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Anticoagulation - oral - Management
How should warfarin be monitored?

  • The anticoagulant effect of warfarin is measured as the international normalized ratio (INR). The INR calculation is based on the ratio between the prothrombin times of the test and control samples.
    • The INR is most accurately measured in venous blood samples, but many anticoagulation clinics use capillary blood samples because these are more convenient to obtain.
    • Intravenous drug users, and people with hepatitis B, hepatitis C, or HIV, may be referred to a specialist clinic. Capillary blood sampling in these populations may cause risk of transmission, and venous access is often difficult. Arrangements for monitoring the INR in such cases may vary with locality.
  • The INR should be measured:
    • Daily, or on alternate days, until it is within the therapeutic range (usually between 2.0 and 3.0, ideally 2.5) on two consecutive occasions. Note: although the INR may be measured each day after starting warfarin, a meaningful INR can only be obtained 3–4 days after starting treatment.
    • Then, twice weekly for 1–2 weeks, followed by weekly measurements until the INR is stable within the therapeutic range.
    • Thereafter, depending on the stability of the INR, at longer intervals (for example, up to every 12 weeks, if agreed locally). Once a stable warfarin dose that controls the INR has been established, changes in dose are seldom required.
  • More frequent monitoring of the INR is recommended if the person:
    • Has an increased risk of over coagulation: people with severe hypertension, liver disease (including alcoholic liver disease) or renal failure.
    • Is at increased risk of bleeding: people on high intensity anticoagulation (INR > 4.0); age 65 years or over; highly variable INRs; history of gastrointestinal bleeding; uncontrolled hypertension; cerebrovascular disease; serious heart disease; risk of falling; thrombocytopenia, anaemia, or coagulation disorders; malignancy; trauma, renal insufficiency; morbidity changes (such as intercurrent illness, or exacerbations of chronic conditions); or has changed their medication (for example, when starting or stopping prescribed or over-the-counter medicines).
    • May find adherence difficult.
  • Warfarin metabolism can be affected by:
    • Thyroid status. People with hypothyroidism or hyperthyroidism should be closely monitored on starting warfarin therapy.
    • Genetic variability. If the person or another family member is known to have polymorphisms of CYP2CP or VKORC1, extra care is warranted.

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