Citrated blood was investigated directly after blood sampling and 1 and 4 days after incubation at 4 degrees C before and after passage of a microaggregate filter (MF 10, Biotest). During the 4 days of incubation the platelet number was reduced from a mean of 200 000/microliter to a mean of 140 000/microliter. After the filter passage the platelet count was reduced in the freshly prepared blood samples by 10% and in the 4-days-old samples by 20%. Filter passage induced only a slight stimulation of platelets (sphering and pseudopode formation) in freshly prepared citrated blood. Aggregate formulation in the samples was small but increased continuously during the 4 days of incubation. In the 4-days-old samples the medium-sized aggregates consisting of 4-12 platelets were reduced while the number of small aggregates consisting of 2-3 single platelets increased after filter passage. Platelet aggregation was not changed by the filter passage but was continuously reduced during the storage time. The filter passage did not change the thromboplastin time, factor VIII values, fibrinogen, thrombin time and the thrombin coagulase time. The partial thromboplastin time values did not differ before and after filter passage in the freshly prepared and 1-day-old samples but were slightly shortened in the samples stored for 4 days with a large variation of the single values. The minimal platelet-stimulating effect, which could be demonstrated in freshly prepared blood samples only is reversible and corresponds to that observed if blood is drawn through a PVC catheter at blood sampling. In so far the microaggregate filter MF 10 had no thrombogenetic effect.
