Comparison Of Different METHODS For Urine Hydration Assessment

Abstract

Several instruments and techniques to assess hydration via urine exist; but which is used for evaluation depends on diagnostic accuracy, reliability, cost, and convenience. Urine specific gravity assessed via refractometry is a common field technique for urine assessment; however, previous comparisons of different refractometers have provided contradicting reports of bias. PURPOSE: To identify agreement between different refractometers (manual and digital) to measure urine specific gravity, as well as assess the diagnostic accuracy of urine specific gravity compared to osmolality. METHODS: Free-living participants (n = 39, 27 males, 12 females, age 25 ± 5 y, ht 1.76 ± 0.09 m, wt 81.1 ± 15.2 kg, BMI 26.2 ± 4.1) volunteered to provide a spot urine sample. Each sample was evaluated in duplicate with an osmometer (Osmo1 Single-Sample Micro-Osmometer), manual refractometer (Atago MASTER-SUR/Nα), and digital refractometer (Atago PAL-10S). Bland-Altman analysis was performed to assess agreement between manual and digital refractometers. Diagnostic accuracy of refractometers to identify concentrated samples (≥1.025) was conducted using receiver operating characteristic curves with osmolality (≥850 mOsm/kg) as the standard. RESULTS: Samples evaluated with the digital (1.018 ± 0.008) and manual (1.018 ± 0.007) refractometers were highly correlated (r = 0.998, P < 0.001). Bland-Altman analysis demonstrated high agreement between manual and digital refractometers (mean difference: 0.0002 ± 0.0004). Compared with osmolality, the digital refractometer identified concentrated samples with an area-under-the curve (AUC) of 0.86, sensitivity of 0.71, and specificity of 1.00. The manual refractometer identified concentrated samples with an AUC of 0.89, sensitivity of 0.79, and specificity of 1.00. CONLUSION: This study demonstrated a high agreement between manual and digital refractometers, suggesting either instrument would be acceptable for use. When compared to osmolality, both instruments provided excellent specificity for identifying diluted samples, but moderate sensitivity for identifying concentrated samples.