Summary

Primary haemostasis consists of platelet adhesion to subendothelialcollagen, their activation and aggregation and finallythe formation of a platelet plug. Erythrocytes are involved in thisprocess because they flow in the center of the vessel and pushplatelets towards the site of action on the vessel wall andenhance shear forces, which activate platelets. In the plateletfunction analyzer PFA-100 ® (Dade Behring, Düdingen, Switzerland),the in vivo situation is simulated in vitro with blood being aspiratedat high shear rates (5000s-1) through a capillary into amembrane pore with a diameter of 150 µ m coated with type Icollagen and either epinephrine or adenosine diphosphate.Aggregating platelets plug the pore and stop the flow, which ismeasured as the closure time.We analysed the influence of erythrocytes on platelet function analyzer measurements by systematicvariation of the haematocrit (20,30,40,and 50%) at constantplatelet counts of 289±61 x10 3 / µ l plasma, or 152±30x103 / µ l blood, 96±9 x103 / µ l blood and 54±5 x103 / µ l blood, respectively.An inverse correlation was found between haematocritand closure time under all circumstances.A decrease of theplatelet count by 50 x103 / µ l could be compensated for by a 10%increase in haematocrit. The haematocrit must, therefore, betaken into consideration for the correct interpretation ofPFA-100® measurements. Our data also provide a pathophysiologicalrationale to reduce the risk of bleeding in patients withthrombocytopenia and anaemia by normalizing the haematocritwith erythrocyte transfusions.