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Introduction: Dehydration can have an immediate negative impact on the performance of Soldiers in training or combat environments. Field expedient methods for assessing hydration status may be valuable for service members. Measurement of urine-specific gravity (USG) via refractometer is inexpensive, simple, fast, and a validated indicator of hydration status. Manual (MAN) and digital (DIG) refractometers are commonly used in laboratory settings however, digital (DIG) devices have not been validated in the field against MAN devices. The purpose of this study was to determine the validity and feasibility of using a DIG refractometer to assess USG compared to a MAN refractometer during a military field training exercise., Materials and Methods: Fifty-six military service members provided 672 urine samples during two 10-day field training exercises in central Texas. USG was assessed using a MAN and a DIG refractometer with cutoff value of ≥1.020 indicating hypohydration. The study received a non-human research determination., Results: The MAN measurements were strongly correlated with the DIG (r = 0.91, p < 0.0001) measurements. Bland-Altman analysis demonstrated agreement between the refractometers. The DIG displayed good sensitivity (93.9%) and specificity (85.8%) compared to the MAN., Conclusion: The DIG refractometer used in this study was reliable and valid compared with a MAN device and was feasible for use in a field environment; however, the DIG refractometer tended to over overestimate hypohydration., (Published by Oxford University Press on behalf of the Association of Military Surgeons of the United States 2019. This work is written by (a) US Government employee(s) and is in the public domain in the US.)

We developed and evaluated the properties of in-house urine reference materials for the verification of laboratory refractometers, which are frequently used in clinical chemistry and doping testing laboratories. Urine was gathered from 26 healthy volunteers (16 male 30 ± 5 years old and 10 female 29 ± 4 years old), from which two urine batches were obtained: one with a low specific gravity (1.012± 0.003) and the other with a high specific gravity (1.027 ± 0.003). Homogeneity studies were conducted over 20 consecutive days. For short-term stability studies, aliquots of both urine batches were stored at -20 ± 2°C; 3 ± 2°C; 20 ± 2°C; 45 ± 2°C for 0, 2, 7, 14 and 35 days, under both light and dark conditions. Similarly, another study was conducted to measure the long-term stability of urine at -20 ± 2°C, over a 24-month evaluation period. Our data showed that the urine was homogeneous and stable at -20 ± 2°C, 3 ± 2°C, 20 ± 2°C, and 45 ± 2°C under both light and dark conditions. In all cases, the urine was evaluated by specific gravity and no statistically significant differences (p ≤ 0.05) were recorded. Additionally, a proficiency test was conducted in collaboration with 15 ISO/IEC 17025 accredited laboratories, and z-scores and performance factors were evaluated. These data indicate that this material could be used for the verification of refractometers., (© 2019 John Wiley & Sons, Ltd.)

Context: Urine specific gravity (Usg), measured by a handheld manual refractometer (MAN), has been recognized as a valid and practical means of assessing hydration status. Newer, digital refractometers are faster and more user friendly but have not been validated against the traditional MAN., Objective: To compare the reliability and validity of 2 digital refractometer models and a MAN., Design: Descriptive laboratory study., Setting: Research laboratory., Patients or Other Participants: Sample of convenience was recruited from the local university and surrounding community (n = 82)., Intervention(s): Participants provided multiple urine samples (n = 124) over a 5-month period under various hydration conditions., Main Outcome Measure(s): Urine specific gravity was compared among a MAN, a digital refractometer requiring the prism to be dipped (DIP) into a urine sample, and a digital refractometer that requires urine to be pipetted (PIP) onto its prism for analysis., Results: The MAN measurements were strongly correlated with the DIP (r = 0.99, P < .001) and PIP (r = 0.97, P < .001) measurements. Bland-Altman analyses revealed slight mean underestimation (95% upper and lower levels of agreement) between MAN and DIP (-0.0012 [0.0028] and PIP -0.0011 [0.0035], respectively) and trends toward increased underestimation at higher Usg. Measurement error ≥ .005 was greater for PIP (4/124, 3.2%) than for DIP (2/124, 1.6%)., Conclusions: Negligible differences were exhibited between PIP and DIP, with both displaying acceptable reliability and validity compared with the MAN. However, the Bland-Altman analysis suggests underestimation bias for the DIP and PIP as Usg increases, with the potential for rare but substantial underestimation when using PIP that should be recognized by clinicians, particularly when used as a screening measure in weight-class sports.

The objective of this study was to evaluate the quality of the measurement of dog urine dilution/concentration by comparing osmolality with three methods of specific gravity (USG) measurement, i.e. weighing, refractometry and test strips. In unselected urine samples from 182 dogs there was a better agreement between osmolality and USG determination by refractometry (r = 0.92) than by weighing (r = 0.82) or by test strips (r = 0.27). There was an almost linear relationship between osmolality and USG: osmolality (mOsm/kg) = 36646(34318/38974) x (USGref - 1) + 25(-39/88); calculated osmolality differed from measured osmolality by more than 500 mOsm/kg in only 8 of 181 samples. There was a good agreement between USG determination by weighing and refractometry: USGref = 1.000(0.905/1.095) x USGweighing - 0.0004(-0.0019/0.0027), with a moderate bias. Only 12% of the differences between the two methods exceeded 0.010. Test strip assessment of USG was unreliable because of systematic underestimation and should not be used for dog urine. Refractometry is the best technique for routine evaluation of urine concentration/dilution when osmometry is not available.

Two methods for the determination of urine density (relative density or specific mass) have been evaluated and compared using NCCLS protocols: the refractometric method and a new method based on the measure of shifts in harmonic oscillation (SHO). Within the range investigated (from 1,000 up to 1,034) the methods were linear. Total imprecision, including the carry-over, was the same for both: the 95% confidence limits, at the level of 1,003, 1,013 and 1,030, were 1,001-1,005, 1,011-1,015 and 1,028-1,032 respectively. In analysing urines having known density, the SHO method was always accurate, while the refractometric method was affected by a significant bias at protein and haemoglobin concentrations greater than 4 and 2 g/L respectively. A direct comparison of the results obtained by the two methods was made on 300 untimed specimens of urine with density ranging from 1,002 to 1,034; the equation of regression line (refractometric method on the x axis, SHO on the y axis), calculated with a non-parametric statistical method, was SHO = refractometric method + 0,001. Despite some urine specimens (24, i.e. 8%) showed density differences (absolute value) of more than 0,003 (between 0,004 and 0,007 relative density units), the good mean agreement observed allows to conclude that both methods give clinically useful analytical results.

A comparison of urine specific gravity by a commercially available multiple reagent strip (Multistix-SG; Ames Division, Miles Laboratory) versus refractometer specific gravity (TS Meter; American Optical Corporation) was performed on 214 routine urine specimens. Agreement to +/- 0.005 was found in 72% of the specimens (r = 0.80). Urine specific gravity by the Multistix-SG showed a significant positive bias at urine pHs less than or equal to 6.0 and a negative bias at urine pHs greater than 7.0 in comparison to refractometer specific gravity. At concentrated (specific gravity greater than or equal to 1.020) specific gravities, up to 25% of urine specimens were misclassified as not concentrated by Multistix-SG specific gravity in comparison to refractometer specific gravity. The additional cost of the specific gravity reagent to a multiple reagent test strip in addition to the poor performance relative to refractometer specific gravity leads to the conclusion that including this specific gravity methodology on a multiple reagent strip is neither cost effective nor clinically useful.