Patient discussions
If warfarin is chosen for anticoagulation, initiate careful discussion with the patient about its proper use and the need for regular follow-up and monitoring of international normalised ratio (INR). As a guide, cover the following points to ensure safe and effective management.
Warfarin makes the blood more difficult to clot and therefore carries a risk of bleeding.
The effect of the drug is measured with a blood-clotting test called the INR.
Warfarin dose frequently changes over time; commonly, dosing varies according to the day of the week.
Patients should be given advice on daily dosing, given the day-to-day fluctuations in dose.
The target INR values are generally between 2 and 3.
Many drugs interact with warfarin, so the clinician who oversees the warfarin treatment must be notified whenever a new medicine (e.g., prescription or over-the-counter medicine, supplement, or herbal therapy) is started for the first time, or when a current medication is stopped or the dose is adjusted. Non-steroidal anti-inflammatory drugs (NSAIDs) should be avoided or used with extreme caution under clinical supervision.
Even when medications do not interact with INR testing, they may still increase the risk of bleeding through pharmacodynamic interactions (NSAIDs, selective serotonin-reuptake inhibitors).
Dietary changes can affect the INR, especially the intake of foods with high amounts of vitamin K (e.g., spinach, broccoli); eating any amount of vegetables or food high in vitamin K is acceptable, so long as the intake is consistent from week to week. Alcohol should be consumed with caution and only in small amounts. Grapefruit juice should be avoided.
Activities that carry a high risk of trauma or serious bleeding should be avoided, or if this is not possible, additional safety precautions should be taken.
The INR must be checked (monitored) frequently, with blood tests, often once or twice weekly until the stable dose is reached, then on an extended interval (4-12 weeks) thereafter.
Advise the patient on how to handle a missed dose (the approach may vary according to the anticoagulation clinic).
Make sure the patient is very clear about the daily dose of warfarin and the colours of their different warfarin tablets (a pill organiser may help).
Although direct oral anticoagulants (DOACs) do not require coagulation assay laboratory monitoring and drug-drug interactions are minimised when compared with warfarin, there are still some medications that interact with DOACs and can lead to either increased risk of bleeding or increased risk of thrombosis (e.g., primidone, amiodarone, diltiazem, verapamil, rifampicin, phenytoin, phenobarbital). Interactions are most commonly mediated via cytochrome P450 enzyme (CYP450) and/or the transporter permeability glycoprotein (P-gp).[161]
Give the patient information about the range of signs and symptoms of bleeding and recurrent thrombosis, and advise them to seek immediate medical attention if these occur.
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