Serum creatinine concentration and creatinine-based estimation of glomerular filtration rate in athletes

Since serum creatinine concentration test results in athletes can sometimes be abnormal, correct test interpretation needs to take account of the sportsman's status, as well as the transient exercise-induced changes in creatinine and the marked differences in creatinine levels among athletes competing in different sport disciplines. Serum creatinine concentration in athletes is related to body mass index, so the use of general population reference ranges should not be recommended in sports medicine. This does not necessarily imply, however, that specific creatinine reference ranges for athletes need to be defined. The individuality index (the ratio between intra- and interindividual variability) for creatinine is about half (0.33 vs 0.60) the generally considered lower limit for a population-based reference interval to be classified as useful. Prediction of glomerular filtration rate in athletes by means of creatinine-based equations is also questionable because of discrepancies among formulae, owing to the particular anthropometric characteristics of athletes (high body mass index). Furthermore, differences in muscle mass need to be entered into the recommended equations for calculating the estimated glomerular filtration rate (eGFR) in athletes. Based on current knowledge about creatinine measurement and eGFR calculation, we suggest that athletes be monitored periodically by consecutive creatinine level assessments, comparing the values reported during the training and competition season with the baseline levels recorded during the recovery period.