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The Athlete's Biological Passport (ABP) is a tool for the indirect detection of blood doping. Guidelines from the World Anti-Doping Agency (WADA) require a 2 hours delay after any physical exercise, and to be seated for 10 minutes before collecting an ABP sample. This study investigated posture-related hematological variations with changes in body position during blood sampling. Ten successive venous blood samples from 38 subjects were collected in three situations: immediately after 10-minutes of normalized activity (B1), after 10-minutes seated (B2, typical reference sample in an anti-doping context), after a 50 m walk (B3), after 5 and 10-minutes in a seated position again (B4 and B5), and finally after 5-30 minutes supine (B6-B10). Hemoglobin concentration [Hb] and hematocrit (Hct) were determined by flow cytometry to assess putative posture-related variations. Reticulocytes percentage was unchanged in all conditions, [Hb] and Hct were stable after at least 10-minutes in a seated position. Due to shifts in plasma volume, [Hb] and Hct increased slightly but significantly higher after changing posture for a short walk (+0.1 gr/dL [P = .008] and +0.4% [P = .01] respectively), but readjusted to previous levels after only 5 minutes. Supine position (>10 minutes) induced decreases of [Hb] (-0.2 g/dL in average, P < .01) and Hct (-1.1%, in average, P < .01). The observed variations in [Hb] and Hct may have minor clinical significance, while they underline the need to follow strict guidelines for posture before and during blood sampling in an anti-doping context.