Abstract: The invention relates to a water-based preparation to be mixed with blood, containing citrate ions which inhibit coagulation, as well as a method applied at collection of blood from a living organism while adding an anticoagulant substance including citrate ions. The new features of the invention are that acetate, being a physiologic energy source, inter alia for platelets, is included in the preparation and is added to the blood during collection in addition to the anticoagulant substance in order to obtain prolonged shelf-life of the platelet concentrate without increased risk for the donor or patient.