This chapter explores the risk factors reported to increase the risk of venous thromboembolism (VTE). The most important risk factors for VTE are a history of previous VTE, recent surgery, hospital in-patient stay and cancer. Pulmonary embolism remains the most widely reported preventable cause of death in patients undergoing surgery. It is the most common cause of death within 30 days of surgery, with 40% of VTE events occurring later than three weeks post operatively. Patients undergoing lower limb surgery are among the highest risk patients (odds ratio > 10), and this includes total hip and knee arthroplasty, hip/leg fractures, major orthopaedic trauma and spinal surgery. Lower limb immobilisation in casts, with or without surgery, increases the risk of VTE. Approximately 70-80% of fatal hospital-acquired thrombosis (HAT) occurs in medical patients. Venous thrombosis is increased in most acute medical conditions, necessitating hospital admission.

Blood group P1 expression was scored by direct agglutination in 32 patients who had previously developed post-enteropathic haemolytic uraemic syndrome (HUS). Sixty-six children of similar ages undergoing venepuncture for other renal disorders acted as controls. The expression of P1 in controls was that expected from the normal caucasian population, 23% being negative. By contrast, there was an excess of HUS patients with weak or absent expression of P1 (χ2 for linear trend 5.45,P<0.02), and this was particularly evident in those with a poor outcome. Verotoxin (VT), which is associated with HUS, requires the terminal disaccharide of the P1 antigen to bind to cells, and after internalization disrupts the transcription of ribonucleic acid. Mature erythrocytes do not synthesize protein and may be toxin resistant. We postulate that strong expression of P1 antigen may promote the binding of VT to red cells and thus reduce the dose to vulnerable nucleated cleated endothelial cells. P1 positivity may be protective, and P1 negativity a risk factor in HUS.