Prothrombin Time (PT)

Prothrombin is a protein made by the liver. It is one of several substances known as clotting (coagulation) factors. When you get a cut or other injury that causes bleeding, your clotting factors work together to form a blood clot. Blood clotting is an important process that prevents you from losing too much blood when you are injured. Clotting factor levels that are too low can cause you to bleed too much after an injury. Levels that are too high can cause dangerous clots to form in your arteries or veins.

A PT/INR test helps find out if your blood is clotting normally. It also checks to see if a medicine that prevents blood clots is working the way it should. It's also used to look into inherited or acquired clotting disorders.

If you were tested because you are taking warfarin or have certain health conditions, your results will probably be in the form of INR levels. INR levels that are too low may mean you are at risk for dangerous blood clots because your blood clots too easily. INR levels that are too high may mean you are at risk for dangerous bleeding because your blood is taking longer than normal to form a clot. If your INR is not normal, your provider will probably change your dose of warfarin.

For most people taking warfarin, the goal INR is between 2.0 and 3.0. People with certain higher-risk conditions, such as some mechanical heart valves, may have a higher target of 2.5 to 3.5. Your provider will tell you which range fits you.

If you are not taking warfarin, your results may be in the form of a PT time, which is the number of seconds it takes for your blood sample to clot (prothrombin time). If you are not taking warfarin and your INR or prothrombin time results are not normal, it may be a sign of a clotting disorder, supplements with vitamin K (or a diet high in vitamin K), certain medicines, a bleeding disorder, liver disease, or vitamin K deficiency.

Be sure to tell your provider if you are taking any medicines or supplements. Some may affect your results. Other things that may affect your PT/INR results include alcohol, foods high in vitamin K (such as green leafy vegetables), soybean and canola oils, and high-fat diets.