Objectives To determine the prevalence of heparin-induced antibodies on admission to the ICU and the development of new heparin-antibodies during the first 7 +/- 2 days of hospitalization. Secondary objectives include: determining the incidence of heparin antibody formation in patients treated with UFH, LMWH and mechanical prophylaxis (MPX); and to compare the incidence of heparin antibodies between different ICU populations.