What Is a Partial Thromboplastin Time Test?

Here’s the routine when you get a cut: apply some pressure, the bleeding stops, slap on a bandage if you need one. Seems easy enough from the outside. But inside your body, even a little nick from shaving kicks off a series of complex steps to stop the bleeding.

One of those steps is to send a bunch of proteins -- called clotting factors -- to the injured area. They fit together in a very specific way to make a blood clot, which is basically a solid lump of blood that stops up the bleeding and helps healing begin.

That’s how it’s supposed to work. When it doesn’t, you may find that you bleed or bruise easily or that you get clots in your blood vessels when you shouldn’t.

That’s when your doctor might order a partial thromboplastin time (PTT) test, which measures how many seconds it takes for your blood to clot.

What Does a PTT Test Look For?

Your body makes several different clotting factors. A problem with any of them -- if it’s missing, broken, or you don’t have enough -- can affect how long it takes a clot to form, if it forms at all.

A PTT test looks at one set of these factors to see how well they’re working. It’s often done along with another test, called the prothrombin time (PT) test, which looks at another set of clotting factors.

Together, they give your doctor a more complete picture of what happens in your body when a clot forms.

Why Would I Need One?

Your doctor might order this test to check for a bleeding disorder, such as hemophilia or von Willebrand disease. Symptoms of bleeding disorders include:

  • Bleeding or bruising easily
  • Blood clots that form when they shouldn’t
  • Blood in your poop or urine
  • Gums that bleed easily
  • Heavy menstrual periods in women
  • Nosebleeds
  • Swelling or pain in your joints

You’d also need this test if you get heparin therapy -- that’s a medicine used to prevent or treat blood clots after you’ve had an issue like a heart attack or stroke. The PTT test helps make sure you get the right dose. You want to prevent dangerous clots, but still let your blood clot when you need it to.

You may also get the test to:

  • Check that your blood clots normally before you go in for surgery
  • Look for a problem with your immune system (some immune system conditions make clots more likely to form -- in women, that can also lead to miscarriages)
  • See how well your liver’s working, since it makes the clotting factors

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How Do I Prepare for It?

There’s nothing special you need to do to get ready.

Let your doctor know about any medicines, herbs, vitamins, or supplements you take, including over-the-counter, prescription, or illegal drugs. Many common meds, such as blood thinners, aspirin, and antihistamines, might affect your results.

What Happens During the Test?

This test is a basic blood draw and takes just a few minutes. A lab tech will:

  • Clean your skin where the needle goes in
  • Wrap a rubber strap around your upper arm -- this creates pressure to make your veins swell with blood
  • Insert a thin needle into a vein, usually on the inside of your arm at your elbow or in the back of your hand
  • Draw the blood
  • Remove the rubber strap and put a bandage on your arm or hand

Are There Any Risks?

Typically, you’ll feel a prick when the needle goes in. That’s usually the worst of it, but since you’re having your blood drawn, there is a small chance of things such as:

  • Bleeding or bruising
  • Feeling dizzy or lightheaded
  • Infection

What Do the Results Mean?

The test tells you how many seconds it took your blood to form a clot. What’s normal varies with different labs, so check with your doctor to help you understand what your numbers mean.

Usually, you get the results within a few hours or a day, but it depends on your lab.

A typical value for a PTT is 60 to 70 seconds. There’s also an activated PTT (aPTT) test that measures the same thing, but they add a substance to your blood to make it clot faster. A typical aPTT value is 30 to 40 seconds.

If you get the test because you’re taking heparin, you’d want your PTT results to be more like 120 to 140 seconds, and your aPTT to be 60 to 80 seconds.

If your number is higher than normal, it could mean several things, from a bleeding disorder to liver disease. You’ll usually get other tests at the same time depending on what your doctor’s looking for.

If your number is lower than normal, which doesn’t happen often, you may have a higher chance of getting blood clots and, for women, having several miscarriages. You’ll likely get more tests to find out what’s going on.

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Will I Get Other Tests at the Same Time?

It depends on what your doctor’s looking for. To learn more about how your blood clots, you may get tests such as:

  • Activated whole thrombin time test (ACT)
  • Prothrombin time test (PT)
  • Thrombin time test (TT)

Other tests you might get include:

  • Dilute Russell viper venom test (DRVVT) to test for lupus anticoagulant, a sign that you have an immune system problem
  • Platelet count, if you’re getting heparin therapy
  • von Willebrand factor to check if you have von Willebrand disease
WebMD Medical Reference Reviewed by Minesh Khatri, MD on January 18, 2017

Sources

SOURCES:

University of Rochester Medical Center: “Activated Partial Thromboplastin Time Test,” “Prothrombin Time.”

KidsHealth: “Blood Test: Partial Thromboplastin Time (PTT).”

Lab Tests Online: “PTT,” “Lupus Anticoagulant Testing.”

Medscape: “Partial Thromboplastin Time, Activated.”

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