International Normalised Ratio (INR) testing is well established as an integral part of warfarin treatment. INR has a critical role in maintaining the warfarin response within a therapeutic range, to provide the benefits of anticoagulation, while avoiding the risks of haemorrhage. The bestpractice Decision Support INR Monitoring module provides clinical support for health professionals in managing patients’ anticoagulant therapy.


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 as it is the most frequent cause of adverse drug reactions in New Zealand.

An individual patient assessment addressing co-morbidities, medications and the ability to comply with monitoring is essential for the safe use of warfarin. Once the decision to use warfarin is made, prevention of bleeding complications relies on maintaining an INR within the recommended therapeutic range. To achieve this, appropriate monitoring and dose adjustments are required, as well as patient education to effectively manage any modifiable risk factors.

To ensure safe and effective anticoagulation for patients, a systematic and practice-wide approach is needed for warfarin therapy and the maintenance of INR levels within appropriate target ranges. The BPAC Inc web-based INR Monitoring module has been designed to support clinicians in achieving these goals.

How it works

The bestpractice Decision Support INR Monitoring module utilises a formula developed by Ryan et al. (BMJ 1989; Vol 299: 1207-1209) to calculate oral anticoagulant doses and recommend appropriate follow up times. By applying the Ryan algorithm (also known as the Coventry equation), dosing schedules are optimised to keep INR levels within the target range for each condition.

The INR monitoring module is designed for INR maintenance and not for initiation of warfarin therapy. If a patient has less than six INR result recorded in their records, the clinician will be prompted to confirm the patient is on a stable regime. The bestpractice Warfarin Initiation Protocol is available through the Resources tab.

Safety questions
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Interactions, diet changes and unintentional over- or under-dosing can all increase INR and bleeding risk.

Initial safety questions help monitor potential areas of concern so these can be investigated and noted in the patient’s record. The bestpractice INR module checks for any adverse reaction to therapy and on the patients’ general wellbeing such as a new illness such as fever or diarrhoea which may affect their warfarin therapy.

An erratic INR may reflect non-adherence to the drug regime often due to misunderstanding of dosage requirements. A missed dose of warfarin is usually reflected in the INR result 2 to 5 days after the missed dose, although a response may be seen within 16 hours.

Warfarin interacts with multiple other prescriptions medicines, as well as nutritional supplements, over-the-counter medicines and some foods e.g. foods that contain high levels of vitamin K such as broccoli, spinach and cabbage. Effects are more marked when starting, changing or stopping the dose. Quality patient information is available through the bestpractice Decision Support tool so patients can receive education about managing their warfarin therapy.

Warfarin drug interactions table
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A warfarin drug interactions table shows some of the important interactions with warfarin and how they will affect the risk of bleeding.
Warfarin dosing history
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INR lab results and warfarin dosing history are plotted on a graph and displayed in a chart to illustrate the patient’s previous results.

When the patient’s diagnosis is entered, the duration of treatment is calculated giving clinicians the INR target range for that condition. The patient’s INR result will be extracted from the practice management system or it can be entered manually. The module then immediately gives a dose recommendation and the date of the next recommended review.

The clinician can override a dose or review date if appropriate.

Suggested dosing and review
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Warfarin dose calculation and recommended review date are calculated immediately.
Patient dose calendar
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A Patient Dose Calendar is also available that can be printed for the patient and saved to the Outbox of the patient record.
Current review summary
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A summary of the current review can be printed for the patient and saved to the practice management system.

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