What it measures
Prothrombin time (PT) is the time, in seconds, that a plasma sample takes to clot after the lab adds tissue factor. It tests a group of clotting factors made by the liver, several of which need vitamin K. It is the basis of the INR.
Why people test it
- Part of a clotting screen when unexplained bleeding or bruising is being looked into.
- Used to check how well the liver is making clotting factors.
- Checked before some operations, and when low vitamin K is suspected.
- Reported as an INR for people taking warfarin.
Typical range
Ranges vary by lab, because they depend on the chemicals and machine used, which is why the INR exists. Your report shows the lab's own range.
Finger-prick or vein?
A vein sample is needed. Clotting tests use a special tube that must be filled to an exact level, and blood from a finger prick starts to clot as it is collected.