Prothrombin time

The range depends on the reagent the laboratory uses (some use 10 to 13 or 11 to 14 seconds); use the range your lab printed.

The prothrombin time measures how many seconds it takes for a sample of plasma to clot after a trigger is added. It tests the part of the clotting system that depends on vitamin K and on the liver, including clotting factors II, V, VII and X and fibrinogen. A longer time means the blood is slower to clot.

Doctors request it before operations and procedures, when someone bruises or bleeds easily, when looking at how well the liver is working, in people on warfarin (where it is reported as the INR), and when checking whether vitamin K is short.

• Warfarin and similar vitamin K blocking medicines
• Liver disease
• Low vitamin K, including from poor absorption or long courses of antibiotics
• Some direct oral anticoagulant medicines
• Inherited shortage of a clotting factor
• A severe illness that uses up clotting factors

• There is no common cause of a short prothrombin time; a slightly short result is not usually of any importance

• A tube that was under-filled or filled slowly reads long; the laboratory usually asks for a repeat
• A sample that took more than a few hours to reach the laboratory can read long
• Different reagents give different times, so results are not comparable between laboratories unless the INR is used
• Some medicines, including some antibiotics and anticonvulsants, lengthen it by interacting with vitamin K

No fasting needed. Taken in a blue-top citrate tube that must be filled to the line. For people on warfarin the INR, not the PT in seconds, is the number followed.

Written by a UK consultant physician. Typical UK adult reference ranges; your laboratory's own printed range takes precedence.