INR

This range applies to people who are not taking warfarin; for people on warfarin the doctor sets an individual target range that sits deliberately above it, and the laboratory's printed range does not apply to them.

The INR is the prothrombin time expressed as a standardised ratio, so that results from different laboratories and reagents can be compared. A result of 1.0 means the blood clots in the usual time; 2.0 means it takes about twice as long. It was designed for monitoring warfarin, and that is still its main use.

Doctors use it to monitor warfarin, where the aim is to keep the blood thinner within a set window. It is also requested before operations and procedures, in liver disease, and when someone bruises or bleeds easily.

• Warfarin and similar vitamin K blocking medicines, as intended
• Liver disease
• Low vitamin K, including poor absorption and long courses of antibiotics
• Many medicines that interact with warfarin, including some antibiotics and antifungals
• Alcohol, in people on warfarin
• A severe illness that uses up clotting factors

• In people on warfarin: missed tablets, a diet high in vitamin K (green leafy vegetables), and some medicines such as rifampicin and some anticonvulsants
• Outside warfarin, a slightly low INR is of no importance

• In people on warfarin the INR is meant to sit above the general population range, and a value in their own target window is the aim
• An under-filled or delayed citrate tube reads high
• A change in diet, alcohol intake or a new medicine commonly shifts it in people on warfarin
• Point-of-care finger-prick INR machines can read a little differently from the laboratory

No fasting needed. For people on warfarin the trend and the individual target window matter; a single value out of that window is common and is discussed with the anticoagulant clinic or doctor.

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