Warfarin Dosage Adjustment
Warfarin is one of the most commonly used oral anticoagulants. It is a vitamin-K antagonist (VKA) that has a narrow therapeutic index. While the newer oral anticoagulants (NOAC) like dabigatran, apixaban, and rivaroxaban are starting to replace warfarin in many clinical situations requiring anticoagulation, they are expensive. Moreover, in valvular atrial fibrillation and in patients who have undergone valve replacement, warfarin is still the drug of choice; guidelines do not recommend NOACs yet, in these indications.
A common situation in Medicine and Cardiology wards is the need for warfarin dosage adjustment. To assist you, here is a handy table with all the details for dosage adjustment.
Identifying high-risk patients
There are some high-risk conditions where the use of warfarin may lead to bleeding risk. Various guidelines suggest the use of scoring systems like CHADS2-Vasc in these situations to assess the need for anticoagulation:
- Age >60 years
- Malnourishment, low BMI
- Liver disease (Child-Pugh B/C)
- Recent major surgery, or high bleeding risk
- Taking medications known to increase warfarin activity or increase bleeding risk (amiodarone, atorvastatin, rosuvastatin, fluconazole, itraconazole, ketoconazole, voriconazole, sulfamethoxazole, aspirin >100mg/day, ciprofloxacin, levofloxacin, metronidazole, sulfasalazine, disulfiram)
We should obtain a baseline (within current admission) INR of each patient before starting therapy. Subsequently, we should overlap oral anticoagulation with either subcutaneous heparin or LMWH for at least five days, and until the INR is within the therapeutic range for two consecutive days. Additionally, we should monitor INR every 24 hours until the newly initiated patients reach the target therapeutic range.
In the long-term, we should follow-up stable patients (and patients with no recent changes in medications with INRs in the desired range) with an INR every 30-60 days.
Warfarin & Diet
Vitamin K is an antagonist of warfarin. Hence, we recommend patients to avoid foods that have high vitamin K content, to achieve a quick and stable INR therapeutic range. In addition, we also recommend patients to take the tablet in the evening, at about 5 or 6 PM, with no food intake one hour before and after. Warfarin needs a food-free interval to improve its bioavailability.
Avoid these vitamin K rich foods when taking warfarin:
- Green leafy vegetables, such as spinach, lettuce etc.
- Vegetables such as spring onions, broccoli, cauliflower, and cabbage
- Green tea
In addition, it is also wise to avoid alcohol and cranberry juice as they unpredictably increase the effect of warfarin.
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