Every undergraduate and many lay persons understand that packed cell volume (PCV l / l or %) rises in dehydration and shock and falls in (extreme) haemorrhage. PCV is however, probably the least useful RBC measure in the elite athlete. It is a labile parameter influenced by splenic contraction following exercise or excitement and blood biochemistry is a better means of evaluating hydration status. Identification of the anaemia that is associated with chronic infection is the entity that is of most importance in evaluating RBC criteria in the elite equine athlete.  From a practical point of view, anaemia can be suspected where haemoglobin (Hb g / l) values are < 12 for thoroughbreds and 11 for sport horses. Care must be exercised here however, as older thoroughbreds e.g. mature National Hunt / jump racing horses can be so relaxed on sampling, that their Hb values can be misleadingly low. Equine erythrocytes do not follow the patterns associated with regeneration in other species. Anisocytosis and reticulocytosis are only seen in the most extreme and long-standing anaemia. Serial evaluation of RBC, Hb and total protein are probably the best way to follow a post-anaemic erythropoietic response.