Prothrombin time test results can be presented in two ways.

In seconds

The average time range for blood to clot is about 10 to 14 seconds. A number higher than that range means it takes blood longer than usual to clot. A number lower than that range means blood clots more quickly than normal.


This ratio — which allows for easier comparisons of test results from different laboratories — is used if you take blood-thinning medications.

In healthy people an INR of 1.1 or below is considered normal. An INR range of 2.0 to 3.0 is generally an effective therapeutic range for people taking warfarin for disorders such as atrial fibrillation or a blood clot in the leg or lung. In certain situations, such as having a mechanical heart valve, you might need a slightly higher INR.

When the INR is higher than the recommended range, it means that your blood clots more slowly than desired, and a lower INR means your blood clots more quickly than desired.

What your results mean

Clotting too slowly

Blood that clots too slowly can be caused by:

  • Blood-thinning medications
  • Liver problems
  • Inadequate levels of proteins that cause blood to clot
  • Vitamin K deficiency
  • Other substances in your blood that hinder the work of clotting factors

Clotting too fast

Blood that clots too quickly can be caused by:

  • Supplements that contain vitamin K
  • High intake of foods that contain vitamin K, such as liver, broccoli, chickpeas, green tea, kale, turnip greens and products that contain soybeans
  • Estrogen-containing medications, such as birth control pills and hormone replacement therapy