Randomized, placebo-controlled trial of dalteparin for the prevention of venous thromboembolism in acutely ill medical patients - Leizorovicz, A; Cohen, A T; Turpie, A GG; Olsson, C G; Vaitkus, P T; Goldhaber, S Z; PREVENT Medical Thromboprophylaxis Study Group (Circulation, 2004)