WARFARIN INITIATION

• The senior clinician decides to anticoagulate orally, including duration and intensity of treatment

Choice of rapid or slow anticoagulation

Choose:

• Rapid anticoagulation with concurrent LMWH or heparin e.g. for VTE, follow RAPID ANTICOAGULATION
• Slow anticoagulation without concurrent LMWH or heparin e.g. for AF, follow SLOW ANTICOAGULATION

Who initiates?

• Refer inpatients to an anticoagulation management service (AMS)
• Only if patient not referred to AMS, follow this guideline

Review clinical condition

Medication history

• Determine any significant interactions with warfarin
• remember herbal remedies
• Consider whether to discontinue or substitute medications
• particularly important for ‘as required’ medications e.g. NSAIDs, where the interaction may be inconsistent
• Seek further information from medicines information or AMS where necessary

Increased sensitivity to warfarin

• Frail, sick, have multiple comorbidities or take multiple medication
• Aged >80 yr
• Significantly underweight
• Congestive cardiac failure
• Abnormal liver function
• Receiving parenteral nutrition or drugs that potentiate warfarin significantly (see BNF Appendix 1)

Counsel patient

• Reason, risks and benefits of oral anticoagulation
• bleeding risk
• drug interactions (including alcohol)
• need for regular INR monitoring
• Provide anticoagulation information pack

Dose prediction

• Anticoagulation with warfarin takes effect only in 72–96 hrs after first dose
• The following algorithm allows the maintenance dose of warfarin to be predicted over 4 days by optimal interpretation of timed daily INR measurements
• Use INR to guide the selection of daily warfarin dose
• even during concurrent anticoagulant treatment with unfractionated heparin, dalteparin or any other low-molecular-weight heparin
• All warfarin tablets are scored, and
• any doses recommended can be administered by use of 1 mg, 3 mg and 5 mg tablets

Increased sensitivity to warfarin

• If patient has increased sensitivity to warfarin, use half the doses recommended below

Day 1

• Take blood for measurement of INR
• If INR ≥1.4, this predictive method cannot be used
• the choice of dose must rely on clinical judgement alone
• seek advice from AMS
• If INR <1.4 and no increased  sensitive to warfarin, give warfarin 10 mg before evening meal between 1700 and 1800 hr
• remember to halve doses in sensitive patients

Day 2

• Take blood between 0900 hr and 1000 hr (16 hr after first dose of warfarin)
• Measure INR and use the result to select next dose
• remember to halve doses suggested in sensitive patients

Day 3

• Take blood between 0900 hr and 1000 hr (16 hr after second dose of warfarin)
• Measure INR and use the result to select next dose
• remember to halve doses suggested in sensitive patients

Day 4

• Take blood between 0900 hr and 1000 hr (16 hr after third dose of warfarin)
• Measure INR and use the result to select next dose
• remember to halve doses suggested in sensitive patients

Subsequent management

• The dose selected on day 4 is the predicted maintenance dose necessary to achieve a stable INR in the range 2–4
• Watch for INR instability due to changing/starting/stopping of interacting medication or diet (see BNF appendix 1)
• Make dose adjustments after end of initiation regimen onward intuitively
• If concerns of overanticoagulation, see Warfarin overanticoagulation

• Refer patients stabilised on warfarin to AMS for ongoing monitoring
• Order TTO for warfarin along with other medication

Who initiates?

• Refer inpatients to an anticoagulation management service (AMS)
• Only if patient not referred to AMS, follow this guideline

Review clinical condition

Medication history

• Determine any significant interactions with warfarin
• remember herbal remedies
• Consider whether to discontinue or substitute medications
• particularly important for ‘as required’ medications e.g. NSAIDs, where the interaction may be inconsistent
• Seek further information from medicines information or AMS where necessary 

Increased sensitivity to warfarin

• Frail, sick, have multiple comorbidities or take multiple medication
• Aged >80 yr
• Significantly underweight
• Congestive cardiac failure
• Abnormal liver function
• Receiving parenteral nutrition or drugs that potentiate warfarin significantly (see BNF Appendix 1)

Counsel patient

• Reason, risks and benefits of oral anticoagulation
• bleeding risk
• drug interactions (including alcohol)
• need for regular INR monitoring
• Provide anticoagulation information pack

Day 1

• Take blood for measurement of INR
• When INR result available:
• if INR ≤1.3 on day 1 and increased sensitivity to warfarin, choose Slow anticoagulation OATES regimen
• if INR <1.5 on day 1 and no factors likely to cause increased sensitivity to warfarin, choose Slow anticoagulation Tait regimen
• if INR fits neither of first two choices, seek senior advice

Slow anticoagulation OATES regimen

Slow anticoagulation Tait regimen

Subsequent management

• Make dose adjustments after end of initiation regimen intuitively
• watch for INR instability due to changing/starting/stopping of interacting medication or diet (see BNF appendix 1)
• If concerns of overcantioagulation, see Warfarin overanticoagulation

• Refer patients stabilised on warfarin to AMS for ongoing monitoring
• Order TTO for warfarin along with other medication