Renewed concerns about safety of PLT concentrates will probably further increase costs and complexity of their production and reduce the availability of donors. New safety requirements are established or in evaluation, for example bacterial testing, pathogen inactivation, or exclusion of females from donation of plasma containing components for prevention of TRALI [5
]. Contemporarily, the need of PLT concentrates obtained from single donors by apheresis is increasing, and with this the interest in the production of multiple PLT concentrates from a single donation.
In our previously reported study [4
], the donations were individually tailored in order to obtain the highest number of standard units containing 2 × 1011
PLT within 100 min. This acceptable duration of donations up to 100 min results in a reduction of the ratio between the initial period (adjustment of interface) and the consecutive stable collection phase and therefore in a higher CR [3
]. However, the donation-induced decrease of the donor's circulating PLT count may represent a safety limit for the production of multiple PLT concentrates. A recruitment of PLT during apheresis may partially counterbalance the PLT extraction and thereby increase the safely achievable maximal PLT yield.
In both PLT apheresis procedures performed with A or T, the actual PLT yield exceeded significantly the calculated yield, i.e. the calculated number of circulating PLT removed from the donor's blood using the pre- and post-PLT count and the blood volume of the donor. This allowed the collection of a high proportion of multiple units per donation [4
] despite a normal or slightly decreased post-apheresis PLT count (in all donors between 120 and 220 × 109
/1). However, the recruitment was significantly higher in the PLT aphereses performed with T than with A. This contributed to a higher yield and hence a higher proportion of multiple donations, despite identical pre- and post-apheresis PLT counts (table ). The donation-induced decrease of the donor's circulating PLT count, which represents one of the main safety limits for the production of multiple PLT concentrates, seems therefore to be markedly influenced by the device and/or procedure utilized. This should be considered in the development of specific guidelines.
In order to support the hypothesis of PLT recruitment by further empirical data, we measured the MPV of the donors before and after PLT apheresis. The increasing MPV during PLT apheresis seems to support the hypothesis of a recruitment of PLT from an extravascular site into the circulating blood with both procedures. This would support the hypothesis formulated 25 years ago that PLT are recruited from the spleen during apheresis, which in particular was based on the observation that splenectomized patients in remission from leukemia did not show such a recruitment [8
]. We did not find a correlation between MPV and recruitment, and the MPV increase did not differ between the 2 procedures. But this should be interpreted with caution because this analysis was not the primary objective of the study and was performed on two small groups of donations.
Interestingly, we further observed a difference in the increase of donor WBCs between the two PLT apheresis procedures tested, being more pronounced in T (table ). This may be related to a different extent of WBC sequestration in the different extra-corporal systems of the two devices. The accumulated WBCs may be flushed back to the donor at the end of the procedure to a different extent. Alternatively, there may be a different mobilization of marginal peripheral blood WBCs (for example related to a different citrate toxicity [3
]) or other unknown factors. The increase of donor WBCs after donation also did not correlate with PLT recruitment and needs further investigation.
Recruitment of collected cells during apheresis is a recently reported phenomenon for peripheral blood progenitor cells [10
]. Like in autologous peripheral blood progenitor cell collection, increasing the processed blood volume may be helpful to increase the yield per donation. With the already mentioned statistical limitations, the absence of a negative correlation between recruitment and parameters defining the ‘magnitude’ of the apheresis (e.g. duration, processed blood volume or yield) may imply that the upper limit of processed blood volume revealing a sustained recruitment still was not reached. However, a further intensification of the PLT apheresis procedure may be limited by other factors sich as side effects for donors or an impaired effectiveness of the transfused PLT [11
In conclusion, both cell separators induced a PLT recruitment during multiple PLT apheresis. This recruitment was significantly higher in T, contributing to a higher PLT yield, CR and RE, in spite of a comparable post-apheresis decrease of circulating PLT counts in the donor. The post-apheresis PLT count, which represents one of the main safety limits for donors, seems therefore to be markedly influenced by the device and/or procedure utilized. Our results and previously reported differences of multiple PLT donations concerning effectiveness of the products [11
] and safety of the procedures [4
] should be considered in the establishment of guidelines limiting the number of yielded units per donation.