Always take into account potential reasons for elevated and subtherapeutic INRs—there is a need to evaluate more than just the number. Ask your patient about “the 4 Ds” that can influence warfarin dose requirements:
- Diet—any major changes (fasting, weight watchers), liver or mango consumption?
- Drugs—any new medicines, discontinued medicines since last INR check?
- Dose—confirm current warfarin dose, any missed doses?
- Disease—any recent illness, fever, N/V/D?
A nice "rule of thumb" for dose adjustments near the target range (generally works for INRs from the high 1s to low 4s): If you want to change the INR by 0.5-1 unit, increase or decrease the weekly dose by a daily dose.
Example: Your patient has been taking warfarin 5mg daily for >2 weeks and INR is 1.8. Adding 5mg to the week (for instance, 5mg Monday-Friday with 7.5mg Saturday-Sunday) should increase the INR into the mid-2s.