Variability among four refractometers for the measurement of urine specific gravity and comparison with urine osmolality in dogs

Background Refractometry is often used in clinical veterinary medicine to estimate urine concentration. Variability among commonly-used refractometers has not been critically evaluated. Objective This study aimed to evaluate the variability of urine specific gravity (USG) among four refractometers and compare results of USG measurements with those of urine osmolality (U-osm), the gold standard for determining urine concentrations. Methods USG was determined in 100 dogs using three optical refractometers, the American Optical Abbe refractometer 10450, Reichert TS 400 refractometer, and Heska Veterinary refractometer 2737-E02, and one digital refractometer, the Misco Palm Abbe Digital refractometer #PA203. Results were compared between each pair of refractometers and between each refractometer and urine osmolality determined by freezing point depression using a Multi-Osmette 2430E osmometer. Results were analyzed with Bland-Altman plots and Passing-Bablok regression analysis. Results The Reichert, Heska, and Misco refractometers provide USG measurements that can be used interchangeably based on small differences, consistently less than 0.002, between the USG measurements. The AO refractometer measured USG values with much larger differences, which were deemed clinically significant by the study parameters. None of the refractometers were able to accurately predict U-osm or vice versa within a clinically acceptable range. Conclusions Varying degrees of differences were seen in the USG measurements among the different refractometers. These differences were refractometer-dependent, and the results from one instrument could affect clinical decisions.