The goal of this observational study is to analyse the association between anti-factor Xa activity (antiXa) and the occurence of venous thromboembolism (VTE; either deep vein thrombosis and/or pulmonary embolism) in critically ill patients who are admitted to an intensive care unit. The main questions it aims to answer are: * What is the association between antiXa and VTE? * What is the association between antiXa and symptomatic, respectively incidental, VTE? * How is pharmacological anticoagulation with enoxaparin related to measured antiXa? * What is the association between antiXa and bleeding complications. * What is the incidence of venous thromboembolism in patients treated at an intensive care unit? * How is the occurence of VTE related to patient-centred outcomes such as mortality, quality of life, length of stay and days outside of the intensive care unit/hospital.