Platelets are more vulnerable to damage than erythrocytes because platelets are easily activated by contact with extracorporeal circuits and by exposure to shear forces. However, the degree of platelet damage caused by centrifugal pumps is unclear. To evaluate platelet damage in different pumping conditions, the rates of increase for specific proteins in platelet alpha-granules, beta-thromboglobulin (beta-TG), and platelet factor 4 (PF-4) were measured in both in vitro simulated left ventricular assist device (LVAD) and cardiopulmonary bypass (CPB) conditions and compared with the erythrocyte trauma. A flow of 5.0 L/min with deltaP of 100 mm Hg for LVAD (low pressure head condition) and a flow of 5.0 L/min with deltaP of 350 mm Hg for CPB (high pressure head condition) were investigated. Each condition was tested 4 times for 3 h in a mock circuit with a Capiox (Terumo, Tokyo, Japan) centrifugal pump using fresh human blood. Blood was sampled at 1 h intervals, measuring plasma free hemoglobin (fHb), beta-TG, and PF-4. To evaluate the degree of damage, the rates of increase of fHb, beta-TG, and PF-4 were calculated for each condition as deltafHb/deltaN, deltabeta-TG/deltaN, and deltaPF-4/deltaN where deltafHb is the increase in plasma free hemoglobin, deltabeta-TG is the increase in beta-TG, deltaPF-4 is the increase in PF-4, and deltaN is the increase in the passing number. The passing number is defined in the following equation: N = Qt/V where t is the time, V is the priming volume, and Q is the flow rate. There was no significant difference between the 2 conditions (low pressure head condition versus high pressure head condition) in the rate of increase of fHb (0.0035+/-0.0004 vs. 0.0034+/-0.0010 g/100 L, NS). Contrary to this, the rates of increase for specific proteins in platelet alpha-granules in the high pressure head condition demonstrated a significantly higher rate of increase than in the low pressure head condition. The mean rate of increase for beta-TG in the low pressure head condition was 0.22+/-0.03 ng/ml and in the high pressure head condition was 0.51+/-0.05 ng/ml (p < 0.05). The rate of increase for PF-4 in the low pressure head condition was 0.11+/-0.02 ng/ml and in the high pressure head condition was 0.30+/-0.06 ng/ml (p < 0.05). These results suggest that measurements of beta-TG and PF-4 may be more sensitive parameters than hemolysis for evaluating blood cell trauma and that platelets are more vulnerable to mechanical damage by a centrifugal pump than erythrocytes.