Comparison of a Schmidt and Haensch refractometer and an Atago PAL-USG Cat refractometer for determination of urine specific gravity in dogs and cats

Background The performance of a digital Atago PAL-USG Cat refractometer (Atago) was compared with a Schmidt and Haensch, Goldberg type refractometer (S+H). Materials and Methods Specific gravity of 47 canine and feline urine samples was determined with both refractometers and the results were compared with Passing–Bablok and Bland–Altman plots. In addition, the specific gravity of dilutions of 10% glucose, 10% NaCl, and 3% albumin solutions was determined and compared with expected values. Results Both refractometers consistently reported 1.000 with distilled water. The correlation between both refractometers based on Passing–Bablok plots of 47 urine samples was excellent (r = .99), but, in the Bland–Altman plots, there was a significant, proportional, negative error for the Atago readouts. This was also illustrated by the fact that 10 urine samples with an S+H result of > 1.030 were read out between 1.023 and 1.028 by Atago. Schmidt and Haensch results of various glucose solutions matched exactly expected values, but Atago results were lower. Likewise, S+H results with diluted NaCl solutions were closer to expected results than Atago results. In contrast, Atago results with dilutions of 3% albumin were closer to expected results than S+H results. Discussion The Atago refractometer reported lower specific gravity results in urine and standard solutions of glucose and NaCl, which could adversely affect clinical decisions concerning normal renal function based on solute concentrations determined in canine and feline urine samples.