All patients admitted to hospital for major trauma (e.g. fracture causing immobilisation), major surgery (e.g. duration over 30 mins), or acute medical illness (e.g. likely to require bed rest for three days or more) should be individually assessed for risk of VTE.

Assessment of individual risk should include: personal risk factors for VTE (see Table 1) past history of VTE (hospitalisation increases risk of recurrent VTE) type of trauma, surgery (and anaesthesia) or medical illness.

Local guidelines should be developed and updated for specific patient groups.

Within local guidelines, individual prophylaxis should be chosen according to the balance of efficacy and risks (especially bleeding), and the patient's preferences.

Routine screening for thrombophilias prior to risk situations such as prescription of oral contraceptives or hormone replacement therapy, pregnancy, or elective major surgery is not recommended.61, 62

Table 1: Risk factors for venous thromboembolism ^{1,46,54,55,59,60}

Age1,46,47,48,50,51	Exponential increase in risk with age. In the general population: < 40 years - annual risk 1/10,000 60-69 years - annual risk 1/1,000 > 80 years - annual risk 1/100 May reflect immobility and coagulation activation52,53
Obesity1,46,50,51,54,55,56	3 x risk if obese (body mass index >= 30 kg/m2) May reflect immobility and coagulation activation52,53
Varicose veins50,51	1.5 x risk after major general / orthopaedic surgery But low risk after varicose vein surgery15,57
Previous VTE1,17	Recurrence rate 5% / year, increased by surgery
Thrombophilias58,59,60,61,62	Low coagulation inhibitors (antithrombin, protein C or S) Activated protein C resistance (e.g. factor V Leiden) High coagulation factors (I, II, VIII, IX, XI) Antiphospholipid syndrome High homocysteine
Other thrombotic states59,60	Malignancy 7 x risk in the general population Heart failure Recent myocardial infarction / stroke Severe infection Inflammatory bowel disease, nephrotic syndrome Polycythaemia, paraproteinaemia Bechet’s disease, paroxysmal nocturnal haemoglobinuria
Hormone therapy	Oral combined contraceptives, HRT, raloxifene, tamoxifen63,64 3 x risk High-dose progestogens 6 x risk (see section 10)
Pregnancy, puerperium	10 x risk (see section 9)
Immobility	Bedrest > 3 days, plaster cast (see section 5), paralysis (see sections 5 & 7), 10 x risk; increases with duration
Prolonged travel	see section 11
Hospitalisation	Acute trauma, acute illness, surgery 10 x risk
Anaesthesia	2 x general vs spinal / epidural65