Definition

Partial thromboplastin time (PTT) is a blood test that looks at how long it takes for blood to clot. It can help tell if you have a bleeding problem or if your blood does not clot properly. This test measures the function of a part of the coagulation (clotting) system.

A related blood test is prothrombin time (PT), which measures the function of a different part of the clotting system.

Alternative Names

APTT; PTT; Activated partial thromboplastin time

How the Test is Performed

A blood sample is needed. If you are taking any blood-thinning medicines, you will be watched for signs of bleeding.

How to Prepare for the Test

Your health care provider may tell you to temporarily stop taking medicines that may affect the test results. Be sure to tell your provider about all the medicines you take. Also, tell your provider about any herbal remedies you take.

How the Test will Feel

When the needle is inserted to draw blood, some people feel moderate pain. Others feel only a prick or stinging. Afterward, there may be some throbbing or slight bruising. This soon goes away.

Why the Test is Performed

You may need this test if you have problems with bleeding or your blood does not clot properly. When you bleed, a series of actions involving many different proteins (clotting factors) take place in the body that helps the blood clot. This is called the coagulation cascade. The PTT test looks at some of the proteins or factors involved in this process and measures their ability to help blood clot.

The test may also be used to monitor patients who are taking heparin, a blood thinner.

A PTT test is usually done with other tests, such as the prothrombin test or PT.

Normal Results

In general, the PTT should be 25 to 35 seconds. If the person is taking blood thinners, clotting takes longer.

Normal value ranges may vary slightly among different labs. Some labs use different measurements or may test different specimens. Talk to your provider about the meaning of your specific test results.

What Abnormal Results Mean

An abnormal (too long) PTT result may also be due to:

• Bleeding disorders, a group of conditions in which there is a problem with the body's blood clotting process
• Disorder in which the proteins that control blood clotting become over active (disseminated intravascular coagulation)
• Liver disease
• Difficulty absorbing nutrients from food (malabsorption)
• Low level of vitamin K
• Drugs such as heparin

Risks

There is little risk involved with having your blood taken. Veins and arteries vary in size from one person to another, and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.

Other risks associated with having blood drawn are slight, but may include:

• Fainting or feeling lightheaded
• Hematoma (blood accumulating under the skin)
• Multiple punctures to locate veins
• Infection (a slight risk any time the skin is broken)

This test is often done on people who may have bleeding problems. Their risk of bleeding is slightly higher than it is for people without bleeding problems.

References

Lee GM, Ortel TL. Antithrombotic therapy. In: McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 24th ed. Philadelphia, PA: Elsevier; 2022:chap 43.

Schafer AI. Approach to the patient with bleeding and thrombosis. In: Goldman L, Schafer AI, eds. Goldman-Cecil Medicine. 26th ed. Philadelphia, PA: Elsevier; 2020:chap 162.