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 ) or placebo (PLA) was injected at SL (SL , n = 25, SL n = 9) and AL (AL , n = 12, AL n = 27) every second day for 3 weeks. Venous blood was collected weekly. Sysmex measurements revealed that IRF and IR/RBC were up to ~70% (P < 0.01) and ~190% (P < 0.001) higher in SL than SL during treatment and up to ~45% (P < 0.001) and ~55% (P < 0.01) lower post-treatment, respectively. Compared with AL , IRF and IR/RBC were up to ~20% (P < 0.05) and ~45% (P < 0.001) lower post-treatment in SL , respectively. In AL , IRF and IR/RBC were up to ~40% (P < 0.05) and ~110% (P < 0.001) higher during treatment and up to ~25% (P < 0.05) and ~40% (P < 0.05) lower post-treatment, respectively, compared with AL . Calculated thresholds provided ~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.