The relationship between specific gravity and refractive index for cats and dogs: an equivalence study

• Autor(a) principal: Mota, Andreia Fernanda Moreira da
• Data de Publicação: 2020
• Tipo de documento: Dissertação
• Idioma: eng
• Título da fonte: Repositório Científico de Acesso Aberto de Portugal (Repositórios Cientìficos)
• Texto Completo: http://hdl.handle.net/10348/10766
• topic: urine concentration; refractometry

The concentration of urine is a frequent clinical assessment in veterinary routine practice. This laboratorial parameter is estimated by measuring the refractive index of the urine on an instrument called a refractometer. For slightly esoteric historical reasons, rather than reporting this on a scale of refractive index (the measurement actually being made), it is reported on a specific gravity scale. Whether the relationship between the measured urine refractive index reported on this SG scale and the actual specific gravity of the urine is true is largely irrelevant as long as everyone uses the same specific gravity scale as a measure of refractivity. A claim has been made, however, that the relationship between the actual urine specific gravity and the urine refractive index differs whether it is urine of a cat or a dog, which led to development of a “urine specific gravity cat scale” and a correction formula to obtain specific gravity of feline urine from human-based specific gravity scale. This work aimed to assess the underpinning relationship rationale behind the published and widely used “specific gravity cat scale” by comparing the relationship between urine specific gravity and urine refractive index in cats with the relationship between urine specific gravity and urine refractive index in dogs. The present study has been carried out on clinical cases attended at the Gortlands Veterinary Clinic (Belfast, Northern Ireland) between October and December 2019. The urine was collected under the scope of routine investigations. Urine specific gravity and refractive index were determined in 47 dogs and 42 cats from different ages, genders and breeds, using a digital density meter and an optical hand-held refractometer, respectively. The relationship between urine specific gravity and urine refractive index in dogs and cats of this study was studied by equivalence testing (paired two one-sided tests) and the difference obtained between cats and dogs was visually compared with the difference suggested in the literature. The paired two one-sided tests showed equivalence between feline and canine urine regarding the relationship between its specific gravity and refractive index (p-value < 0.01). The results also indicate that, for the same urine refractive index value, a difference between feline urine specific gravity and canine urine specific gravity of much lesser magnitude than the one suggested in the published literature. This study showed that feline urine is not more refractive than canine urine, paving the way to abandon the use of the specific gravity scale for cats and the application of the correction formula.