Coagulation
INR / Prothrombin time
INR (international normalised ratio) is a standardised expression of the prothrombin time (PT), the time taken for plasma to clot through the tissue-factor (extrinsic) and common coagulation pathways. At the point of care it is measured from a fingerstick drop of whole blood, chiefly to monitor patients on vitamin K antagonist therapy such as warfarin.
Why it is measured
INR allows clinicians to keep oral anticoagulation within a safe therapeutic window, balancing the risk of clotting against the risk of bleeding. Near-patient testing supports same-visit dose review in anticoagulation clinics, pharmacies and at home.
| Typical range | Indicative adult values: normal INR approximately 0.8 to 1.2 (a dimensionless ratio), with the underlying PT approximately 11 to 13.5 seconds. The usual therapeutic target on warfarin is INR 2.0 to 3.0 for most indications, or about 2.5 to 3.5 for higher-risk indications such as some mechanical heart valves. Ranges and targets vary by method, thromboplastin ISI and local protocol. |
|---|---|
| Sample | Capillary whole blood from a fingerstick, typically about 8 microlitres applied directly to a test strip. Some meters also accept fresh, non-anticoagulated venous whole blood. No separate citrate tube or plasma separation is needed for the fingerstick mode. |
| Turnaround | Result typically within about 1 minute, generally under 2 minutes, with an on-strip quality check on many meters. |
Point of care devices that report it
- Roche CoaguChek (XS, XS Pro, Pro II, INRange)
- Siemens Healthineers / Universal Biosensors Xprecia Stride and Xprecia Prime
- CoaguSense / Coag-Sense PT2 PT/INR Monitoring System
- Abbott i-STAT (PT/INR cartridge)
- LumiraDx INR test
Questions, answered
Why does a point-of-care INR sometimes differ from the laboratory INR?
The two use different specimens and methods: meters read capillary whole blood with a device-specific thromboplastin, while the laboratory uses citrated venous plasma. Agreement is generally close, but a difference of up to around 0.5 INR units can occur, more often at higher INR values. Where the result will materially change management, it is good practice to correlate against a paired laboratory sample per local policy.
Can point-of-care INR be used to monitor direct oral anticoagulants such as apixaban or rivaroxaban?
No. Point-of-care INR is designed and validated for vitamin K antagonist (warfarin) monitoring. Direct oral anticoagulants affect the PT/INR unpredictably, and INR is not used to dose or monitor them. A normal INR does not exclude a clinically relevant DOAC effect.
How often should INR be checked on warfarin?
This is a clinical decision set by the prescriber and local anticoagulation protocol, not by the device. Operationally, testing tends to be more frequent at initiation or after dose changes and less frequent once results are stable. Follow your service's monitoring schedule rather than any fixed device default.