Formation of occlusive platelet aggregates in whole blood caused by low concentrations of ADP

Minute concentrations of ADP are released when platelets are exposed to she ar stress during extracorporeal flow. However, based on current methods, th ese low concentrations have not been shown to have a significant impact on platelet function. We report here the formation of rigid microaggregates (M A) in response to low concentrations of ADP. A newly developed light scatte ring whole blood aggregometer (LSWBA) was used to detect an aggregation dos e response to ADP (0-2 muM) in heparinized (1.5 u/ml) human blood. Although the LSWBA showed that ADP induced MA were reversible, evidence provided by constant pressure filtration (50 mm Hg) suggested that aggregates existed as rigid particles in the blood for up to 6 minutes. The possible implicati ons of these findings to extracorporeal circulation are discussed.