Reticulocyte counts in sports medicine

Reticulocytes are juvenile red blood cells (RBCs) containing remnant ribonucleic acid (RNA). Their percentage in the peripheral blood (PB) is a useful indication of erythropoiesis in the bone marrow. In the context of sport, the reticulocyte count and other complete blood count (CBC) parameters can provide useful information about the potential for athlete performance for trainers and sports medicine clinicians. In order to obtain reliable measurements, pre-analytical variables have to be controlled, including the timing of blood collection and the standardisation of phlebotomy procedures. Although the manual reticulocyte count with supravital staining is recommended as the reference method, the absolute and percentage reticulocytes in whole blood are today better analysed using modern haematology cell counters. The comparability of the results from the manual and automated methods remains in doubt mostly due to the lack of suitable calibration and control bloods for both methods. Mild haemolysis and increased oxygen demand are associated with exercise, especially during endurance training. The production of erythropoietin is enhanced in this setting inducing erythropoiesis resulting in an elevated PB reticulocyte count. Athlete training at high altitudes produces a physiological PB hypoxic effect in an attempt to maximise oxygen carrying capacity and performance. Studies into the reticulocyte response in athletes show that minimal intensity and duration of training will initiate the erythropoietic response to increase PB reticulocyte numbers. Efforts to raise athlete performance by increasing the number of circulating RBCs have led to the banning of autologous and/or homologous blood transfusions and the use of recombinant human erythropoietin (rHuEpo). The illegal use of these methods to boost performance is a major focus for some athletes and a major challenge for anti-doping agencies trying to keep sport clean from the use of banned performance enhancers. As part of the screening for the use of performance boosting activities, the OFF score is used for the detection of recent usage of rHuEpo and is based on the PB reticulocyte percentage. More research and studies are still necessary to standardise reticulocyte measurement, particularly for its application in blood doping in sports.