Detailed information about methods and PLT donations are reported elsewhere [4]. Briefly, the duration of donations was individually tailored to obtain the highest possible number of units containing at least 2 × 10¹¹ PLTs within a maximal apheresis duration of 100 min. At the beginning of the donation the setting was based on the PLT count and hematocrit (HCT) level of the previous donation. 15–20 min later it was adjusted regarding the current pre-collection analyses. For procedures using A we adjusted to a standard anticoagulant (AC) ratio of 10, a citrate infusion rate of 1.25 mg/kg/min and a maximal cycle volume of 200 ml. The settings for the T procedures included an AC ratio of 12, an AC infusion rate level of 5, a maximal draw flow of ‘high’, a draw flow of 6 and a return flow of 3.

The 2 × 60 donations of the before mentioned study [4] were further compared for different variables, including pre- and post-apheresis PLT and white blood cell (WBC) counts, mean PLT volume (MPV), HCT, PLT yield/h (= collection rate; CR), and PLT yield per processed blood volume of the donor (relative efficiency; RE). We measured the MPV before and after apheresis in order to find out if a release of younger PLT (typically with a higher MPV) in the circulating blood occurs, being consistent with a PLT recruitment from an extravascular site. The PLT recruitment was defined as the ratio of the total PLT yield and the depletion of circulating PLT in the donor:

Recruitment = PLT yield / (PLT_before − PLT_after) × donor blood volume (1).

A recruitment was assumed if the ratio was > 1. Its correlation with donor and donation variables was separately analyzed for A and T donations.

The basic characteristics of the donors and the laboratory values before and after PLT collection are summarized in table ​table1.1. Although the PLT counts before and after donation and the difference of pre- and post-apheresis PLT counts were comparable for T and A, the PLT yield was higher for T, in spite of a shorter apheresis duration (table ​(table2).2). This corresponded to a higher collection rate and relative efficiency during apheresis with T (table ​(table3).3). A relevant PLT recruitment (lower threshold of the 95% CI >1) was observed with both collection procedures but was significantly higher with T (table ​(table33).

The MPVs before and after collection were comparable in the two groups. However, they differed significantly before and after PLT apheresis. In the A donations, mean MPV_before was 7.59 (95% CI 7.40–7.78) and mean MPV_after 7.96 (95% CI 7.74–8.17; p < 0.001). In the T donations mean MPV_before was 7.56 (95% CI 7.38–7.73) and mean MPV_after 7.86 (95% CI 7.66–8.07; p < 0.001).

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, 4, 7]. 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 ​(table1).1). 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, 9]. 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 ​(table1).1). 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].