A reliable and cost-effective screening test for erythropoietin (EPO) doping is still unavailable. Thus a new approach by estimating mean red blood cell (RBC) age by means of erythrocyte aspartate aminotransferase activity (eAST) was developed. We investigated 201 women and 169 men residing at low altitude for peak oxygen uptake, EPO, and eAST. Additionally, we investigated 63 women and 42 men residing at 2600 m above sea level for EPO and eAST. Furthermore, 22 female and 28 male patients with renal failure receiving recombinant human EPO (rhEPO) were investigated for eAST levels. There was no difference in eAST between trained (women, 2.9+/-0.7 U x gHb(-1); men, 2.5+/-0.4 U x gHb(-1)), moderately trained (women, 2.8+/-0.6 U x gHb(-1); men, 2.4+/-0.4 U x gHb(-1)), and untrained subjects (women, 3.0+/-0.5 U x gHb(-1); men, 2.5+/-0.4 U.gHb-1) at low altitude. Participants receiving rhEPO had a dose-dependent increase in eAST (r=0.25; P< .05). Trained high-altitude residents (women, 2.8+/-0.8 U x gHb(-1); men, 3.0+/-1.1 U x gHb(-1)) had higher eAST than untrained high-altitude residents (women, 2.5+/-0.6 U.gHb-1; men, 2.4+/-0.4 U x gHb(-1); P< .05). Since eAST was sensitive to RBC rejuvenation, eAST elevation could indicate EPO use in lowlanders. eAST values above the 95% confidence interval (>3.3 U x gHb(-1) for men; >4.1 U x gHb(-1) for women) are suspected of EPO use.