The Leiden Thrombophilia Study (LETS)

Study Design (5) LETS was designed to assess the importance at the population level of various risk factors for thrombosis, many of which had been identified by family studies. Generally, two approaches are possible: cohort studies and case-control studies. In the former, patients with and without a risk factor are followed over time, and absolute risks (probability of disease, incidence) and relative risks (incidence rate ratio) can be calculated. In a case-control study, patients with thrombosis and healthy controls are included, and relative risks (odds ratios) can be calculated. Due to the rarity of the genetic risk factors for thrombosis, a follow-up study of a truly unselected sample from the