 The main purpose of the NORIDES study is to investigate the effect of pharmacological thromboprophylaxis with low molecular weight heparins LMWHs in critically ill patients, and how it is affected by presence of acute kidney injury AKI and treatment with hemodialysis. The main objective is to compare the prophylactic effect of dalteparin in intensive care unit ICU patients with AKI and Citrate-Calcium dialysis CiCa-dialysis with a control group of ICU patients with normal kidney function. Our main hypothesis is that CiCa-dialysis reduces dalteparin effect, and that patients undergoing CiCa-dialysis do not achieve adequate prophylaxis against venous thromboembolism VTE. The primary endpoint is development of DVT during ICU stay, the secondary endpoint inadequate heparin effect measured in blood samples. 