Digital VS Optical Refractometer to measure urine specific gravity in cats

The evaluation of renal concentrating ability is an important function in veterinary medicine. The gold standard for the assessment of urine concentration is the measurement of urine osmolality, but this requires expensive equipment that’s not readily available in clinical practice. The measurement of urine specific gravity (USG) is commonly used since it closely correlates with osmolality and can measure renal concentrating ability with reasonable accuracy.

In this study, both a cat-specific digital refractometer and an optical hand-held refractometer were used to evaluate urine samples of 55 cats, the goal being to determine the difference in USG measurements between both methods and how each compared with urine osmolality. Each sample was analysed using both instruments simultaneously. After that, ten random samples were submitted for an osmolality assay.

The results showed that the optical refractometer has consistently higher readings than the digital cat-specific one. However, as both instruments were well correlated with osmolality, the authors concluded that the difference between the digital and optical readings is not clinically relevant.

In Comparison of digital and optical hand-held refractometers for the measurement of feline urine specific gravity; Alexander D. Bennett, Grace E. McKnight, Steve J. Dodkin, Kerry E. Simpson, Anita M. Schwartz and Danièlle A. Gunn-Moore; Journal of Feline Medicine and Surgery Vol 13 (2011)