Immature Reticulocytes Are Sensitive And Specific To Low-Dose Erythropoietin Treatment At Sea Level And Altitude

We investigated whether immature reticulocyte fraction (IRF) and immature reticulocytes to red blood cells ratio (IR/RBC) are sensitive biomarkers for low-dose recombinant human erythropoietin (rhEpo) treatment at sea level (SL) and moderate altitude (AL) and whether multi (FACS) or single (Sysmex-XN) fluorescence flow cytometry is superior for IRF and IR/RBC determination. Thirty-nine participants completed two interventions, each containing a 4-week baseline, a 4-week SL or AL (2,230 m) exposure, and a 4-week follow-up. During exposure, rhEpo (20 IU kg(-1)) or placebo (PLA) was injected at SL (SLrhEpo, n = 25, SLPLA n = 9) and AL (AL(rhEpo), n = 12, AL(PLA) n = 27) every second day for 3 weeks. Venous blood was collected weekly. Sysmex measurements revealed that IRF and IR/RBC were up to similar to 70% (P < 0.01) and similar to 190% (P < 0.001) higher in SLrhEpo than SLPLA during treatment and up to similar to 45% (P < 0.001) and similar to 55% (P < 0.01) lower post-treatment, respectively. Compared with AL(PLA), IRF and IR/RBC were up to similar to 20% (P < 0.05) and similar to 45% (P < 0.001) lower post-treatment in SLrhEpo, respectively. In AL(rhEpo), IRF and IR/RBC were up to similar to 40% (P < 0.05) and similar to 110% (P < 0.001) higher during treatment and up to similar to 25% (P < 0.05) and similar to 40% (P < 0.05) lower post-treatment, respectively, compared with AL(PLA). Calculated thresholds provided similar to 90% sensitivity for both biomarkers at SL and 33% (IRF) and 66% (IR/RBC) at AL. Specificity was >99%. Single-fluorescence flow cytometry coefficient of variation was >twofold higher at baseline (P < 0.001) and provided larger or similar changes compared to multi-fluorescence, albeit with smaller precision. In conclusion, IRF and IR/RBC were sensitive and specific biomarkers for low-dose rhEpo misuse at SL and AL.