Serum Creatinine Values in Elite Athletes Competing in 8 Different Sports: Comparison with Sedentary People

The concentration of creatinine in serum is the most widely used and commonly accepted measure of renal function in clinical medicine. Reference values of biochemical variables specific for sportsmen have never been defined, and those used for the general population are also applied to athletes. The common reference interval for creatinine in the general population corresponds to 62–115 μmol/L (0.7–1.3 mg/dL) for adult males (1). In our experience with athletes, we frequently observed high creatinine values, near or higher than 115 μmol/L (1.3 mg/dL). We studied 220 elite athletes: 15 triathletes of the Italian National Team, 29 basketball players of an Italian First Division team, 35 cyclists from 2 professional teams, 13 racing motorcyclists of a professional team, 27 soccer players of an Italian First Division team, 23 sailors of an America’s Cup yacht, 34 alpine skiers of the Italian National Team, and 44 rugby players of the Italian National Team. The athletes were all males, and the age range was 17–36 years. Serum was always analyzed within 5 h from blood drawing on an Aeroset c8000 (Abbott) by an alkaline picrate reaction method. Blood samples were obtained before the start of training for the competitive season, when athletes were not undergoing specific training loads or psycho-physical stresses. We selected age-matched sedentary, nonobese, apparently healthy males, without biochemical and hematologic signs of diseases, as controls (n = 100). We found mean (SD) creatinine values of 97 (18) μmol/L [1.1 (0.2) mg/dL] in the athletes and 88 (9) μmol/L [1.0 (0.1) mg/dL] in controls. The frequency distributions of serum creatinine in the 2 groups are shown in Fig. 11 . The mean (SD) values in the different sports groups were as follows: 88 (9) μmol/L [1.0 (0.1) mg/dL] in triathletes, 97 (9) μmol/L [1.1 (0.1) mg/dL] in basketball players, 80 (9) μmol/L [0.9 (0.1) mg/dL] in cyclists, 80 (9) μmol/L [0.9 (0.1) mg/dL] in motorcyclists, 115 (9) μmol/L [1.3 (0.1) mg/dL] in soccer players, 97 (9) μmol/L [1.1 (0.1) mg/dL] in sailors, 106 (9) μmol/L [1.2 (0.1) mg/dL] in skiers, and 115 (9) μmol/L [1.3 (0.1) mg/dL] in rugby players. There were significant differences (Student t-test, P <0.01) between each group of athletes in the different sports and the control group. Our data seem to be discordant with those reported for endurance athletes (Nordic skiers and cyclists), who showed creatinine values lower than controls: 71–88 μmol/L [0.8–1.0 mg/dL] for skiers and 62–80 μmol/L [0.7–0.9 mg/dL] for cyclists. However, these values were measured during periods of competition (2). To our knowledge, no other study has been published describing creatinine values in elite athletes at rest. In general, creatinine is not influenced by training and competition (3), even during extreme effort (4). We found that the creatinine values in elite athletes were generally higher than in controls, as expected in part because of their higher muscle mass. The athletes competed in 8 different sports with different characteristics of aerobic/anaerobic metabolism, different training loads and frequency of competitions, different lengths of competitions, and different periods of training and competitions. The athletes’ diets were monitored and controlled by team physicians; no creatinine supplementation was administered to the athletes. The use of reference intervals based on general populations is not recommended in sports medicine, to avoid misinterpretation of data and additional, unnecessary investigations. This does not necessarily imply that specific reference intervals should be calculated for the creatinine concentrations of athletes. The individuality index (ratio between intra- and interindividual variability) of creatinine is 0.33, lower than the value of 0.60, which is universally considered the threshold for classifying a reference interval as useful in a population. We recommend that for each athlete, consecutive creatinine assessments be monitored, with one of the values determined before the start of training and competitions used as the basal value. Frequency distribution of serum creatinine concentrations in the 2 groups. Data are divided considering as threshold the median value of the control group [88 μmol/L (1.0 mg/dL)].