What it measures
INR (international normalised ratio) is the prothrombin time, the time a plasma sample takes to clot, set against a standard. Because it adjusts for differences in the chemicals labs use, an INR from one lab can be compared with one from another. A higher INR means blood takes longer to clot.
Why people test it
- Monitored regularly in people taking warfarin, where the anticoagulation clinic sets a target range.
- Used to check how well the liver is making clotting factors.
- Checked before some operations and procedures, and when bleeding is being looked into.
Typical range
Ranges vary a little by lab and method, and your report shows the lab's own range. For people taking warfarin, the anticoagulation clinic sets a personal target range instead.
Finger-prick or vein?
Lab tests use a vein sample, collected into a clotting tube that must be filled to an exact level. Handheld finger-prick meters are also used by anticoagulation clinics and by some people who test at home.