The present thesis aim to contribute to the anti-doping research within blood manipulation, as it appear to be a continuous problem. Specifically, the present thesis aimed to establish the natural fluctuations in plasma volume during a high water ingestion as well as during and after a period of increased endurance exercise load.

It was hypothesized that the natural fluctuations of plasma volume are important for the sensitivity and specificity of the Athlete Biological Passport.

It was further hypothesized that plasma albumin, soluble transferrin receptor and pro-atrial natriuretic peptide are sensitive markers for the plasma volume fluctuations and that adding the plasma volume independent reticulocyte percentage increase the sensitivity of the passport.

Finally, the hypothesis that an autologous blood transfusion of half a blood bag (~135 ml red blood cells) improve cycling time-trial performance and that an autologous blood transfusion of two blood bags (~500 ml red blood cells) is detectable by an untargeted metabolomics analysis was investigated.