In reply

Strasser and colleagues are renowned experts in apheresis technology, and hence their interest in our publication and their insightful comments are very much appreciated. In their recent letter they comment on a rather minor point mentioned in our article, that is, the hypothesis that even with automatic apheresis systems specifics of device operation may influence apheresis outcomes. The point by Strasser and colleagues that more thorough analyses of log file data may offer additional insight is well taken, yet the modest purpose of our paper was solely to report on feasibility and efficacy of unstimulated leukapheresis in donors and patients in a routine clinical setting. Our data speak of an overall very similar performance of the new and old technologies, including a considerable and for an automatic device possibly unexpected variability in apheresis outcomes that is no smaller than with the old device generation. We had previously made similar observations for mobilized stem cell apheresis procedures. Of relevance in this context, our recent data surprisingly identify the donor as a systematic variable influencing apheresis outcomes. Strasser and colleagues’ analyses go much deeper than ours, however, as they seek to identify causes for this variability. Indeed the careful analyses by Strasser and coworkers unravel differences in extraction efficiency between monocytes and lymphocytes, and they offer an attractive hypothesis for this observation, that is, overflow of some of the relatively low-density lymphocytes from the full elution chamber. Although in our series, monocyte collection was actually slightly less efficient than lymphocyte collection, in light of Strasser’s exciting observation, we analyzed whether for our data set a greater number of processed mononuclear cells (MNCs) between chamber flushes would be associated with a lesser collection efficiency CE1 for MNCs. For this purpose, the number of processed MNCs was calculated from MNC concentration in blood and processed volume between flushes. Indeed a very strong, significant negative correlation between processed MNCs and CE1 became apparent (Fig. 1A). This was observed, however, despite the systematic markedly premature triggering of the chamber flush at a point when the chamber is only approximately half full, on which we comment in our article. Because of constant flush volumes, MNC concentration is a direct measure of elution chamber filling. Therefore, if overflow of the elution chamber were the primary cause of this variability, then the expectation would probably be that products with higher MNC concentration (fuller elution chambers) should be associated with lower CE1, yet that clearly was not the case (Fig. 1B). Whether this indicates that in such donors many MNCs bypass the apheresis system altogether, are processed through the apheresis system without being collected, or are collected and then lost we can only speculate. Importantly, in the reported series we had allowed apheresis procedures to proceed with manufacturer-defined default settings and a minimum of operator intervention; our data indicate that with that approach apheresis outcomes are just fine. The article’s claims for the novel technology— feasibility and essentially similar performance as the comparator technology in the routine clinical setting—we believe are well supported by the data provided. Detailed analyses of variables affecting apheresis outcomes and approaches to their mitigation are beyond the scope of our abilities.