Prothrombin Time (PT) Blood Test

Also called: Pro Time Test

What Is a Prothrombin Time Blood Test?

A prothrombin time (PT) blood test measures how long it takes for a blood clot to form in a blood sample. Clotting is important to help prevent too much bleeding. Proteins called clotting factors are needed for blood to clot properly. If there aren't enough of them, or any of them don't work as they should, it can take longer than normal for blood to clot.

Prothrombin is a clotting factor that the liver makes. A prothrombin time test checks the activity of prothrombin and other clotting factors.

Why Are Prothrombin Time Tests Done?

Doctors do PT tests to check for bleeding problems. Kids might have the test if they:

• have had a lot of bleeding or bruising
• have a medical condition that can lead to problems with clotting
• are having surgery or a procedure that might cause bleeding
• take blood-thinning medicines and doctors want to check the clotting levels 

Top Things to Know

• The prothrombin time (PT) blood test checks the time it takes for blood to start clotting.
• Doctors use it to check for bleeding or clotting problems and to see if blood-thinning medicine is working.
• Kids might need to stop taking certain medicines or not eat or drink before the blood test.
• Your doctor can explain what it means if PT is low or high.

How Can I Help My Child Prepare for a Blood Test?

To help kids get ready for a blood test, find out if they need to fast (not eat or drink) or should stop taking medicines before the test. Explain what to expect during the test. If your child is nervous about it, work together on ways to stay calm. Deep breathing or even imagining being in a place that makes your child feel safe and relaxed can help.

What Do Prothrombin Time Test Results Mean?

Your doctor will explain what the results of the PT test mean for your child. “Normal” PT varies based on things like age and gender.

High Prothrombin Time

High PT means that blood is taking longer than usual to clot. This could be due to things like:

• blood-thinning medicine
• a bleeding or blood clotting condition
• a liver problem
• too little vitamin K

Low Prothrombin Time

Low PT means that blood is clotting faster than usual. This could be due to:

• eating a lot of foods that have vitamin K, like broccoli, kale, soy, chickpeas, and green tea
• using medicines that have the hormone estrogen in them, like some birth control pills or hormone replacement therapy
• taking vitamin K supplements

What Else Should I Know?

When kids have a PT test, they may also have a partial thromboplastin time (PTT) blood test. The two tests check for different clotting factors.

If you have questions about the PT test or what the results mean, talk to your doctor.