published in  Huisarts Nu februari 2010 39(1) S1 - S36

How to initiate warfarin treatment in family practice?

Prior to initiation

·        Do not start warfarin in any patient at severe risk of bleeding or with recent severe bleeding, or if there is insufficient guarantee of correct use or treatment compliance.

·        Ask about any medications and diseases that may possibly cause interference. Establish the indication, INR target value, use and duration of treatment prior to commencing therapy.

Initiation

·        Start warfarin at 5 mg per day (Grade 1B); 2.5 mg for subjects over 60 years of age and risk patients (Grade 1C). Do not adjust the dose until after day 3 onwards, based on INR (Grade 1B.)

LMWH

·        Start LMWH as monotherapy if there is a strong clinical suspicion or diagnosis of DVT or lung embolism (Grade 1A). Warfarin is initiated upon confirmation of diagnosis.

·        Stop LMWH when INR is within the therapeutic range, but no sooner than after 5 days (Grade 1C) and continue treatment with warfarin.

What is the INR target value?

An INR target value of 2.5 applies for almost all indications (Grade 1A). INR must not deviate from the target value by more than 0.5 INR units (Grade 1B).

What is the duration of treatment?

·        The duration of anticoagulant treatment depends on the indication, clinical progression, previous history and the ratio between risk of relapse vs. bleeding risk.

·        A three-month treatment period is sufficient in DVT or lung embolism as a result of a transient ‘triggering’ factor such as surgery or cast immobilisation (Grade 1A) or for initial DVT (Grade 2B) in the presence of additional risk.

·        In long-term treatment, regularly reassess the benefits and risks of warfarin continuation (Grade 1C). Stop if it is clear that the benefits (reduction of VTE relapse) no longer outweigh the risks (increased risk of bleeding).

·        Long-term (life-time) anticoagulant treatment is required in atrial fibrillation (chronic or paroxysmal). 

How often should patients on warfarin be followed up with INR determination?

·        Determine INR on day 3 after the start of warfarin treatment. Then every 3 to 4 days until a stable maintenance dose is reached (Grade 2C).

·        Determine INR at least every 4 weeks (Grade 2C). Do so more frequently in patients over 75 years, particularly in cases of comorbidity and co-medication with several agents.

Which patient-related factors may influence INR values and require possible adjustment of the approach?

Drug interactions

·        Always consider the need for co-medication. Remember that any medication may interact.

·        Determine INR values within one week after initiating and when stopping or adjusting the dose of any co-medication; adjust the warfarin dose if necessary.

·        Advocate a healthy and varied diet (Grade 2B). Ask about dietary habits if unexpected INR values occur (including supplements, natural foods, vitamins, etc) (Grade 2C).

Pathology that may cause interference

·        Before initiating anticoagulation, discuss the underlying risk disorder with the treating specialist.

·        Patients with chronic disorders associated with an increased risk of bleeding should be monitored very carefully, in order to maintain INR within the therapeutic range.

·        If acute disorders occur (e.g. diarrhoea, fever), INR determination within 3 days is desirable.

Age

·        Monitor patients >75 years very carefully, to maintain INR within the therapeutic range.

·        Old age alone does not sufficiently warrant the initiation of warfarin.

How to manage abnormal INR values as a result of warfarin treatment?

·        First investigate whether the unusual or unexpected INR may have been caused by improper blood sampling, incorrect intake of medication, changes in comorbid disorders, effects of co-medications or self-medication, dietary changes or intake of dietary supplements containing vitamin K.

·        If INR is between 3.0 and 5.0 and in the presence of significant bleeding, reduce the warfarin dose by 10 to 20% (Grade 1 C). Check after one week (Grade 1C).

·        If INR is between 5,0 and 9,0 and without significant bleeding: skip one or two daily doses, monitor INR every two days and restart warfarin at a 30% lower dose once the INR is £ 3.

When and how to discontinue warfarin and switch to LMWH in the event of (dental) surgery?

·        Discuss with the surgeon and anaesthetist what type of perioperative management will be followed, depending on the bleeding and thrombotic risk.

·        Stop warfarin 5 days before surgery (Grade 1B). Administer an additional dose of 2 mg oral vitamin K if INR is still ≥1.5 one or two days before surgery (Grade 2C).

·        If warfarin is discontinued, the nature of bridging anticoagulation with LMWH will be determined by the thrombotic risk.

Continue warfarin treatment in cases of minor dental, dermatological or ophthalmic surgery (Grade 1C).