Measures of risk and their relationship to the relative size of a high-risk group: application to medical thromboprophylaxis

Prophylaxis of venous thromboembolism in medical patients: too much or too little?

Venous thromboembolism (VTE) is a potentially serious complication occurring in 1%–2% of hospitalized medical patients. Despite this low absolute risk, as many as 82% of medical patients are considered to be at increased risk of developing VTE and are eligible for medical thromboprophylaxis. In this commentary, The author will discuss the main findings of a recent paper published in Clinical Epidemiology that questions the large proportion of individuals who are eligible for medical thromboprophylaxis, and also discuss the potential implications for the prevention of VTE. The recent paper demonstrated that when a population is divided into high- and low-risk groups, the maximum absolute risk depends on the inverse of the proportion of patients that is considered to be high risk. Consequently, even an effective treatment will only result in a small reduction in the absolute risk when the high-risk group comprises the largest proportion of this population. For medical thromboprophylaxis, this implies that even patients considered to be at high-risk for developing VTE have a maximum absolute VTE risk of 2% when the overall risk is 1.6%. Therefore, even an effective preventive initiative will only result in a small risk reduction. This small potential benefit should be weighed against potential harms associated with prophylaxis, mainly bleeding events. Still, there may be a reasonable overall balance between prevention of pulmonary embolism and major bleeding, mainly because major bleeding events are rare. Nonetheless, this discussion underscores that future risk prediction models should aim to predict the benefits and harms in individual patients in order to provide optimal care for the right patients.