activity test and the antithrombin antigen test. When looked at together, they are called a functional antithrombin III assay. These tests show how much healthy (functional) antithrombin your body is making. If your levels of functional antithrombin are too low, this 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 stops your blood from clotting too much. If your body does not make enough antithrombin or makes antithrombin that does not work like it should, your blood may 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 person having a blood sample taken from the arm.

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 bleeding problems 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 levels of functional antithrombin can be a sign of:

Hepatitis.
Blockage of a bile duct.
A lack (deficiency) of vitamin K.
A history of kidney transplant.

Lower than normal levels of functional antithrombin can be a sign of:

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?

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.