Venous Thromboembolism Prophylaxis

Last updated 20 August 2026.

Key points

Why it matters: hospital-associated thrombosis is a leading cause of preventable death in hospital, and a substantial proportion of all VTE occurs during or within 90 days of an admission. Most of it is preventable.
The core duty: every patient admitted to hospital must have a documented VTE risk assessment, balancing their thrombosis risk against their bleeding risk, and this must be reassessed as their condition changes.
Pharmacological prophylaxis: low molecular weight heparin (LMWH) is first line for most patients. Fondaparinux is the alternative where heparin is unsuitable, and DOACs are used after elective hip and knee replacement.
Renal impairment: LMWH is renally cleared and accumulates in renal failure - use unfractionated heparin (or a reduced LMWH dose per local policy) when eGFR is severely reduced.
Mechanical prophylaxis: intermittent pneumatic compression or anti-embolism stockings, used when pharmacological prophylaxis is contraindicated by bleeding risk, or alongside it in higher-risk patients.
The stroke exception: anti-embolism stockings are contraindicated after stroke - the CLOTS trials showed no benefit and significant harm from skin breakdown. Intermittent pneumatic compression is used instead.
Extended prophylaxis: continued after discharge in specific groups - typically 28 days after major abdominal or pelvic cancer surgery, and 28-35 days after hip replacement or hip fracture surgery.
Pregnancy: risk is assessed antenatally, on admission and postnatally. LMWH is the agent of choice; DOACs and warfarin are contraindicated.

Introduction

Venous thromboembolism - deep vein thrombosis and pulmonary embolism - is one of the most significant preventable causes of death and morbidity in hospitalised patients. Hospital-associated thrombosis is defined as VTE occurring during admission or within 90 days of discharge, and accounts for a large share of all VTE events.¹

The essential principle is that thromboprophylaxis is not a specialist activity but a routine, universal safety process: every admitted patient is assessed, the balance of thrombosis and bleeding risk is documented, appropriate prophylaxis is prescribed, and the assessment is repeated as circumstances change.

Risk assessment

NICE requires that all patients admitted to hospital (and those having day surgery where relevant) are assessed for VTE risk on admission, and reassessed within 24 hours and whenever the clinical situation changes.² The assessment weighs two opposing risks.

Factors increasing thrombosis risk

Reduced mobility - anticipated significantly reduced mobility for 3 days or more relative to normal
Active cancer or cancer treatment
Age over 60
Obesity (BMI over 30)
Dehydration
Previous VTE, or a first-degree relative with VTE
Known thrombophilia
Surgery - especially major surgery, total anaesthetic plus surgical time over 90 minutes (or 60 minutes for pelvic/lower limb surgery), and hip or knee replacement
Acute medical illness - heart failure, respiratory failure, acute infection, inflammatory conditions
Critical care admission
Significant comorbidities - cardiac, metabolic, endocrine or respiratory disease
Oestrogen-containing contraception or HRT
Varicose veins with phlebitis
Pregnancy or within 6 weeks postpartum

Factors increasing bleeding risk

Active bleeding
Acquired bleeding disorders (e.g. acute liver failure) or inherited bleeding disorders (haemophilia, von Willebrand disease)
Concurrent anticoagulation at treatment dose, or use of agents increasing bleeding risk
Acute stroke - haemorrhagic stroke, and ischaemic stroke within the initial period
Thrombocytopenia (platelets below 75 x10^9/L)
Uncontrolled systolic hypertension (230/120 mmHg or higher)
Untreated inherited or acquired bleeding disorder
Lumbar puncture, epidural or spinal anaesthesia expected within the next 12 hours or performed within the previous 4 hours
Recent neurosurgery, spinal surgery or eye surgery

Pharmacological prophylaxis

Pharmacological prophylaxis options.
Agent	Role	Notes
Low molecular weight heparin (LMWH)	First line for most medical and surgical patients	Once daily subcutaneous dosing (e.g. enoxaparin, dalteparin); dose adjusted for body weight and renal function; no routine monitoring required
Unfractionated heparin (UFH)	Preferred in severe renal impairment	Not renally cleared to the same degree; shorter half-life and reversible with protamine, useful where bleeding risk is a concern
Fondaparinux sodium	Alternative where heparin is unsuitable	A synthetic factor Xa inhibitor; also used where there is a history of heparin-induced thrombocytopenia
DOACs (apixaban, rivaroxaban, dabigatran)	After elective hip and knee replacement	Oral, convenient for extended post-discharge prophylaxis; not used routinely for general medical prophylaxis
Aspirin	Limited role	Used in some orthopaedic protocols, but not a substitute for anticoagulant prophylaxis in general medical or surgical patients

Timing around neuraxial anaesthesia is critical: LMWH must be appropriately spaced from insertion and removal of an epidural or spinal catheter, because of the risk of spinal haematoma causing permanent paralysis. Follow local anaesthetic protocols precisely.

Mechanical prophylaxis

Intermittent pneumatic compression (IPC) - inflatable sleeves that cyclically compress the calves, promoting venous return. Used when pharmacological prophylaxis is contraindicated, and alongside it in higher-risk surgical patients. Continue until the patient is mobile
Anti-embolism (graduated compression) stockings - apply graded pressure to promote venous return. Require correct measurement and fitting, and daily skin inspection
Early mobilisation and adequate hydration - simple, effective and frequently overlooked components of prophylaxis

Contraindications to mechanical prophylaxis

Peripheral arterial disease or an absent/impaired peripheral pulse - compression risks critical ischaemia
Peripheral neuropathy (e.g. diabetic), where pressure damage may go unnoticed
Severe leg oedema, particularly from acute heart failure
Local skin conditions - dermatitis, cellulitis, recent skin graft, fragile skin or ulceration
Known or suspected existing DVT in that limb
Severe leg deformity preventing correct fitting

Prophylaxis in specific settings

Prophylaxis by clinical setting.
Setting	Approach
General medical admission	LMWH if the patient has significantly reduced mobility and no contraindication; continue until mobility is restored or discharge
General/abdominal surgery	LMWH plus mechanical prophylaxis, starting per local protocol relative to surgery timing
Major abdominal or pelvic cancer surgery	Extended prophylaxis with LMWH for 28 days after surgery
Elective hip replacement	LMWH, or a DOAC; extended prophylaxis for 28-35 days
Elective knee replacement	LMWH or a DOAC (or aspirin in some protocols) for 10-14 days
Hip fracture surgery	LMWH or fondaparinux; extended prophylaxis for 28-35 days
Stroke	Intermittent pneumatic compression (not stockings). Pharmacological prophylaxis only once haemorrhage is excluded and bleeding risk is acceptable
Critical care	Assess and reassess daily, as both thrombotic and bleeding risks change rapidly
Pregnancy and puerperium	Risk assessed antenatally, on admission and postnatally; LMWH is the agent of choice - DOACs and warfarin are contraindicated
Palliative care	Individualised - consider prognosis, symptom burden and patient wishes rather than applying routine protocols

Patient information and discharge

Explain the reason for prophylaxis and the signs of DVT and PE to look out for, both in hospital and after discharge
Advise on the importance of mobilisation and hydration
Ensure patients on extended prophylaxis understand duration and technique - most will need to self-administer subcutaneous LMWH at home, with training and sharps disposal arranged
Review oestrogen-containing contraception or HRT before major elective surgery, considering stopping 4 weeks beforehand where appropriate
Document the plan clearly in the discharge summary, including duration, so that primary care can support and follow up

Red flags

Points not to miss

Most failures here are process failures - the assessment not done, not repeated, or the wrong agent chosen for the patient in front of you.

Every admitted patient needs a documented VTE risk assessment, reassessed within 24 hours and whenever their condition changes - a one-off tick on admission is not sufficient
A high bleeding risk means mechanical prophylaxis, not no prophylaxis - and reassess so pharmacological cover can start once it is safe
Check renal function and weight before prescribing LMWH - accumulation in renal impairment is a common cause of avoidable bleeding
Never use anti-embolism stockings in stroke patients - use intermittent pneumatic compression instead
Check for peripheral arterial disease and neuropathy before applying any compression device - compressing an ischaemic limb can cause critical ischaemia
Observe strict timing intervals around epidural or spinal anaesthesia to avoid spinal haematoma and permanent neurological injury
Do not forget extended prophylaxis after hip/knee arthroplasty and after major abdominal or pelvic cancer surgery - the risk period continues well beyond discharge
New breathlessness, pleuritic chest pain or unilateral leg swelling in an inpatient - investigate for VTE promptly; prophylaxis reduces but does not eliminate the risk

Outcomes

Systematic VTE risk assessment and appropriate prophylaxis are among the most effective patient safety interventions in hospital medicine. Since the introduction of mandatory risk assessment in England, coupled with national audit and reporting of hospital-associated thrombosis, there has been a measurable reduction in VTE-related deaths.¹

It is important to counsel patients that prophylaxis reduces but does not abolish the risk: VTE can still occur despite correctly prescribed prophylaxis, so new symptoms suggestive of DVT or PE must always be investigated on their merits rather than dismissed because prophylaxis was given. Equally, every case of hospital-associated thrombosis warrants a root cause review to identify whether assessment, prescribing or administration failed at any point.

References

House of Commons Health Committee. The prevention of venous thromboembolism in hospitalised patients / NHS England VTE prevention programme. Available here
NICE. Venous thromboembolism in over 16s: reducing the risk of hospital-acquired deep vein thrombosis or pulmonary embolism (NG89). 2018 (updated 2019). Available here
CLOTS Trials Collaboration. Thigh-length versus below-knee stockings and intermittent pneumatic compression after stroke. Lancet. 2009 and 2013. Available here
Royal College of Obstetricians and Gynaecologists. Reducing the risk of venous thromboembolism during pregnancy and the puerperium (Green-top Guideline No. 37a). Available here
BNF. Venous thromboembolism - prophylaxis treatment summary. Available here

This article is written for revision and education. It is not clinical guidance and must not be used to make decisions about the care of a patient. Always check current NICE guidance and local protocols.