Before starting

Required

• 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

Required

• Daily or on alternate days; then twice weekly for 1-2 weeks; then weekly
  • INR · at 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

Required

• Every 12 weeks; more frequently if needed
  • INR · increase 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

Low 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

Bibliography

• Scottish Intercollegiate Guidelines Network (SIGN). Antithrombotics: indications and management (Guideline129). August 2012. [cited 30/07/2020]
• British Journal of Haematology. Guidelines on oral anticoagulation: third edition. Br J Haematol 1998; 101: 374–387 [cited 30/07/2020]
• Keeling D, Baglin T, Tait C, et al. British Committee for Standards in Haematology. Guidelines on oral anticoagulation with warfarin – fourth edition. Br J Haematol 2011; 154: 311–324 [cited 30/07/2020]
• National Patient Safety Agency. Actions that can make anticoagulant therapy safer. March 2006 [cited 15/05/2023]
• NICE Clinical Knowledge Summaries (CKS). Anticoagulation – oral. Updated Jan 2020 [cited 18/08/2020]
• GP notebook. HAS-BLED score for bleeding risk on oral anticoagulation in atrial fibrillation (AF) [cited 07/08/2020]
• Medicines and Healthcare products Regulatory Agency. Warfarin: reports of calciphylaxis. Drug Safety Update Volume 9 Issue 12 July 2016: 1. [cited 30/07/2020]
• Joint Formulary Committee. British National Formulary (online) London: BMJ Group and Pharmaceutical Press [cited 20/06/2020]

Update history