EPIDEMIOLOGY AND RISK PREDICTION OF VENOUS THROMBOEMBOLISM
Authors: Lowe G D O 
Published: August 2018

Uses of epidemiology. Venous thromboembolism continues to be an important cause of death and disability in Western Countries. Its epidemiology may provide clues to etiology, e.g. the increased incidence in oral contraceptive users, and the low prevalence at autopsy in Central Africa or Japan compared to the U.S.A. A second use is the monitoring of time-trends: the diagnosis of pulmonary embolism increased during the 1970s, although the case fatality decreased. A third use is the identification and quantification of risk factors: these could be modified in the hope of prevention, or else used to select high risk groups for selective prophylaxis, e.g. during acute illness. Prevention is the only feasible approach to reducing the burden of venous thromboembolism, since most cases are not diagnosed, and since the value of current treatment is debatable.

Case definition. Presents problems: clinical diagnosis is unreliable, and should if possible be supported by objective methods. Autopsy studies are performed on selected populations, at a decreasing rate; the frequency of thromboembolism depends on technique; and pathologists cannot be blinded and are open to bias. It can also be difficult to judge whether a patient dying with pulmonary embolism died from pulmonary embolism. 125I-fibrinogen scans indicate minimal disease, and now present ethical problems in screening due to risks of viral transmission. Venography is invasive and is not readily repeatable, which limits its use as a screening method. Plethysmography merits wider evaluation, since it is non-invasive, and sensitive to major thrombosis.

Community epidemiology. Data on the community epidemiology are limited. The risk increases with age. When age is taken into account, there is little sex difference. Overweight in women, use of oral contraceptives and blood group A increase the risk: smoking, varicose veins, blood pressure, cholesterol and glucose do not, on current evidence. Long-term follow-up of patients with proven thromboembolism shows an increased risk of malignancy, hence occult cancer may also be a risk factor. Polycythaemia and certain congenital deficiencies (e.g. antithrombin III) are also well-recognised risk factors, although uncommon.

Hospital epidemiology. Data on hospital epidemiology are derived largely from autopsy prevalence, and from short-term incidence of minimal thrombosis detected by 125I—fibrinogen scanning. Old, immobile and traumatised patients are most at risk. Previous thromboembolism, polycythaemia, antithrombin III deficiency, hip and leg fractures, elective hip and leg surgery, hemiplegia, paraplegia, and heart failure carry high risks, and merit consideration for routine prophylaxis. The risk in elective surgery precedes the operation, and increases with age, overweight, malignancy, varicose veins, non-smoking, and operative factors (duration, approach, general anaesthesia, intravenous fluids). Diabetics appear to have no extra risk. Combinations of clinical variables can be used to predict high risk groups for selective prophylaxis, but combination indices require further study. Laboratory variables may increase the predictability of deep vein thrombosis, but the results of published studies are conflicting, and the cost-effectiveness of laboratory prediction should be evaluated.