Athletes and coaches are constantly pursuing legal means, such as training at altitude to augment oxygen carriage through an increase in [Hb] and thereby improving sea-level performance. However, recent revelations relating to high profile individuals within professional cycling, including Floyd Landis, Tyler Hamilton [25] and Lance Armstrong [26], have highlighted the illegal methods used by some athletes to improve performance, often in advance of the efforts of regulatory authorities to constrain them and of their adoption into clinical medicine [27]. It is legitimate to question whether such methods are safe (or at least fall within the broad margins of safety), if they are effective and if they could have wider applicability within clinical medicine.

Whilst a variety of agents have been used to manipulate haemoglobin levels (e.g. blood, recombinant human erythropoietin (rHuEPO), Continuous Erythropoietin Receptor Activator (CERA), hypoxia-inducible factor (HIFs) agents and possibly even ‘gene doping’ (although we do not yet have definitive evidence of this [28, 29]), the basic aim remains the same that increasing oxygen delivery (DO 2 ) through elevating haemoglobin levels will augment maximum oxygen uptake (\({\dot{\text{V}}}\)O 2max ) and perhaps more importantly (for endurance events) increase the workload at which anaerobic threshold (AT) is reached. There is still debate around the factors that limit \({\dot{\text{V}}}\)O 2max , with candidate mechanisms including central control, cardiac limitation, mitochondrial utilisation and total oxygen delivery (the product of cardiac output and blood oxygen content). However, whilst there remains uncertainty about the dominant controlling factor, many authorities agree that in highly trained athletes, DO 2 is a factor that contributes to \({\dot{\text{V}}}\)O 2max limitation [30–32] and that \({\dot{\text{V}}}\)O 2max is also, at least in part, dependent on a number of underlying genetic factors that are not amendable to modification through training [33]. Therefore, blood manipulation to augment DO 2 has been seen as a logical, albeit illegal, approach to augmenting \({\dot{\text{V}}}\)O 2max and thereby improving athletic performance. It is notable in this regard that tHb-mass displays a much stronger relationship with \({\dot{\text{V}}}\)O 2max than does [Hb] [34, 35] and may therefore be a more useful marker of intervention efficacy. Here, the relationship between different physiological measures of physical fitness and performance merits consideration. Whilst the majority of sports research has focused on \({\dot{\text{V}}}\)O 2peak or \({\dot{\text{V}}}\)O 2max as the accepted gold standard indices of cardiorespiratory fitness, other variables may have an important role in determining performance, particularly in endurance events. As exercise increases above a threshold submaximal work rate, anaerobic respiration begins to contribute to Adenosine Triphosphate (ATP) production and this is both inefficient (relative to aerobic respiration) and unsustainable (due to progressive lactic acidosis). Therefore, when discussing performance, although a high total aerobic capacity (\({\dot{\text{V}}}\)O 2peak / \({\dot{\text{V}}}\)O 2max ) is important for success in endurance sports, submaximal indices of fitness, such as the lactate or anaerobic threshold (LT/AT) and exercise efficiency/economy, may also be critical determinants of performance. For example, two athletes with the same \({\dot{\text{V}}}\)O 2max do not necessarily perform to the same level in an endurance performance test or race: the athlete with the higher \({\dot{\text{V}}}\)O 2AT is likely to perform better. Furthermore, the efficiency or economy with which work is done relative to energy expenditure may be important. For example, Lucia et al showed that a range of \({\dot{\text{V}}}\)O 2max levels amongst elite cyclists could be compensated for by differences in efficiency [36]. Whilst improvements to \({\dot{\text{V}}}\)O 2max are important, very few athletic competitions are performed at \({\dot{\text{V}}}\)O 2max and it cannot therefore be assumed that performance will be enhanced to the same degree as \({\dot{\text{V}}}\)O 2max increases. Intriguingly, the premise that improvement in physiological variables (i.e. aerobic capacity) enhances athletic performance (i.e. races or gold medals won) has not been well investigated. Having said that, the effects of blood manipulation on a range of physiological variables, including to \({\dot{\text{V}}}\)O 2max/peak and \({\dot{\text{V}}}\)O 2AT , are both of relevance for athletes and may have significance in clinical contexts [37].

What is ‘blood manipulation’/‘blood doping’?

The World Anti-Doping Agency (WADA) defines blood manipulation as the reintroduction of blood or blood products allogenic (homologous) or heterologous, the artificial enhancement of oxygen transportation or delivery and any form of intravascular manipulation of the blood or its components by physical or chemical means [3]. Blood doping is complex and rapidly evolving, as highlighted by the recent WADA amendments to the 2014 prohibited list consequent on the emergent use of Xenon and Argon as HIF activators. It was reported that Russian athletes used HIF activators at the 2014 winter Olympics in Sochi [3]. The earliest reports of ‘blood doping’ in the scientific literature date back to 1945–1947 [38, 39]. The first alleged use in elite sport was in the 1960s, when a French four times Tour de France winner (1961–1964) was named as one of the first cyclists to use the technique [40]. The first reported use in athletics comes from around the time of the 1968 Mexico City Olympic Games.

It has been said, “Increasing the oxygen transport capacity of the exercising skeletal muscles, either by means of training or doping, is the most powerful tool for improving athletic performance in aerobic sports [41]”. Below we will briefly outline some of the methods of blood manipulation used in sport.

Autologous blood transfusion

The link between the O 2 -carrying capacity of the blood and indices of exercise capacity such as \({\dot{\text{V}}}\)O 2max has recently been reviewed elsewhere [34]. Haematocrit (Hct) is also known as packed cell volume (PCV) or erythrocyte volume (ECV) and is the volume percentage of red blood cells within the blood. There does not appear to be a simple linear correlation between haematocrit and increased \({\dot{\text{V}}}\)O 2max . Brun et al showed that a “low” haematocrit (Hct) (<40 %) was associated with a higher aerobic capacity [42]. However, this must be interpreted with caution, as the lowest Hct was only 36.8 % (i.e. not actually that low). It is probable that lower Hct levels, such as those seen in patients rather than athletes or healthy volunteers, would result in a reduced oxygen carrying capacity and therefore reduced \({\dot{\text{V}}}\)O 2max . By the 1970s, it was becoming well known that increasing tHb-mass could increase \({\dot{\text{V}}}\)O 2max . It later became clear that other factors were also important, for example, changes in diastolic function and changes in blood volume (BV) [43].

A 1982 review documented all published studies comparing exercise testing variables pre-phlebotomy, and post transfusion, at that time. It is apparent from Table 2 that a significant increase in [Hb] was associated with an increase in \({\dot{\text{V}}}\)O 2max . The author concluded that at least 2 units of blood were needed with frozen blood being superior to refrigerated blood [44]. Of the 14 studies in Table 2, only 5 of them showed statistically significant improvements in [Hb] and \({\dot{\text{V}}}\)O 2max post autologous transfusion [39, 45–48]. The results of studies failing to find such a relationship between [Hb] and exercise capacity may in part be explained by the small quantity of blood re-infused, insufficient time for the body to achieve equilibrium [Hb] after venesection, and inadequate storage of the RBCs [44].

Table 2 Summary of studies of blood doping and exercise Full size table

In general, autologous blood transfusion seems to improve performance, but there are very few studies addressing this question directly. Improved 5-mile treadmill run times (mean improvement of 44 s) with reduced self-reported perceived exertion after autologous blood transfusion were demonstrated by Williams et al [47]. Berglund et al demonstrated a significant fall in the race times of cross-country skiers when compared to matched controls pre- and post autologous blood transfusion [49]. Brien et al took 6 well-trained runners and improved their 10 km time by an average of 1 min. Using a double-blind cross-over design, each runner received a 400 ml autologous transfusion of blood or saline repeated again 5 days apart with a 10 km race 5 days after each treatment. Five out of the 6 runners had faster race times after transfusion [50].

Recombinant human erythropoietin: rHuEPO

There are more data available for rHuEPO and a number of studies have shown correlation between improved performance and rHuEPO use. In 1991, Ekblom et al showed an improved \({\dot{\text{V}}}\)O 2max post rHuEPO injection in 15 volunteers [51]. Similar results were shown by Audran et al. Table 2 from this paper shows the increase in Hct and [Hb] from day 0 to day 24 and subsequent rise in \({\dot{\text{V}}}\)O 2max with reduction in maximum heart rate [52]. Parisotto et al attempted to develop a blood profile to detect athletes who were abusing rHuEPO and were able to demonstrate a predictable blood profile post rHuEPO usage. They measured tHb-mass (using Burge and Skinner’s method) and found a consistent increase in Hct, [Hb] and tHb-mass 3 weeks after rHuEPO administration, which persisted for 21 days. They also found a 6.3 and 6.9 % increase in \({\dot{\text{V}}}\)O 2max compared to placebo. After a 4-week washout period, tHb-mass and \({\dot{\text{V}}}\)O 2max had returned to baseline [53]. Birkeland et al showed in a double-blind placebo-controlled trial that injection of 5000 IU of rHuEPO thrice weekly for 4 weeks improved \({\dot{\text{V}}}\)O 2max by 7 %. They found that Hct rose from a mean of 42.7–50.8 and peaked 1 day after rHuEPO was stopped. Haemoglobin concentration also increased in the rHuEPO group [54].

However, data supporting an improvement in performance following rHuEPO usage in athletes were still limited. Russell et al were the first to characterise the submaximal and maximal exercise adaptations to prolonged use of low dose rHuEPO. They compared 3 groups, (1) intravenous (i.v.) iron + rHuEPO, (2) oral iron + rHuEPO and (3) placebo. They performed exercise tests on a cycle ergometer at weeks 0, 4, 8 and 12. The relative increases in \({\dot{\text{V}}}\)O 2max at weeks 4, 8 and 12 were 7.7, 9.7 and 4.5 %, respectively, for the rHuEPO + i.v. iron group; 6.0, 4.7 and 3.1 % for the oral iron + rHuEPO group; and −0.5, −0.1 and −1.0 % for the placebo group [55].

In 2007, Thomsen et al stated that “Although the positive effect of rHuEPO treatment on \({\dot{\text{V}}}\)O 2max is clearly established, it remains unknown as to what its impact is on endurance performance”. They investigated the effect of rHuEPO on \({\dot{\text{V}}}\)O 2max and time to exhaustion during cycle ergometry in healthy volunteers. rHuEPO significantly increased \({\dot{\text{V}}}\)O 2max by 9.1 and 8.1 % in week 4 and 11, respectively, with no changes in the placebo group [37].

Emerging strategies

The range of interventions aimed at increasing tHb-mass, both in development and currently available, is large and has been extensively reviewed elsewhere [2, 4, 6, 56].

Towards the end of the 1990s, interest had grown within clinical medicine and the sporting world in using artificial oxygen carriers and perfluorocarbon emulsions [57]. However, neither has been adopted in either setting, probably due to well-recognised adverse effects and ease of detection [58].

HIF stabilisers/activators are compounds that act by mimicking hypoxia and thereby stimulating EPO synthesis via the HIF pathway. When the partial pressure of oxygen is low, HIF1α undergoes a stabilisation process, which leads to gene transcription, including that of the erythropoietin gene [59, 60]. HIF stabilisers/activators are oral compounds, which are potentially advantageous as they are simply administered and are less immunogenic than erythrocyte-stimulating agents such as rHuEPO. The number of agents available is beyond the scope of this review and has recently been summarised elsewhere [2]. There is already clear evidence of their abuse within elite sport [3–5, 56]. Cobalt is one such agent, which has adverse effects including heart, liver, kidney and thyroid toxicity as well as cancer promotion [61]. There is evidence of cobalt being abused in horse racing [62]. Xenon and argon are also both HIF activators and have both been reportedly used as performance-enhancing agents in the recent years [3, 63].

Gene therapy is also theoretically possible, but some early reports highlighting significant safety concerns including life-threatening red cell aplasia and extreme erythrocytosis have probably limited its use [64]. There are also EPO-mimetic peptides such as Peginesatide that are not currently in production but are nevertheless candidates for abuse [2].

Harms: what are the downsides?

Not only is blood manipulation/doping illegal, but also many of the agents used may pose health risks to the athlete. Despite this, some athletes are apparently prepared to accept such risks to increase their chances of success. As has already been noted, manipulation of haemoglobin may be associated with a variety of adverse effects including, for example, hyper-viscosity from rHuEPO and the toxic effect of cobalt. The risks associated with blood transfusion are summarised in Table 3.