Clinical medicine defines dehydration using blood markers that confirm hypertonicity (serum sodium concentration ([Na+])>145 mmol/L) and intracellular dehydration. Sports medicine equates dehydration with a concentrated urine as defined by any urine osmolality (UOsm) ≥700 mOsmol/kgH2O or urine specific gravity (USG) ≥1.020.
Objective To compare blood versus urine indices of dehydration in a cohort of athletes undergoing routine screenings.
Methods 318 collegiate athletes (193 female) provided blood and urine samples and asked to rate how thirsty they were on a 10-point visual analogue scale. Serum was analysed for [Na+], while serum and UOsm were measured using an osmometer. USG was measured using a Chemstrip. Data were categorised into dehydrated versus hydrated groupings based on these UOsm and USG thresholds.
Results Using UOsm ≥700 mOsmol/kgH2O to define dehydration, 55% of athletes were classified as dehydrated. Using any USG ≥1.020 to define dehydration, 27% of these same athletes were classified as dehydrated. No athlete met the clinical definition for dehydration (hypertonicity; serum [Na+]>145 mmol/L). Normonatremia (serum [Na+] between 135 mmol/L and 145 mmol/L) was maintained in 99.7% of athletes despite wide variation in UOsm (110–1298 mOsmol/kgH2O). A significant correlation was confirmed between serum [Na+] versus UOsm (r=0.18; P<0.01), although urine concentration extremes did not reflect derangement in serum markers or thirst rating.
Conclusion Urine concentration thresholds classified 27%–55% of collegiate athletes as dehydrated, while no athlete was dehydrated according to blood [Na+] measurement. Practitioners should caution against using urine indices to diagnose or monitor dehydration, because urinary output is a response rather than a reflection of (tightly regulated) blood tonicity.
- fluid balance
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Contributors TDH-B participated in study design, data acquisition and analyses and drafting of the manuscript. CE participated in data acquisition, analyses and drafting of the manuscript. JB, MR and MVM participated in data acquisition. All authors participated in revising the manuscript, and all authors gave final approval on the paper.
Funding This study was supported by Oakland University’s Provost’s undergraduate (MVM) and graduate (MR) awards and Prevention Research Center award (TDH-B).
Competing interests None declared.
Ethics approval Michigan, USA.
Provenance and peer review Not commissioned; externally peer reviewed.