In order to discuss blood manipulations, one has to appreciate the reference values. At sea level, healthy males in the United States (n= 7426) have an Htc between 40 and 49% (5th and 95th percentile, respectively), whereas the normal limits for females (n= 7704) are 35 and 44%, with a slight tendency to increase following menopause. The 50th percentile for males and females in the ‘exercise competitive age’ is 44.5 and 39%, respectively. For young children, the normal limits are 33 and 40% (Fulwood et al., 1982). Based on the Second National Health and Nutrition Examination Survey (n= 2515), major [Hb] differences exist between black and white males (144.8 vs. 153.2 g·L−1), females (128.4 vs. 133.9 g·L−1) and children (120.3 vs. 126.8 g·L−1) (Perry et al., 1992). Smoking more than 10 cigarettes per day significantly increases [Hb], and when comparing never-smokers to smokers, an increase in [Hb] from 133 to 137 and from 152 to 156 g·L−1 is seen in females (n= 2454) and males (n= 2250), respectively (Nordenberg et al., 1990). Based on this and similar data, and in an attempt to limit doping practice, the upper Htc limit (cut-off value) allowed for males and females by most sports federations was set to 50 and 47%, respectively. The reference values used for these limits as noted earlier, however, are based on normal healthy individuals. With exercise training, haematological adaptations occur, leading to an initial expansion of plasma volume. However, after approximately 30 days of endurance training, the increase in red cell and plasma volume are approximately equal, and hence also [Hb] normalizes (Sawka et al., 2000). The haematological profile of an elite endurance athlete is less well described. To compare the distribution of blood haemoglobin levels in healthy blood donors and elite athletes specifically for anti-doping purposes, a retrospective (2001–2005) cohort study was performed in 85 846 Danish (sea level) blood donors (males = 36 962; females = 48 884, age 18–65 years) and compared to 1406 national team rowers (males = 1116; females = 290, age 16–32 years). In this data set, 3.9% of the male blood donors had a blood [Hb] of above 10.5 mM (corresponding to a Htc of 51%, i.e., above the cut-off value), and 1.6% of the females had a [Hb] above 9.7 mM (equivalent to a Htc of 47%, the female cut-off value). Surprisingly, this data set also demonstrated that the % distribution for the cut-off values are higher (P < 0.0001) in elite athletes, i.e., 10.4 and 8.3% for the male and females, respectively, and thereby demonstrating that high [Hb] levels in blood are seen regularly in normal people and especially in competitive athletes (Johansson et al., 2009). It needs to be stated, however, that it not can be ruled out that at least some of the athletes included in this study may have made use of doping practice, and that this could have affected the values. In another study, moderately trained subjects (n= 44) were compared to trained runners (n= 19), highly trained cyclists (n= 17) and world class cross-country skiers (n= 21), and in that study, only two of the moderately trained subjects surpassed the upper limit for [Hb], whereas none of the better trained subjects surpassed the upper accepted level. Data from this study also suggest that Htc in Olympic calibre athletes and non-athletes are rather similar, but that the nHb and plasma volumes are increased in highly trained athletes (Jelkmann and Lundby, 2011). Nonetheless, it needs to be acknowledged that some athletes may have blood values above the cut-off values, and this gives national sports federations the very important task to pinpoint such athletes from an early time point in their careers, and then to ensure the needed documentation in order to avoid a ‘no start’ penalty or potential mis-accreditaion of the athlete by sports colleagues or the press.
It is also important to realize that blood values undergo biological variation. Based on results from 12 studies of 638 normal healthy adults, the coefficient of within-subject biological variation of Htc is 3%. The normal within-subject biological variation (3%) and analytical variation (3%) may explain a relative change of approximately 12%, e.g., a change from 42 to 47% between two successive Htc values, measured with a time interval between 1 day and 1–2 months (Thirup, 2003). Partly due to haemodilution in warm weather, Htc often has a seasonal variation in normal healthy adults, and, based on results from 18 studies of 24 793 participants, the population mean is approximately 3% lower in summer than in winter. Population mean values that are 7% lower in summer than in winter have been found in some studies, although no seasonal effect may also be seen, especially in temperate climates (Thirup, 2003). Besides ambient temperature, also altitude exposure influences blood values. As compared to sea level, no changes in Htc or red cell mass (nHb) are usually observed with exposure to 1.600 m altitude (Weil et al., 1968), but starting from around 2350 m altitude (Schuler et al., 2007), an increase is noted which becomes gradually more increased with further altitude gain (Weil et al., 1968; Calbet et al., 2003; Lundby et al., 2006). Other external factors affecting plasma volume, and hence Htc and [Hb], include posture, hydration status and acute exercise (Harrison, 1985). In contrast to this, Schumacher and colleagues (Schumacher et al., 2010) concluded from a specifically anti-doping designed study performed on endurance-trained subjects (mostly cyclists) and appropriate controls that fluid intake and ambient temperature over the course of a ‘typical training day’ had no significant effect on [Hb]. The conducted training resulted in an average [Hb] increase of 0.46 g·dL−1 which however disappeared 2 h into recovery, and, thus, minor changes in [Hb] can be expected following training/racing, but unless a given athlete is extremely dehydrated (and hence not able to train/race in a meaningful manner) (Harrison, 1985), this likely resembles no limitation to anti-doping work. An example of a similar study approach in national team cross-country skiers is shown in Figure 2B where [Hb] is seen not to be significantly influenced by various interventions associated to normal life of such athletes. In more extreme exercise events (Pugh, 1969), however, a decrease in Htc may be observed secondary to an increase in plasma volume while red cell mass remains stable.