Warfarin monitoring

Before starting

Required

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

Calculating ORBIT score

Renal function, haemoglobin and haematocrit are required to calculate ORBIT bleeding risk score

Consider

• Baseline
  • Thyroid function tests · if thyroid disease suspected
  • Blood pressure · if uncontrolled hypertension suspected

After started or dose changed

Required

• Daily or on alternate days; then twice weekly for 1-2 weeks; then weekly, depending on local protocol
  • 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

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, for example every 1 to 2 weeks, where the patient has an increased risk of over-coagulation or bleeding, or may have difficulties with adherence.

Aggravating factors for over-coagulation include:

• severe hypertension
• liver disease including alcoholic liver disease
• renal failure
• concomitant use of interacting medicines
• reduction in dietary vitamin K

Aggravating factors for increased risk of bleeding include:

• high intensity anticoagulation
• older age (65 year of older)
• 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)
• concomitant use of interacting medicines or those that increase the risk of bleeding
• excessive alcohol consumption
• highly variable INRs

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 to 5 days.

Abnormal results

Establish the reason

Where an abnormal INR reading is recorded, establish the reason for it. Possible reasons may include:

• missed or inadvertent change in dose
• interacting medicines
• change in alcohol intake
• change in smoking status
• significant change in diet
• weight changes
• 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 between 5 to 8 with no bleeding

• omit 1 or 2 doses of warfarin
• reduce subsequent maintenance dose

INR between 5 to 8 with minor bleeding

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

• stop warfarin
• refer to local anticoagulation guidelines on giving phytomenadione (vitamin K)
• restart warfarin when INR less than 5

INR greater than 8 with no or minor bleeding

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

• stop warfarin
• give phytomenadione (vitamin K) either orally (off-label use of intravenous formulation) or intravenously depending on presence of bleeding
• repeat dose of phytomenadione (vitamin K) if INR still too high after 24 hours
• restart warfarin when INR less than 5

Bibliography

• British National Formulary. Oral anticoagulants – treatment summary. Available at: https://bnf.nice.org.uk/treatment-summaries/oral-anticoagulants/ [cited 05/08/25]
• British National Formulary. Phytomenadione. Available at: https://bnf.nice.org.uk/drugs/phytomenadione/ [cited 12/09/25]
• British National Formulary. Warfarin sodium. Available at: https://bnf.nice.org.uk/drugs/warfarin-sodium/ [cited 01/08/25]
• Keeling D, Baglin T, Tait C, Watson H, Perry D, Baglin C, Kitchen S, Makris M, The British Committee for Standards in Haematology. British Society for Haematology. Guidelines on oral anticoagulation with warfarin – fourth edition. 14/06/2011. Available at: https://onlinelibrary.wiley.com/doi/full/10.1111/j.1365-2141.2011.08753.x [cited 04/08/25]
• Makris M, Van Veen JJ, Tait CR, Mumford AD, Laffan M, The British Committee for Standards in Haematology. British Society for Haematology. Guideline on the management of bleeding in patients on antithrombotic agents. 01/11/2012. Available at: https://onlinelibrary.wiley.com/doi/full/10.1111/bjh.12107 [cited 04/08/25]
• MDCALC. ORBIT Bleeding Risk Score for Atrial Fibrillation. Available at: https://www.mdcalc.com/calc/10227/orbit-bleeding-risk-score-atrial-fibrillation [cited 05/08/25]
• MHRA Drug Safety Update. Smoking and smoking cessation: clinically significant interactions with commonly used medicines. 11/12/2014. Available at: https://www.gov.uk/drug-safety-update/smoking-and-smoking-cessation-clinically-significant-interactions-with-commonly-used-medicines [cited 05/08/25]
• MHRA Drug Safety Update. Warfarin: be alert to the risk of drug interactions with tramadol. 20/06/2024. Available at: https://www.gov.uk/drug-safety-update/warfarin-be-alert-to-the-risk-of-drug-interactions-with-tramadol [cited 05/08/25]
• MHRA Drug Safety Update. Warfarin and other anticoagulants: monitoring of patients during the COVID-19 pandemic. 22/10/2020. Available at: https://www.gov.uk/drug-safety-update/warfarin-and-other-anticoagulants-monitoring-of-patients-during-the-covid-19-pandemic [cited 05/08/25]
• NICE CKS. Anticoagulation – oral. Last revised March 2025. Available at: https://cks.nice.org.uk/topics/anticoagulation-oral/ [cited 01/08/25]
• NICE CKS. Atrial fibrillation: Scenario: New-onset or acute AF. Last revised May 2025. Available at: https://cks.nice.org.uk/topics/atrial-fibrillation/management/new-onset-or-acute-af/ [cited 05/08/25]
• NICE Guideline 196. Atrial fibrillation: diagnosis and management. 27/04/2021. Available at: https://www.nice.org.uk/guidance/ng196 [cited 05/08/25]
• SIGN Guideline 129. Antithrombotics: indications and management. August 2012. Available at: https://www.sign.ac.uk/our-guidelines/antithrombotics-indications-and-management/ [cited 04/08/25]
• Summary of Product Characteristics. Warfarin 0.5mg Tablets. Ranbaxy UK Limited a Sun Pharmaceutical Company. Last updated on emc 10/12/2021. Available at: https://www.medicines.org.uk/emc/product/3064/smpc [cited 01/08/25]

Update history