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Using this page · Individualise medicines monitoring

This medicines monitoring page has been written using publications and expert opinion. It is designed to save clinician time, but not replace professional responsibility. When using this page you should: ensure an individualised monitoring plan is developed in partnership with the patient and take account of any locally agreed advice and guidance.

Before starting


  • Baseline
    • Blood pressure
    • Clotting screening
    • Estimated glomerular filtration rate or Serum creatinine (for creatinine clearance)
    • Full blood count
    • Liver function tests
    • Thyroid function tests

Calculating HAS-BLED score

Renal function, liver function, and BP required to calculate HAS-BLED score

After started or dose changed


  • Daily or on alternate days; then twice weekly for 1-2 weeks; then weekly
    • INRat each point, obtain two consecutive within target INRs before reducing testing frequency

Patient groups requiring particularly care

A number of patient groups require particular care and close monitoring in the early stages of warfarin therapy. These include patients with:

  • hypothyroidism or hyperthyroidism
  • familial history of polymorphisms of CYP2CP or VKORC1
  • HASBLED score more than 3

Moving to ongoing once stable monitoring

Once a stable warfarin dose that controls INR has been established, consider moving to ongoing once stable monitoring.

Ongoing once stable


  • Every 12 weeks; more frequently if needed
    • INRincrease frequency if high risk patient, poor control, or interacting medicine

Increased frequency monitoring in high risk patients

Consider more frequent monitoring, e.g. every 1-2 weeks, where the patient has an increased risk of over-coagulation or bleeding.

Aggravating factors for over-coagulation include:
  • severe hypertension
  • liver disease including alcoholic liver disease
  • renal failure
  • highly variable INRs
Aggravating factors for increased risk of bleeding include:
  • history of gastrointestinal bleeding
  • uncontrolled hypertension
  • cerebrovascular disease
  • serious heart disease
  • risk of falling
  • thrombocytopenia
  • anaemia
  • coagulation disorders
  • malignancy
  • trauma
  • renal insufficiency
  • morbidity changes (such as intercurrent illness, or exacerbations of chronic conditions)
  • recent change in medication
  • difficulties with adherence

Poor control

Where there is poor control, reassess anticoagulation and increase testing frequency.

Poor control may occur where:

  • Two INR values higher than 5, or one INR value higher than 8, occur within the past 6 months
  • Two INR values less than 1.5 occur within the past 6 months
  • Time in therapeutic range (TTR) is less than 65%

Interacting medicines

Patients who are prescribed a drug that may interact with warfarin should have an INR test performed after 3–5 days.

Those who have had a change in warfarin dose as a result of an interacting drug will need to resume usual maintenance dose following cessation of that drug.

Abnormal results

Establish the reason

Where an abnormal INR reading is recorded, establish the reason for that, (e.g. missed or inadvertent change in dose, interacting drug, changed alcohol intake, significant change in diet, intercurrent illnesses).

Take appropriate action for INR


Refer to local anticoagulation guidelines for use of booster doses and how to increase maintenance dose if needed.

INR greater than 5

Risk of bleeding increases greatly once INR is greater than 5. You should:

  • Refer to local anticoagulant guidelines for advice on number of days to stop therapy
  • Consider adjusting maintenance dose
  • Take further action if there is minor or major bleeding

INR greater than 8

Risk of bleeding increases further once INR is greater than 8. You should:

  • Stop oral anticoagulants should be stopped
  • Give phytomenadione (vitamin K) either orally or intravenously depending on presence of bleeding
  • Repeated dose of phytomenadione (vitamin K) if INR still too high after 24 hours
  • Restart warfarin when INR less than 5


Enquiries about this page

Contact us if you have any enquiries about the drug monitoring information on this page.