Evaluation and analytical validation of a handheld digital refractometer for urine specific gravity measurement

Abstract

ObjectivesRefractometers are commonly used to determine urine specific gravity (SG) in the assessment of hydration status and urine specimen validity testing. Few comprehensive performance evaluations are available demonstrating refractometer capability from a clinical laboratory perspective. The objective of this study was therefore to conduct an analytical validation of a handheld digital refractometer used for human urine SG testing.Design and methodsA MISCO Palm Abbe™ refractometer was used for all experiments, including device familiarization, carryover, precision, accuracy, linearity, analytical sensitivity, evaluation of potential substances which contribute to SG (i.e. “interference”), and reference interval evaluation. A manual refractometer, urine osmometer, and a solute score (sum of urine chloride, creatinine, glucose, potassium, sodium, total protein, and urea nitrogen; all in mg/dL) were used as comparative methods for accuracy assessment.ResultsSignificant carryover was not observed. A wash step was still included as good laboratory practice. Low imprecision (%CV, <0.01) was demonstrated using low and high QC material. Accuracy studies showed strong correlation to manual refractometry. Linear correlation was also demonstrated between SG, osmolality, and solute score. Linearity of Palm Abbe performance was verified with observed error of ≤0.1%. Increases in SG were observed with increasing concentrations of albumin, creatinine, glucose, hemoglobin, sodium chloride, and urea. Transference of a previously published urine SG reference interval of 1.0020–1.0300 was validated.ConclusionsThe Palm Abbe digital refractometer was a fast, simple, and accurate way to measure urine SG. Analytical validity was confirmed by the present experiments.

Clinical utility of urine specific gravity, electrical conductivity, and color as on‐farm methods for evaluating urine concentration in dairy cattle

“…The strong association between U SG‐R and U Osm observed in this study ( r s = 0.85) was consistent with the findings of studies involving calf urine ( r = 0.72), dairy cow urine ( r = 0.92), human urine ( r = 0.83), and dog urine ( r = 0.93) . The slope of the U SG‐R ‐U Osm relationship was 0.000030, which approximated that for calf urine (0.000024) .…”

“…Mixed models regression of the relationship between U Creat and U Osm (reference method) for 217 urine samples obtained periodically from 20 periparturient multiparous Holstein-Friesian cows indicated a linear relationship ( Figure 6), such that U Creat = 0. The strong association between U SG-R and U Osm observed in this study (r s = 0.85) was consistent with the findings of studies involving calf urine (r = 0.72), 1 dairy cow urine (r = 0.92), 3 human urine (r = 0.83), 9 and dog urine (r = 0.93). 5 The slope of the U SG-R -U Osm relationship was 0.000030, which approximated that for calf urine (0.000024).…”

“…The gold standard method for measuring UC is urine osmolality (U Osm ), and the consensus definition for hypohydration in children and adults, where hypohydration represents a general deficit in body‐water content, is U Osm ≥800 mOsm/kg . Measuring U Osm using freezing point depression osmometry is expensive, and a point‐of‐care (POC) device for measuring osmolality is not currently available . Urine specific gravity is therefore commonly used as a screening method for evaluating UC because specific gravity can be measured accurately using low‐cost handheld analyzers such as a refractometer …”

“…[6][7][8] Measuring U Osm using freezing point depression osmometry is expensive, and a point-of-care (POC) device for measuring osmolality is not currently available. 9 Urine specific gravity is therefore commonly used as a screening method for evaluating UC because specific gravity can be measured accurately using low-cost handheld analyzers such as a refractometer. 1,4,10 Urine specific gravity is a measure of the density of urine relative to the density of distilled water.…”

Clinical performance evaluation of total protein measurement by digital refractometry and characterization of non-protein solute interferences

“…In a companion study to the present report [23] we conducted a clinical validation of a handheld digital refractometer to serve as a replacement for manual urine SG measurements in our laboratory. As a human blood (serum/plasma) TP scale was also available on this handheld device, we conducted the present experiments to: a) evaluate the performance of a digital refractometer versus a manual refractometer and a biuret method of TP measurement, and b) to investigate the potential for non-protein substances to interfere with TP measurement when using refractometry.…”