The serum concentration of creatine kinase (CK) is used widely as index of skeletal muscle fiber damage in sport and exercise. Since athletes have higher CK values than nonathletes, comparing the values of athletes to the normal values established in nonathletes is pointless. The purpose of this study was to introduce reference intervals for CK in athletes. Creatine kinase was assayed in serum samples from 483 male athletes and 245 female athletes, aged 7-44. Samples had been obtained throughout the training and competition period. For comparison, CK was also assayed in a smaller number of nonathletes. Reference intervals (2.5th to 97.5th percentile) were calculated by nonparametric method. The reference intervals were 82-1083 U/L (37 °C) in male and 47-513 U/L in female athletes. The upper reference limits were two times the limits reported for moderately active nonathletes in the literature or calculated in the nonathletes of this study. The difference from the limits reported for inactive individuals in the literature was up to sixfold. When reference intervals were calculated specifically in male football (soccer) players and swimmers, a threefold difference in the upper reference limit was found (1492 vs. 523 U/L, respectively), probably resulting from the different training and competition demands of the two sports. In conclusion, sport training and competition have profound effects on the reference intervals for serum CK. Introducing sport-specific reference intervals may help to avoid misinterpretation of high values and optimize training.
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