What’s a Partial Thromboplastin Time Test?

PTT/APTT Test Overview

A partial thromboplastin time (PTT) test measures how long it takes for your blood to clot.
A partial thromboplastin time (PTT) test measures how long it takes for your blood to clot.

Blood clots are your body’s natural defense against blood loss. They are thick clumps of blood that plug cuts, scrapes, and leaky injured blood vessels. When your body takes too long to form them, you may bleed too much.

A partial thromboplastin time (PTT) test measures how long it takes for your blood to clot. Doctors sometimes call this test an activated partial thromboplastin time (aPTT) test. Here’s information to help you understand more about a PTT test, what you can expect while having one done, and what the results mean.

How Does a PTT Test Measure the Time It Takes for Blood to Clot?

When you start to bleed, sticky cells called platelets cork up the broken blood vessel. At the same time, more than a dozen special proteins called clotting factors work together to form a mesh-like clot that gives the platelet cork extra strength.

If one or more clotting factors are damaged or missing, the clot can’t form properly, if at all. The PTT test focuses on how well a specific group of these proteins works. Your doctor may order additional tests to get a fuller picture of your blood’s clotting ability.

Who Gets a PTT Test?

Your doctor may order a PTT test if you have signs or symptoms of a bleeding disorder. These include:

  • Big bruises that happen because of a minor bump or appear out of nowhere
  • A small injury that causes a big bleed that takes a long time to stop
  • Gums that bleed easily
  • Frequent nosebleeds that are hard to stop
  • Bloody urine
  • Heavy menstrual periods in women
  • Swollen, painful joints or muscles

You might also get this test if you:

  • Take heparin: This drug prevents clots. Doctors use the PTT test to see if the drug and dose are working as they should. Too little heparin puts you at risk of dangerous clots. This could cause heart attacks, strokes, or other problems. Too much can cause heavy bleeding.
  • Have unexplained blood clots: Your doctor may order the PTT test to help diagnose a specific immune system disorder that can cause clots. In pregnant women, the disorder can lead to miscarriages and other problems.
  • Need surgery: Your doctor may want to make sure your blood can clot normally before a major operation.

How to Prepare for the Test

There’s no special prep. You won’t need to fast, but don’t eat a high-fat meal beforehand. It can affect the results.

Tell your doctor about all the medications and supplements you take. Aspirin, blood thinners, antihistamines, and some other medicines can interfere with the results.

During the Test

A PTT test involves a simple blood draw that takes just a few minutes. A medical tech will:

  • Clean the skin where the needle goes in (usually on the inside of your arm at the elbow)
  • Put an elastic band on your upper arm so your veins swell with blood
  • Insert the needle and draw the blood into an attached tube that contains chemicals to keep it from clotting before it’s tested
  • Take off the elastic band and remove the needle
  • Put a bandage over the area

What Are the Risks?

There are very few risks with this test. You may feel lightheaded or dizzy afterward. Tell the tech if you do. You may get a bruise at the needle-stick site, but that should fade quickly.

Your vein could swell and hurt after a blood draw. Usually, warm compresses and anti-inflammatory medications like ibuprofen help. You’ll be better in a few days.

What Do the Results Mean?

Different labs have slightly different ranges for normal clotting time. But generally, clotting time is fine if it’s within 25 to 35 seconds.

If you’re getting the test to monitor heparin, your “normal” will be higher -- usually between 60 and 100 seconds.

If your results are above the normal range, your blood clots more slowly. Doctors call this “prolonged” PTT. Many conditions could cause this result. They include:

  • Liver disease: Your liver makes most of your clotting factors. If you have liver damage from either illness or injury, it might not churn out clotting factors in the right amounts. Severe liver disease can prolong PTT.
  • Low levels of vitamin K: You need this vitamin to form several clotting factors. Deficiencies are rare, but you can run low if you eat a very bad diet. Sometimes, you get enough vitamin K in your diet, but your body can’t absorb it. That can happen, for example, if you use certain antibiotics for a long period of time.
  • Hemophilia: This is a rare bleeding disorder. It usually affects males and tends to run in families. People who have the condition have a problem with the proteins that make their blood clot. In serious forms of the disease, bleeding can happen a lot and seemingly for no reason.
  • Von Willebrand disease: This is the most common bleeding disorder. It also runs in families, and it affects both males and females. Severe cases of this condition can prolong PTT.
  • Disseminated Intravascular Coagulation: This is a rare but serious condition caused by injury, infection, cancer, or other illnesses. This complex problem first creates small blood clots throughout the body followed by severe bleeding. Early stage disease may shorten PTT. Later stages can prolong it.

Your doctor will discuss with you what your PTT results mean and whether you need more tests to diagnose a possible problem.

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Johns Hopkins Medicine: “Blood Test: Partial Thromboplastin Time (PTT).”

The U.S. National Library of Medicine: “Partial Thromboplastin Time (PTT) Test.”

American Association for Clinical Chemistry: “Partial Thromboplastin Time (PTT, aPTT).”

Up-to-Date.com: Patient education: “The Antiphospholipid syndrome (Beyond the Basics).”

Cleveland Clinic: “Hemophilia,” “Von Willebrand Disease.”

Stat Pearls: “Partial Thromboplastin Time.”

National Health Services (UK): “Phlebitis (superficial thrombophlebitis).”

Canadian Society of Intestinal Research: “The Liver-An Amazing Organ.”

National Heart, Lung, and Blood Institute: “Disseminated Intravascular Coagulation.”