The Effect of Instrument Type on the Measure of Hydration Status

Context: Although some instruments have been validated for clinical measure of hydration status, new and currently invalid instruments are available for purchase and clinical use. Athletic trainers commonly use these instruments to assess hydration status for weight checks and body mass loss charts due to their ease of use. However, the validity of these popular instruments has not yet been established. Objective: To determine the validity of urine specific gravity (USG) for the assessment of hydration status via the following instruments: handheld clinical refractometer, pen style digital refractometer, and midget urinometer as compared to the gold standard urine osmometer (OSMO). Design: Descriptive diagnostic validity study. Setting: Biochemical research laboratory. Patients or Other Participants: Healthy active men and women (n=108; mean age=22+/-4yrs; self reported height=174+/-20cm and mass=75+/-17kg) were recruited among faculty and students on a university campus. Interventions: The independent variable was instrument type with four levels: osmometer, handheld clinical refractometer, pen style digital refractometer, and midget urinometer. After recruitment, participants completed an informed consent and a short health history questionnaire to rule out any exclusionary criteria such as kidney disease or chronic urinary tract infection. Participants were then given a clean standard urine cup and asked to provide as much sample as possible, providing more than one cup when possible. Main Outcome Measures: Hydration status was measured by USG and OSM. USG was evaluated by a handheld clinical refractometer, pen style digital refractometer, and midget urinometer. The gold standard OSM was calculated by a freezing point depression osmometer. Z scores were calculated for each instrument and Pearson product-moment correlation coefficients were evaluated to examine the relationship between each instrument of USG and OSM. Results: Strong significant correlations were identified for the digital refractometer (r=0.814, p< 0.001) and handheld clinical refractometer (r=0.943, p< 0.001) with OSM. A weak statistically insignificant correlation was established between the midget urinometer (r=0.133, p< 0.142) and OSM. Average hydration status indicated variability among some of the instruments: digital refractometer USG=1.0194+/-0.0075, clinical refractometer USG=1.020+/-0.007, urinometer USG=1.028+/-0.091, osmometer OSM=743+/-271) Conclusions: Handheld clinical refractometry can be used confidently for assessing hydration status as it shows a strong significant correlation with the gold standard osmometer, which is consistent with previous literature. Additionally, the use of the pen style digital refractometer showed a strong, significant correlation with the gold standard osmometer and provides clinicians with another option for the clinical assessment of USG and hydration status. The findings of this also study suggest that the use of a midget urinometer should be performed with extreme caution, as it showed a weak correlation with the gold standard osmometer, indicating it might not provide accurate results when used to determine hydration status.