Prothrombin time (PT)

This lab test measures how long it takes for your blood to clot. It becomes very important in monitoring the safety and how effective the medication warfarin is.

Prothrombin time (PT) Overview

Reviewed: April 22, 2014

Prothrombin time (PT) measures the time it takes for blood to clot. In addition, PT evaluates how warfarin is working at thinning the blood.

Prothrombin time (PT) is measured in seconds. The normal ranges for prothrombin time (PT) are the following:

  • Goal range: 9 - 12.5 seconds
  • If being treated with warfarin, goal for PT is 24 seconds


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Fasting is not required before the test

What the results mean

If the value falls below the normal range for a patient on anticoagulation, blood clots more quickly than expected. The dose may need to be increased.

If the value is greater than 30 seconds, it takes blood longer than usual to clot. In turn, the risk for bleeding is increased.