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The partial thromboplastin time (PTT) test helps measure the ability of the blood to clot normally. It is very similar to the prothrombin time (PT) test. These two tests are often ordered together. The PT and PTT tests each measure the function of a different subset of the 12 or more proteins involved in blood clotting.

Who is a candidate for the test?

• when a person has a bleeding problem
• to monitor a person who is taking blood-thinning medicine
• before surgery to make sure a person will not bleed too much during the operation

How is the test performed?

A blood sample is usually taken from a vein on the forearm or hand. The skin over the vein is first cleaned with an antiseptic. Next, a strong rubber tube, or tourniquet, is wrapped around the upper arm. This restricts blood flow through the veins in the lower arm and causes them to enlarge.

A small needle is gently inserted into a vein and the tourniquet is removed. Blood flows from the vein through the needle. It is collected in a syringe or vial. After the needle is withdrawn, the puncture site is covered to prevent bleeding. Pressure is held on the puncture site for a bit longer than usual if a bleeding disorder, especially one involving the blood platelets, is known or suspected. The blood sample is sent to a lab for testing.

What is involved in preparation for the test?

Generally, no preparation is needed for this test. Because test preparation details may vary, a person should ask his or her healthcare professional for specific instructions.

What do the test results mean?

The normal range for a PTT test generally falls between 25 to 45 seconds, depending on technical variables unique to the particular medical lab.

Abnormally high PTT values may occur when a person:

• is taking blood-thinning medicines, especially heparin
• is taking other medicines, such as certain antibiotics, that interfere with the test
• has severe liver disease
• has disseminated intravascular coagulation (DIC), a condition usually accompanying an episode of acute critical illness, during which clotting mechanisms are abnormally activated throughout the body
• has certain inherited bleeding disorders, such as hemophilia
• has a vitamin K deficiency

Abnormally low values are usually not important. Occasionally they are a sign of widespread cancer.