reply

We appreciate the interest in our article shown by Sanz and colleagues. We accept the statement that the amount of activated platelets depends on the collection device and apheresis kit employed. Recently, we demonstrated that only a minimal degree of platelet activation occurred during apheresis with a new type of cell separator.' Sanz et al. also show that a low degree of activation occurs during apheresis with the Dideco device. Their data, however, appear to be unclear, as it is not known whether CD42a (glycoprotein [GPI IX), CD42b (GPIba), or CD42c (GPIbP) has been analyzed. This is important, as the single domains of the GPIb-IX-V complex respond differently to stimulation. Furthermore, it is not surprising that no changes occurred in CD41 during apheresis, as this finding is consistent with the other preapheresis and postapheresis results. However, the difference between preapheresis and postapheresis CD41 results in the donor and the enormous increase in CD41 in the storage container remains unexplained. Regarding the comment of Sanz et al. on MCFI, the expression of flow cytometric results by MCFI, either by setting a threshold or by subtracting specific fluorescence from nonspecific MCFI, represents a commonly accepted approach? This is particularly true for the analysis of so-called activation-dependent markers. In contrast, when results are expressed as a percentage, the number of percentagepositive platelets may be underestimated, as the amount of antigen expressed on the single platelet may be too low for detection of a marker-positive platelet.* This may explain the unexpectedly low CD62p (p-selectin) postapheresis results of Sam et al. The results of Sanz et al. regarding the percentage of CD62p-positive platelets also show that an average of 8.6 f 8.9 percent of the platelets in the collection bags are already activated immediately after apheresis. Therefore, as activated platelets continue to circulate in donors after plateletpheresis, it can be expected that this percentage would increase in the components after repeated apheresis procedures, if the interval between the donations is too short. In this case, previously activated platelets of the donor will be harvested in subsequent procedures. Sanz et al. agree with our previous statement3 that platelet activation depends on the apheresis procedure, as long as no other'underlying conditions in the donor induce platelet activation. Knowing this, we chose a group of apparently healthy blood donors for our study. However, these factors cannot be anticipated in regular apheresis donors. A higher baseline level of platelet activation may be generated in some donors-for instance, by smoking, oral contraceptives, or physical activity-that contributes to further accumulation of activated platelets in the platelet component. Platelet activation, particularly the expression of activation-dependent antigens, is important to the quality of the platelet components. Today, there is no doubt that the in vivo efficacy of platelet components is compromised by platelet activation; this has been shown in several s t ~ d i e s . ~ , ~ The upregulation of p-selectin (CD62p) on the cytoplasmic membrane is predominantly associated with a reduction in the viability of the platelets and the quality of the platelet ~omponen t .~ -~ The upregulation and the increase in CD62p on the platelets result in the binding of activated platelets to white cells and in their subsequent sequestration. Therefore, the interval between repeat plateletpheresis procedures is an important factor in considering the quality of the platelet component. However, additional individual biocompatibility profiles are necessary, to estimate the degree of platelet activation of the various devices currently employed.