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Comparison of a digital and an optical analogue hand-held refractometer for the measurement of canine urine specific gravity
  1. J. K. Paris, BSc, BVSc, MRCVS1,
  2. A. D. Bennett, DVM2,
  3. S. J. Dodkin, BSc, MSc3 and
  4. D. A. Gunn-Moore, BSc, BVM&S, PhD, MACVSc, MRCVS, RCVS, Specialist in Feline Medicine1
  1. Royal (Dick) School of Veterinary Studies and the Roslin Institute, Division of Clinical Veterinary Sciences, University of Edinburgh, Easter Bush Veterinary Centre, Roslin, Midlothian EH25 9RG, UK
  2. North Carolina State University, Department of Clinical Sciences, Raleigh, NC, 27607, USA
  3. Biochemistry Laboratory, Langford Veterinary Services, Langford House, Langford, North Somerset BS40 5DU, UK
  1. E-mail for correspondence jparis{at}

Urine specific gravity (USG) is used clinically as a measure of urine concentration, and is routinely assessed by refractometry. A comparison between optical analogue and digital refractometers for evaluation of canine urine has not been reported. The aim of this study was to compare a digital and an optical analogue hand-held refractometer for the measurement of canine USG, and to assess correlation with urine osmolality. Prospective study. Free-catch urine samples were collected from 285 hospitalised adult dogs, and paired USG readings were obtained with a digital and an optical analogue refractometer. In 50 dogs, urine osmolality was also measured using a freezing point depression osmometer. There was a small but statistically significant difference between the two refractometers (P<0.001), with the optical analogue refractometer reading higher than the digital refractometer (mean difference 0.0006, sd 0.0012). Paired refractometer measurements varied by <0.002 in 91.5 per cent of cases. The optical analogue and digital refractometer readings showed excellent correlation with osmolality (r=0.980 and r=0.977, respectively, P<0.001 in both cases). Despite statistical significance, the difference between the two refractometers is unlikely to be clinically significant. Both instruments provide an accurate assessment of USG in dogs.

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  • Provenance not commissioned; externally peer reviewed

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