Urine specific gravity (Usg) is widely used to detect hypohydration in athletes. However, often athletes are incorrectly classified as hypohydrated by Usg > 1.020 g·mL-1 while their plasma osmolality (Posm) is < 290 mOsml·kg-1 (i.e., false positive). Out of the 4% solutes in urine, 60% are protein metabolites and 40% are electrolytes. PURPOSE: To determine if differences in protein metabolites in urine contribute to the high incidence of false positives. METHODS: We measure first morning urine Usg, body weight (BW) and collected blood in nine rugbiers and nine runners (body mass: 90 ± 12 vs. 61 ± 5 kg; muscle mass: 42 ± 6 vs. 32 ± 3 kg; VO2max: 51 ± 6 vs. 66 ± 8 mL-1·kg-1·min-1; respectively; p < 0.05) during six consecutive days. The last training bout, meal and hydration were standardized. We analyzed blood serum and urine for osmolality (Sosm, Uosm), urine electrolytes (U[Na+], U[K+], U[Cl-]) and urine protein metabolites (U[Creatinine], U[Urea], U[Uric acid]). RESULTS: During the 6 days, BW and Usg were remarkably constant (ICC of 0.99 and 0.80 vs. 0.99 and 0.76 for rugbiers vs. runners, respectively). Pearson's coefficients between Usg and U[Creatinine], U[Urea], U[Uric acid], were 0.94, 0.91, 0.89, while were lower for U[K+], U[Cl-] and U[Na+] (0.80, 0.60 and 0.43, respectively). Table 1 shows the six days mean ± SD of urinary and blood measurements and false positives. CONCLUSIONS: Despite all subjects were euhydrated by Sosm (< 290 mOsml·kg-1), Usg was higher for rugbiers than runners due to higher urea and uric acid concentration. The larger incidence of false positives (low specificity) seems to be influenced by protein metabolites in urine which was higher in the athletes with the largest muscle mass (i.e., rugbiers). Supported by GSSI