Purpose: To evaluate within-subject variability in the serum erythropoietin (EPO) response to multiple simulated altitude exposures. Methods: Seven physically active men and women (age 27 ± 3 years, body mass index = 24.6 ± 4.0 kg/m2) were exposed to normobaric hypoxia (fraction of inspired oxygen [FiO2] = 0.14) for 12 hours on three separate occasions. Serum EPO concentrations were measured before exposure (0 hour), after 6 hours, and after 12 hours in hypoxia. The EPO response to hypoxia was calculated as percent change from 0 to 12 hours (DeltaEPO0-12). Results: Exposure time had a significant effect on EPO (p < 0.001) with concentrations increasing 3.2 ± 1.3 mIU/ml from 0 to 6 hours (p = 0.034) and 4.7 ± 1.2 mIU/ml from 0 to 12 hours (p = 0.001). Group mean DeltaEPO0-12 remained unchanged (p = 0.688) between the three exposures; however, there was considerable intraindividual variability in EPO responses. The intrasubject coefficient of variation for DeltaEPO0-12 was 61% ± 28% (range: 17%-103%) with intrasubject associations ranging r = 0.052 to r = 0.651 between repeated exposures. Conclusions: Athletes who routinely supplement training with simulated altitude methods (e.g., hypoxic tents) should expect inconsistent EPO responses to intermittent exposures lasting =12 hours.
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