Creatine-kinase reference intervals at rest and after maximal exercise in Standardbred racehorses

Due to the high variability of data drawn from the literature, aim of the study was to evaluate the effects of exercise on creatine-kinase (CK) serum activity and to determine CK reference intervals (RIs) at rest and post-exercise in healthy Standardbred racehorses. Data concerning history, physical examination, laboratory evaluation and ECG were collected retrospectively from a population of 258 Standardbred racehorses in training that underwent an incremental-maximal treadmill exercise. Those subjects with alterations potentially influencing CK serum activity were excluded. Finally, a reference sample of 194 horses was selected. Blood samples were collected 1 hour before exercise and 6 hour post-exercise and analysed with a spectrophotometric method. Values were compared by Wilcoxon test for paired samples. The effect of age and sex was evaluated by Kruskal-Wallis and Dunn post-test. Statistical significance was set at P<0.05. RIs were determined following Clinical and Laboratory Standard Institute guidelines (CLSI), approved by the American Society for Veterinary Clinical Pathology. Using a macroinstruction set for Microsoft Excel (RefValAdv), RIs were determined with a non-parametric method. A significant increase (P<0.0001) in CK activity post-exercise was observed. Partition by sex and age did not show any statistical difference, either at rest or post-exercise. In RIs determination no outliers were identified. RIs ranged from 25 to 394 U/l at rest and from 44 to 735 U/l post-exercise. To our knowledge, this is the first study considering CK post-exercise RIs in racehorses using CLSI's guidelines and specific CK-related exclusion criteria. These RIs could be useful to discriminate between physiological and pathological CK post-exercise increase.