In the fight against doping in sports, indirect detection methods using Hb and Hct are starting to play a more and more important role. The goal of this investigation was to quantify the changes in PV during a 2 week study of endurance exercise simulating a cycling stage race and to test for PV changes relative to the circadian rhythm of the athlete. 15 endurance trained triathletes and cyclists performed a standardised 3 day taper (no exercise), followed by a 9 day cycling stage race simulation. Hb showed a mean increase of 4% during the 3 day taper period which was caused by a reduction in PV. During the exercise phase Hb dropped by 1.5 g/dl or 11%, which relates to an average expansion of PV by 0.6 l or 16%. The fluctuations in morning PV showed a wide inter-subject variability. The smallest change was 493 ml (10%), which was accompanied with a drop in Hb from 14.8 g/dl to 13.7 g/dl. The largest increase was 1277 ml (or 25%). The corresponding Hb values for this subject dropped from 15.8 g/dl to 13.1 g/dl (figure 2). The Hb concentration of the morning and evening samples showed a mean difference of approximately 0.6 g/dl or 3.7%, which was observed both during the taper (without exercise) and stage race phases. This study showed that exercise induced changes of PV in healthy trained endurance athletes can be up to 1277 ml or 25% when tested at the same time of the day. This fact needs to be considered in the interpretation of blood profiles as this value might even be higher if the test was to be performed at different times of the day. In our study we were able to demonstrate that the circadian rhythm of Hb concentration remains stable even under maximal PV expansion and reaction to exercise. This finding could be implemented in the athlete biological passport model to help reduce the variation of biological markers.
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