The present invention relates to a method for the anticoagulation of human blood ex vivo in which blood calcium concentration remains the same, no thrombin is formed and thrombocyte function is not affected. The invention further relates to a blood coagulation factor Xa inhibitor that can be advantageously used in this method, an agent that contains an inhibitor of the blood coagulation factor Xa, and a kit for diagnosing cellular blood components. The anticoagulated blood obtained according to the invention can be used for examining cellular blood components, for example for the diagnosis of thrombocyte function.