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INR Testing

Key Messages INR PDF
Initiation of warfarin therapy
Monitoring INR
Other issues for INR management


Warfarin is the most widely used anticoagulant in New Zealand. It has a valuable role in the prevention of thrombosis but the use of warfarin is associated with serious risks. Warfarin is the most frequent cause of adverse drug reactions in New Zealand (Didham, 2006).

To ensure safe and effective anticoagulation, a systematic and practice-wide approach is needed for warfarin therapy and the maintenance of INR levels within appropriate target ranges.

The role of INR

INR testing is used to maintain warfarin response within the therapeutic window. Maintaining the INR within a target range is the key to minimising the risks of bleeding while providing the benefits of anticoagulation (Blann, 2003).

What is INR?

The basis of the INR is the one-stage prothrombin time (PT) using the reagent thromboplastin. In the past, PT values varied dependent on the particular thromboplastin used. To avoid this, all thromboplastins are now standardised against a WHO standard, and are assigned an International Sensitivity Index (ISI). This enables the International Standardised Ratio (INR) to be calculated. The development of INR has enabled patients on warfarin therapy to be managed more effectively as the INR result is independent of the thromboplastin used and therefore comparable across all laboratories.

Some people are at particular risk from warfarin therapy

The large number of biological and other variables involved means that achieving good control of INR levels is not a simple task.
  • There is no standard response to warfarin - some people are particularly sensitive to the effects of warfarin while others can be relatively resistant.
  • Elderly people require lower doses of warfarin to achieve target INRs and may find attending for regular blood tests and adhering to complex warfarin regimens difficult.
  • Poor literacy or numeracy skills are associated with poorer control of INR levels.

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