Definition

Anticoagulation is the therapeutic reduction of coagulation used to prevent or treat thromboembolism. UK practice commonly uses DOACs, warfarin, low-molecular-weight heparin or unfractionated heparin, with choice determined by the indication and patient factors.

Epidemiology

Anticoagulants are widely prescribed and bleeding is their principal serious adverse effect. DOACs have replaced warfarin for many eligible people with non-valvular atrial fibrillation and venous thromboembolism, but warfarin remains essential for mechanical heart valves and selected specialist indications.

Pathophysiology

Warfarin inhibits vitamin K epoxide reductase and reduces synthesis of factors II, VII, IX and X and proteins C and S. Apixaban, rivaroxaban and edoxaban inhibit activated factor Xa; dabigatran inhibits thrombin. Heparins potentiate antithrombin. The target, clearance and onset of each agent determine its monitoring, interactions and reversal pathway.

First principles

The indication determines the anticoagulant

Direct-acting oral anticoagulants (DOACs) are preferred for most eligible people with non-valvular atrial fibrillation and are first-line options for many adults with confirmed proximal DVT or PE. Warfarin remains important when a DOAC is contraindicated, unsuitable or not tolerated, and is required for mechanical heart valves; people with triple-positive antiphospholipid syndrome need a specialist VKA pathway.^1,2,3

Warfarin and DOACs require different monitoring

Warfarin inhibits vitamin K recycling, so its effect builds and changes with illness, diet and interacting medicines; INR monitoring is essential. DOACs have predictable fixed-dose pharmacology and do not need routine anticoagulant-effect monitoring, but renal function, liver function, weight, adherence, bleeding and interactions still need planned review.^1,4,5

A normal routine clotting test does not exclude DOAC effect

INR is the monitoring test for warfarin, not a reliable measure of apixaban, dabigatran, edoxaban or rivaroxaban activity. If the result will change urgent management, use a calibrated drug-specific anti-Xa assay for factor-Xa inhibitors or an appropriate thrombin-time assay for dabigatran where available, alongside the time of the last dose and renal function.^4,6,7,8,9

Reversal is for serious bleeding or urgent procedures, not an abnormal number alone

First stop the anticoagulant, assess the bleed, resuscitate and obtain urgent senior or haematology help. Warfarin reversal replaces clotting factors rapidly with PCC and restores synthesis with vitamin K. Dabigatran has idarucizumab; andexanet alfa is a NICE-recommended option for life-threatening or uncontrolled bleeding from apixaban or rivaroxaban under its technology-appraisal criteria. Edoxaban has no specific authorised reversal agent in the current MHRA advice.^10,4,11,12

Presentation

Anticoagulation is prescribed to prevent or treat thromboembolism, most commonly in atrial fibrillation, venous thromboembolism or selected valve and thrombophilia conditions. The urgent complications are major bleeding, unsafe interruption and thrombosis from missed doses or an unsuitable drug.^1,2,4

Cardinal features

A current indication: atrial fibrillation, confirmed or suspected VTE, mechanical valve or another specialist indication
Bleeding symptoms or signs, anaemia, haemodynamic compromise or neurological change
Warfarin exposure with an unexpected INR or a new interacting medicine, illness, diet or alcohol change
DOAC exposure with renal deterioration, low body weight, interacting medicines, missed doses or an urgent procedure
Thromboembolic symptoms suggesting under-anticoagulation: focal neurological deficit, pleuritic pain, dyspnoea or a swollen painful leg

Red flags

Suspected intracranial haemorrhage, new focal neurology, severe headache, reduced consciousness or head injury
Haemodynamic compromise, major gastrointestinal or retroperitoneal bleeding, rapidly falling haemoglobin or uncontrolled bleeding
Bleeding requiring urgent surgery or an invasive procedure that cannot safely wait
A mechanical valve patient taking a DOAC, or a patient with triple-positive antiphospholipid syndrome on an unsuitable DOAC
New stroke, acute limb ischaemia, DVT or PE after missed doses or interruption

Investigations

Confirm the indication, drug, dose and last dose

The risk of stopping or reversing anticoagulation depends on why it was prescribed, the exact agent, dose, renal function, time since the last dose and whether other antithrombotics are present.

Expected finding: A documented indication, treatment duration, current prescription, adherence, last dose, interacting medicines and patient-held anticoagulant alert card.

^1,2,4

Full blood count, renal and hepatic function

Baseline tests identify anaemia or thrombocytopenia, assess organ function relevant to drug selection and dosing, and provide a comparator if bleeding occurs. NICE recommends baseline tests when starting anticoagulation for suspected or confirmed VTE, without delaying interim treatment while results are awaited.

Expected finding: Haemoglobin and platelet count guide bleeding assessment; renal and hepatic results determine whether a DOAC is appropriate and whether its dose remains safe.

^2,1,4

INR and coagulation screen for warfarin or major bleeding

INR measures VKA effect and helps guide warfarin reversal. PT, APTT, fibrinogen and other haemorrhage tests may be needed in major bleeding, but a normal INR does not exclude clinically relevant DOAC activity.

Expected finding: The INR is interpreted against the indication-specific target for warfarin; haemoglobin, platelets, fibrinogen and serial observations show the severity and trajectory of major bleeding.

^5,11,4

Drug-specific DOAC assay when the result changes urgent care

Routine coagulation tests cannot reliably quantify DOAC exposure. A calibrated anti-Xa assay may help for apixaban, edoxaban or rivaroxaban, and an appropriate thrombin-time assay may help for dabigatran, particularly in overdose, major bleeding or emergency surgery.

Expected finding: A result is interpreted with the last dose, renal function and clinical context; do not use anti-Xa assays to measure the effectiveness of andexanet alfa.

^4,6,7,8,9

Targeted assessment for bleeding or thrombosis

Investigate the complication rather than treating the anticoagulant result in isolation: use urgent brain imaging for suspected intracranial bleeding, group and crossmatch when transfusion may be needed, and appropriate imaging for suspected DVT, PE or other internal bleeding.

Expected finding: The anatomical source and severity of bleeding or thrombosis determine reversal, source control, specialist referral and when anticoagulation can be safely restarted.

^11,12,2

Management

Step	Detail	Source
Confirm the indication and choose the appropriate agent	For atrial fibrillation, offer a DOAC at a CHA2DS2-VASc score of 2 or more and consider one for men with a score of 1, taking bleeding risk and patient preference into account. Use a VKA when a DOAC is contraindicated, unsuitable or not tolerated. Do not substitute a DOAC for warfarin in a mechanical-valve patient; use specialist VKA management for triple-positive antiphospholipid syndrome.^1,2,3	NICE NG196 recommendations 1.6.2 to 1.6.6; NICE NG158; NHS/MHRA mechanical-valve safety alert
Check baseline safety and prescribe using current BNF details	Before starting or changing treatment, review full blood count, renal and hepatic function, weight, pregnancy possibility where relevant, contraindications, interacting medicines, adherence and the planned duration. Use the current BNF monograph and local anticoagulation service for agent-specific dose, renal thresholds, administration and switching instructions; do not copy a dose across indications.^{1,2,4,13,6,7,8,9}	NICE NG196; NICE NG158; BNF anticoagulant monographs; MHRA DOAC safety advice
Start and monitor warfarin deliberately	Use the anticoagulation service's initiation protocol, check INR frequently until stable, then at the interval specified by the service. Recheck promptly after new medicines, acute illness, vomiting or diarrhoea, substantial diet or alcohol change, missed monitoring or symptoms of bleeding. Explain that INR reflects warfarin effect only and that the patient should carry the treatment-specific alert card.^1,2,5,14	NICE NG196 and NG158; NHS SPS warfarin monitoring; MHRA anticoagulant monitoring advice
Treat VTE and review the duration	For confirmed proximal DVT or PE, NICE recommends at least 3 months of anticoagulation. Offer apixaban or rivaroxaban when suitable; use the specified LMWH-to-dabigatran/edoxaban or LMWH-plus-VKA pathway when they are unsuitable. At 3 months, discuss stopping, continuing or changing treatment: a provoked event may allow stopping when the provoking factor has resolved, while an unprovoked event often favours continuation when recurrence risk outweighs bleeding risk.²	NICE NG158 recommendations 1.3.5 to 1.3.9 and 1.4.1 to 1.4.8
Manage a major bleed as an emergency	Stop further doses, call senior and haematology support, activate major-haemorrhage or critical-care pathways when indicated, obtain the last-dose and renal history, control the bleeding source and provide blood-product or haemodynamic support. Do not delay life-saving resuscitation while waiting for a drug level.^11,12,4	NHS Highland anticoagulant-reversal guidance; NHS England PCC patient-safety advice; MHRA DOAC advice
Reverse the specific anticoagulant when criteria are met	For life-threatening warfarin bleeding or urgent surgery, use PCC with intravenous vitamin K according to the current local haematology protocol. For life-threatening or uncontrolled bleeding on dabigatran, use idarucizumab; for apixaban or rivaroxaban, use andexanet alfa only within NICE TA697 criteria and local availability. Edoxaban has no specific authorised antidote in current MHRA advice; discuss PCC or another specialist option urgently. Recheck clinically and with appropriate tests after reversal.^10,4,11,13,7	NICE TA697; MHRA DOAC safety update; NHS Highland anticoagulant-reversal guidance
Plan interruption, restart and follow-up with the indication in view	For an operation, invasive procedure or non-major bleed, balance procedural bleeding risk against thromboembolic risk and use the current agent-specific peri-procedural or local haematology protocol; avoid a universal stop interval or automatic bridging rule. After bleeding or surgery, restart only when haemostasis is secure and the responsible team has reviewed the indication, recurrence risk, renal function and patient preferences.^{1,2,13,6,7,8,9}	NICE NG196 and NG158; BNF anticoagulant monographs; local anticoagulation and haematology guidance
Give practical safety-netting and annual review	Give verbal and written information about the indication, duration, doses, missed doses, bleeding symptoms, interactions, alcohol, dental treatment, travel, pregnancy and when to seek urgent help. Provide the treatment-specific anticoagulant alert card. Review the ongoing need and quality of anticoagulation at least annually, or sooner after clinically relevant events affecting anticoagulation or bleeding risk.^1,2,15	NICE NG196 recommendation 1.6.18 and NG158 recommendations 1.5.1 to 1.5.2

Illustrations

Coagulation cascade and anticoagulant targetsDiagram of the clotting cascade highlighting the vitamin K-dependent factors targeted by warfarin and the single activated factor targeted by each DOAC.PassFinals · original

Warfarin versus DOAC reversal pathwaysSide-by-side diagram showing vitamin K and PCC reversing warfarin against idarucizumab and andexanet alfa reversing selected DOACs, with specialist escalation for edoxaban.PassFinals · original

Indication, monitoring and safety-netting mapFlow diagram linking atrial fibrillation, VTE and valve indications to drug choice, INR or renal monitoring, bleeding red flags and specialist review.PassFinals · original

Differentials

Warfarin

Vitamin K antagonist with delayed onset and offset, interaction-sensitive dosing and mandatory INR monitoring.

Direct oral anticoagulant

Fixed-dose direct factor Xa or thrombin inhibition without routine anticoagulant-effect monitoring, but with renal, interaction, adherence and bleeding review.

Low-molecular-weight heparin

Parenteral anticoagulation used in selected VTE, pregnancy, cancer, renal or bridging pathways according to specialist guidance.

Unfractionated heparin

Parenteral option when rapid titration or reversibility is important, or in selected renal-failure and unstable pathways.

Complications

Major bleeding, including intracranial and gastrointestinal haemorrhage
Thrombosis after missed doses, inappropriate interruption or an unsuitable drug
Drug interactions and renal deterioration causing over-anticoagulation
Warfarin skin necrosis, teratogenicity and unstable INR
Heparin-induced thrombocytopenia with heparin exposure

Prognosis

Appropriately selected anticoagulation reduces thromboembolic events, but benefit depends on adherence, renal and hepatic safety, interaction review, monitoring where required and rapid recognition of bleeding. The balance changes over time, so indication, duration and safety should be reviewed after major events and at least annually.

Guidelines

Atrial fibrillation: diagnosis and management (NG196) (NICE, 2021)
Venous thromboembolic diseases: diagnosis, management and thrombophilia testing (NG158) (NICE, 2020)
Andexanet alfa for reversing anticoagulation from apixaban or rivaroxaban (TA697) (NICE, 2025)

Anticoagulation (warfarin and DOACs)