Comparison of canine urine specific gravity measurements between various refractometers in a clinical setting

Veterinary facilities might use multiple refractometers and individuals to measure urine specific gravity (USG). Previous comparison studies show conflicting results. Furthermore, the clinical significance of measurement differences and interobserver variabilities has not been assessed.

We aimed to determine statistically and clinically significant differences between four refractometers in measuring canine USG and subsequent categorization of urine concentrations and azotemia and determine the variability between different observers performing USG measurements.

Fifty-nine specimens were included for the USG measurements with four refractometers by different observers. Each refractometer pair was compared using Spearman's rank correlation, Bland-Altman difference plots, and Deming regression analyses. Calculated bias was compared to set performance goals. Interobserver agreement was evaluated, and intraclass correlation coefficients were used to determine differences in the categorization of urine concentrations and azotemia (prerenal or renal).

There was excellent correlation (r_s  = .99-1.00) between refractometers. All comparisons involving R4 showed significant constant and proportional biases. Mean bias met the clinical performance goals for all refractometers, except for comparisons with R4, where up to 17 results were outside the allowable bias. There was almost perfect agreement (r_s  = .999) between observers and excellent agreement (ICC = .96-.99) for the classification of urine concentrations. In azotemic patients (22%), there was perfect agreement (ICC = 1.00) for the categorization of azotemia.

In most cases, three of the refractometers evaluated in this study can be used interchangeably at all USG values, without affecting clinical decision-making. Multiple observers did not significantly affect decision-making.