BACKGROUND Platelet transfusions are being used increasingly in patients with thrombocytopenia to improve hemostatic function before surgery and invasive procedures. However, there are limited data on the immediate quantitative and qualitative platelet response after transfusion. Some authors have suggested that transfused platelets require time in vivo to regain maximal function, which is dependent on the duration of platelet storage. Therefore, the timing of surgery and invasive procedures with optimal platelet function may not be occurring. METHODS Twenty-five patients with thrombocytopenia from ablation chemotherapy and total body irradiation (before bone marrow transplantation), were randomized to receive either 1-day (fresh) or 4-day stored platelets. No patient had infection, organ system dysfunction, or previous platelet transfusion. Single-donor platelets were transfused (1 unit/10 kg body weight) over 60 min. Whole blood from an indwelling central venous catheter was obtained before, immediately after, and 1, 2, and 24 h after transfusion. Platelet number was measured on a Coulter counter and platelet reactivity was measured on a Coulter counter and platelet reactivity was measured using agonist-induced whole blood impedance aggregometry (ohms) and dense granule release (adenosine triphosphate [ATP]). RESULTS Platelet number increased similarly (21,000 +/- 2,000/mm3 to 76,000 +/- 7,000/MM3 AND 20,000 +/- 1,000/MM3 TO 65,000 +/- 4,000/MM3) after transfusion in the 1- and 4-day stored platelets, respectively. These levels were maintained for 2 h after transfusion in both groups and then decreased similarly (26% and 27%) at 24 h. Agonist-induced platelet aggregation increased immediately after transfusion to 5 micrograms/ml collagen (0.7 +/- 0.4 to 11.4 +/- 1.0 ohms and 0.1 +/- 0.1 to 5.2 +/- 1.0 ohms), 10 micrograms/ml collagen, (1.5 +/- 0.7 to 18.0 +/- 1.9 ohms and 0.6 +/- 0.4 to 10.0 +/- 1.6 ohms) and ristocetin (0.7 +/- 0.4 to 10.1 +/- 1.7 and 0.1 +/- 0.7 to 6.2 +/- 1.0 ohms), in 1- and 4-day, stored platelets, respectively and persisted unchanged in both groups for 2 h. Fresh platelets were hyperaggregable compared to 4-day stored platelets for collagen-induced (5 micrograms/ml and 10 micrograms/ml) aggregation. Agonist-induced platelet dense granule release (ATP) increased immediately after transfusion to 5 micrograms/ml collagen (42 +/- 18 to 410 +/- 49 picomoles ATP and 20 +/- 7 to 186 +/- 22 picomoles ATP), 10 micrograms/ml collagen (60 +/- 22 to 449 +/- 53 picomoles ATP and 44 +/- 13 to 219 +/- 25 picomoles ATP in 1- and 4-day platelets, respectively. Ristocetin-induced ATP release increased immediately after transfusion of fresh platelets only (0 +/- 0 to 69 +/- 17) and remained unchanged for 2 h. Fresh platelets also demonstrated greater dense granule release to collagen (5 micrograms and 10 micrograms/ml) and ristocetin than 4-day stored platelets. CONCLUSIONS In patients with chemotherapy-induced thrombocytopenia, platelet transfusion causes an immediate increase in number and function, which is independent of storage time. This quantitative and qualitative increase persists unchanged for 2 h after transfusion, suggesting that there is no acute "warm-up-time" necessary for transfused platelets to regain maximal function. Fresh platelets demonstrate increased aggregation and dense granule release compared to 4-day stored platelets and may impart improved hemostatic function in vivo.