Warfarin

NICE CKS Anticoagulation – oral: Scenario: Warfarin. Last revised: April 2024

BNF Treatment Summaries: oral anticoagulants

BSH Guidelines Oral Anticoagulation with Warfarin – 4th Edition. Last revised: Oct 2016.

Article Last Updated:25/01/2026

Background Information

Mechanism of Action

Warfarin is a vitamin K antagonist

Inhibit vitamin K-dependent clotting factors (II, VII, IX, X), and
Inhibit protein C & S

Indications

Embolism prophylaxis in atrial fibrillation and rheumatic heart disease
Embolism prophylaxis in patients with prosthetic heart valves
Embolism treatment or prophylaxis of VTE
TIA

Warfarin Prescription Information

Patient Education

Patients who take warfarin should be advised of the following:

Adhere strictly to the prescribed dosing schedule
Report any signs of bleeding (e.g. unusual bruising, haematuria, melena, headache)
Dietary vitamin K intake should remain consistent (avoid sudden increase or decrease in intake of food high in vitamin K – e.g. leafy green vegetables)
Ensure moderate and consistent alcohol intake (excessive / variable consumption can alter warfarin effect)

If the patient missed a warfarin dose:

Do not double any subsequent doses
Take the missed dose ASAP on the same day

Initiating Warfarin

Warfarin has an initial paradoxical pro-coagulation state, therefore it should be introduced together with heparin (heparin lead-in) and continued until:

At least 5 days, and
INR ≥2 for at least 24 hours

To start warfarin → give warfarin AND heparin at day 0. Continue heparin for at least 5 days. Once INR ≥2, stop heparin and continue warfarin alone.

Target INR

BSH Guidelines (endorsed by NICE CKS and BNF) recommend the following:

INR target is generally 2.5 +/- 0.5 units (note target range is no longer recommended)
Some exceptions
- Mechanical mitral valve → 3.0 / 3.5
- Recurrent DVT / PE whilst anticoagulated → 3.5

Some patients require a higher INR target due to additional risk factors for thrombosis, including mitral valve positioning, mitral stenosis and concurrent atrial fibrillation.

Monitoring and Follow-Up

The INR is used to assess and monitor the anticoagulant effect of warfarin.

Recommended INR measurement frequency:

Before therapeutic range reached	Daily or on alternate days (note a meaningful INR can only be obtained 3-4 days after starting treatment)
Once within therapeutic range	Twice weekly for 1-2 weeks, then Weekly until at least 2 INR measurements within range
Once 2 measurements within therapeutic range	Measure at longer intervals, BNF recommends up to every 12 weeks

More frequent monitoring is recommended if there is:

↑ Risk of over-coagulation
↑ Risk of bleeding
Adherence to treatment may be difficult

INR is a short-term measurement of anticoagulation status – it tells you the current level of anticoagulation at the time of blood draw.

Prothrombin time (PT) and INR are closely related. PT measures the time (in sec) it takes for a clot to form after adding thromboplastin to evaluate the extrinsic + common pathways of the clotting cascade. While INR is a standardised way of expressing PT (calculated by patient’s PT divided by normal PT), allowing results from different laboratories and regents to be compared.

Therefore, warfarin would raise both PT and INR.

Out-of-Range INR Management

Management depends on:

Bleeding severity
INR

Definition of major bleeding (in the context of bleeding patient on warfarin), ANY of the following:

Bleeding that is fatal
Bleeding in a critical area / organ (e.g. intracranial, intraspinal, intraocular, retroperitoneal, intra-articular, intramuscular with compartment syndrome)

In the BNF, and many guidelines, “vitamin K” is listed as “phytomenadione“.

Phytomenadione is simply the proper chemical name for vitamin K1, which is one specific form within the broader vitamin K family (vitamin K = K1 + K2 forms).

Major Bleeding

Regardless of INR:

Stop warfarin
IV vitamin K (5 mg)
Prothrombin complex concentrate (PCC)

If PCC is not available, fresh frozen plasma (FFP) can be given instead, but it is less effective than PCC.

Minor Bleeding

Regardless of INR:

Stop warfarin (restart when INR <5.0)
IV vitamin K (1-3 mg)

No Bleeding

Depends on INR:

INR >8.0 → stop warfarin (restart when INR <5.0) + oral vitamin K (1-5 mg)
INR 5.0-8.0 → withhold 1-2 doses of warfarin + ↓ subsequent doses

Pre-Operative Preparation

Elective Surgery

Warfarin is usually stopped 5 days before elective surgery

Oral vitamin K should be given the day before surgery if INR is ≥1.5
If the patient is at high-risk of thromboembolism (e.g. VTE within the last 3 months, AF with previous stroke / TIA, mitral mechanical heart valve), they may also require LMWH bridging (treatment dose) once warfarin is stopped and to be stopped at least 24 hours before surgery (or 48 hours if the surgery carries a high risk of bleeding)

If warfarin cannot be stopped 3 days before the surgery, anticoagulation should be reversed with low-dose vitamin K.

General surgical literature and perioperative guidelines commonly advise aiming for an INR <1.5 (and ideally ≤1.3 for major procedures with significant bleeding risk) before proceeding with most elective surgeries, especially where the risk of bleeding is high.

NICE CKS states that INR <2.5 is acceptable for minor, low-risk procedures.

For dental procedures, warfarin does NOT need to be stopped routinely. However, INR should be checked 24 hours beforehand, and the procedure should only go ahead if INR <4.

Emergency Surgery

If possible, delay the surgery for 6-12 hours to give IV vitamin K to reverse the anticoagulant effect of warfarin.