Creatinine Normalization Over-Corrects Urinary Electrolytes but Not Protein: Implications for Hydration Monitoring in...

Background: Creatinine normalization is the default adjustment for urine dilution in spot-urine monitoring of athletes, yet its appropriateness is seldom tested in field data. We tested whether it removes or introduces concentration dependence across a panel of urinary analytes in soccer players training in extreme heat and whether raw versus normalized values change the inferred response. Methods: Sixteen male soccer players (age 19.9 ± 0.8 y) provided spot urine at three daily time points (morning, pre-training, post-training) over six training days in Riyadh, Saudi Arabia. Sodium, potassium, chloride, calcium, magnesium, glucose, urea nitrogen, uric acid, total protein, and creatinine were measured. For each analyte we quantified its scaling exponent with creatinine, the correlation of raw and normalized values with creatinine, and the within-day pattern on raw versus normalized values using linear mixed-effects models. Results: Urinary creatinine varied 22.7-fold (coefficient of variation 39%). Only protein and glucose scaled proportionally to creatinine (exponents ≈1.0) and normalized cleanly (ratio-creatinine r = -0.06 and -0.21). The remaining seven analytes, including all electrolytes, scaled weakly (exponents 0.16-0.49), so normalization over-corrected them, producing strong negative ratio-creatinine correlations (uric acid -0.74, sodium -0.59, chloride -0.55). For four of nine a the inferred within-day response differed in direction or significance between raw and normalized values; restricting creatinine to the recommended 0.3-3 g·L^-1 range attenuated but did not abolish the over-correction. Conclusions: Creatinine normalization suited protein and glucose but over-corrected every electrolyte and could reverse inferred responses. In this setting of hot-environment team-sport athletes, creatinine normalization should not be assumed adequate for electrolytes, which instead warrant interpretation against an independent index of urine concentration such as osmolality or specific gravity.