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Background: Small sample volumes may artificially elevate plasma osmolality (Posm) measured by freezing point depression. The purpose of this study was to compare two widely different sample volumes of measured Posm (mmol/kg) to each other, and to calculated osmolarity (mmol/L), across a physiological Posm range (~50 mmol/kg)., Methods: Posm was measured using freezing point depression and osmolarity calculated from measures of sodium, glucose, and blood urea nitrogen. The influence of sample volume was investigated by comparing 20 and 250 μL Posm samples (n = 126 pairs). Thirty-two volunteers were tested multiple times while EUH (n = 115) or DEH (n = 11) by -4.0% body mass. Protinol™ (240, 280, and 320 mmol/kg) and Clinitrol™ (290 mmol/kg) reference solutions were compared similarly (n = 282 pairs)., Results: The 20 μL samples of plasma showed a 7 mmol/kg positive bias compared to 250 μL samples and displayed a nearly constant proportional error across the range tested (slope = 0.929). Calculated osmolarity was lower than 20 μL Posm by the same negative bias (-6.9 mmol/kg) but not different from 250 μL Posm (0.1 mmol/kg). The differences between 20 and 250 μL samples of Protinol™ were significantly higher than Clinitrol™., Conclusions: These results demonstrate that Posm measured by freezing point depression will be ~7 mmol/kg higher when using 20 μL vs 250 μL sample volumes. Approximately half of this effect may be due to plasma proteins. Posm sample volume should be carefully considered when calculating the osmole gap or assessing hydration status., (© 2018 The Authors Journal of Clinical Laboratory Analysis Published by Wiley Periodicals, Inc.)

Background: The efficacy of different commercial beverage compositions for meeting oral rehydration therapy (ORT) goals in the treatment of acute dehydration in healthy humans has not been systematically tested. The objective of the study was to compare fluid retention, plasma volume (PV), and interstitial fluid (ISF) volume restoration when using 1 popular glucose-based and 1 novel amino acid-based (AA) commercial ORT beverage following experimental hypertonic or isotonic dehydration., Methods: Twenty-six healthy adults (21 males, 5 females) underwent either a controlled bout of hypertonic (n = 13) or isotonic (n = 13) dehydration (3%-4% body mass) via eccrine or renal body water and electrolyte losses induced using exercise-heat stress (EHS) or Lasix administration (LAS), respectively. Rehydration was achieved over 90 minutes by matching fluid intake to water losses (1:1) using a sports drink (SP) or AA commercial ORT beverage. Fluid retention (water and electrolytes), PV, and ISF volume changes were tracked for 180 minutes., Results: AA produced significantly (P <0.05) greater fluid retention (75% vs 57%), ISF volume restoration, and tended (P = 0.06) to produce greater PV restoration in trial EHS. In trial LAS, neither beverage exceeded 65% retention, but AA replaced electrolytes and preserved ISF volume better than SP (P <0.05)., Conclusion: The results of this study demonstrate superior rehydration when using AA compared with SP for both hypertonic and isotonic dehydration., (© 2018 American Society for Parenteral and Enteral Nutrition.)