Standardization of Reticulocyte Counts in the Athlete Biological Passport: A Practical Update.

Introduction The athlete biological passport monitors blood variables over time to uncover blood doping. With the phasing in of a new series of blood analyzers, the Sysmex XN series, it was necessary to examine the comparability of results with the previously employed XT/XE series. A previous comparison between XN and XT/XE series suggested a small but significant bias between the two instruments in the measurements of RET%. Here, we examined the comparability of RET% on the XN and XT/XE platform using data collected over the first year since the transition. Methods The comparability of results obtained from XN and XT/XE instruments was assessed using three datasets: (i) 767 blood samples measured on both instrument series in 22 WADA-accredited laboratories, (ii) 27 323 samples measured on either instrument across 31 laboratories, and (iii) 119 clinical samples and 110 anti-doping samples measured on both instruments in a single laboratory. Results Analysis of the three datasets confirms the previous observation of a bias toward higher RET% values for samples measured on Sysmex XN instruments compared with the XT/XE series. Using data across a larger number of XN instruments and a larger athlete population, the current work suggests that the bias is proportional and slightly higher than previously observed across most of the range RET% values. Conclusion A model is proposed for the comparison of data across XN and XT/XE technologies whereby the instrument bias increases proportionally with RET% measured on Sysmex XN Series, but where the rate of increase is negatively related to IRF%.