WARFARIN, oral

  • Warfarin, oral, 5 mg daily adjusted to maintain INR between 2 and 3.
    • Warfarin interactions:

A large number of medicines interact with warfarin leading to under- or over-anticoagulation, and careful evaluation of all new medicines, herbal and over-the counter products is critical. This includes (but is not an exhaustive list):

  • Medicines altering platelet function e.g. NSAIDs, aspirin, clopidogrel, etc.
  • Food or medicines altering vitamin K synthesis e.g. antibiotics.
  • Medicines interfering with warfarin metabolism e.g. efavirenz, rifampicin, macrolide antibiotics, simvastatin, phenytoin, carbamazepine, etc.
  • Grapefruit juice.

Unless INR is markedly out of range the modest adjustments recorded below should be followed:

Initiation

Warfarin initiation dosing protocol (week 1) with INR target: 2–3
Day therapy INR Value Total daily dose
Day 1 5 mg daily
 
(2.5 mg daily for high sensitivity)
2 to 3
days after initiation 		< 1.5
 
1.5 – 1.9
 
2.0 – 2.5
 
> 2.5 		5–7.5 mg daily
 
2.5–5 mg daily
 
2.5 mg daily
 
Hold warfarin and recheck INR next day
2 to 3
days after last INR check 		< 1.5
 
1.5 – 1.9
 
2.0 – 3.0
 
> 3.0 		7.5–10 mg daily
 
5–10 mg daily
 
2.5–5 mg daily
 
Hold warfarin and recheck INR in1–2 days
Frequency of INR monitoring after initiation of warfarin
Check INR
Every
2–3 days 		Until INR within therapeutic range on 2 consecutive INR checks
Then
every week 		Until INR within therapeutic range on 2 consecutive INR checks
Then
every 2 weeks 		Until INR within therapeutic range on 2 consecutive INR checks
Then
every 4 weeks 		When dose is stable, check monthly

Maintenance

Warfarin maintenance dosing protocol to maintain an INR 2-3:

INR<1.5 INR:
1.5-1.9 		INR:
2.0-3.0 		INR:
3.1-4.0 		INR:
4.1-5 		INR:
5.1-9.0 		INR>9.0
Extra Dose.
 
Increase weekly
dose 10%. 		Increase
weekly dose 5%. 		No
change. 		Decrease weekly
dose 5%. 		Withhold
1 dose.
 
Decrease weekly
dose 10%. 		*Withold
2 doses.
 
Decrease weekly
dose 20%. 		Admit.

*History and examination to exclude bleeding. Admit persons with additional risks for bleeding.

Frequency of INR monitoring for maintenance of warfarin
Check INR
Every 3–5 days If start/stop interacting medication, change in diet, change in activity level or other change that could affect INR.
Every 1–2 weeks If dose needed adjustment by 5–10%.
Every 4 weeks If maintained on same stable dose < 6 months.
Every 6–8 weeks If maintained on same stable dose ≥ 6 months.