Purpose: We assessed the possibility of using soluble transferrin receptor (sTfR) as an indicator of doping with recombinant erythropoietin (rhEPO). Methods: A double-blind, placebo-controlled study was conducted with the administration of 5000 U of rhEPO (N = 10) or placebo (N = 10) three times weekly (181-232 U·kg-1·wk-1) for 4 wk to male athletes. We measured hematocrit and the concentration of hemoglobin, sTfR, ferritin, EPO, and quantified the effects on performance by measuring time to exhaustion and maximal oxygen uptake (V(dot)O2max) on a cycle ergometer. Results: Hematocrit increased from 42.7 ± 1.6% to 50.8 ± 2.0% in the EPO group, and peaked 1 d after treatment was stopped. In the EPO group, there was an increase in sTfR (from 3.1 ± 0.9 to 6.3 ± 2.3 mg·L-1, P < 0.001) and in the ratio between sTfR and ferritin (sTfR·ferritin-1) (from 3.2 ± 1.6 to 11.8 ± 5.1, P < 0.001). The sTfR increase was significant after 1 wk of treatment and remained so for 1 wk posttreatment. Individual values for sTfR throughout the study period showed that 8 of 10 subjects receiving rhEPO, but none receiving placebo, had sTfR levels that exceeded the 95% confidence interval for all subjects at baseline (= 4.6 mg·L-1). V(dot)O2max increased from 63.6 ± 4.5 mL·kg-1·min-1 before to 68.1 ± 5.4 mL·kg-1·min-1 2 d post rhEPO administration (7% increase, P = 0.001) in the EPO group. Hematocrit, sTfR, sTfR·ferritin-1, and V(dot)O2max did not change in the placebo group. Conclusion: Serum levels of sTfR may be used as an indirect marker of supranormal erythropoiesis up to 1 wk after the administration of rhEPO, but the effects on endurance performance outlast the increase in sTfR.
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