BACKGROUND AND OBJECTIVES: Despite recent progress in our understanding of platelet function in vitro, there remains a remarkable paucity of methods to study platelet function in vivo.

MATERIALS AND METHODS: We have developed novel three color whole blood flow cytometric methods for tracking of infused platelets and measurement of their function in vivo.

RESULTS: These methods were used to demonstrate that circulating P-selectin-positive (degranulated) platelets rapidly lose surface P-selectin to the plasma pool, but continue to circulate and function.

CONCLUSIONS: 1) These studies strongly suggest that the measurement of platelet surface P-selectin in platelet concentrates stored in the blood bank should not be used as a predictor of platelet survival or function in vivo. 2) The described methods provide the means to answer many previously difficult to address questions about in vivo platelet function in transfusion medicine.