Measurement of urine concentration is an important part of a urinalysis and crucial to differentiation of prerenal and renal azotemia. Generally, osmolality is considered a more exact measurement of urine concentration than is specific gravity. However, because of the ease of use, USG is most commonly used to evaluate renal concentrating ability. Osmolality is a measure of the number of dissolved particles in a fluid. In comparison, specific gravity represents the ratio of the weight of a solution to the weight of an equal volume of water.1
Refractometers typically are used in clinical settings for estimation of USG. Use of urine test strips to determine USG is not recommended because of poor reliability.2–4 For refractometers, weight of a solution is not measured; instead, it is estimated on the basis of refraction of light.5 The amount of refraction can be affected by solute concentration, chemical composition of the solute, and temperature. A strong linear correlation for human urine exists between urine osmolality and USG as measured by refractometry in samples that do not contain protein or glucose.1,6
The presence of glucose in urine has the potential to affect USG measurements. Conditions that commonly cause glucosuria are diabetes mellitus and primary renal glucosuria. The ability to accurately evaluate urine concentrating ability in diabetic patients is extremely important because many have concurrent disorders.7 Because of the perception that urine glucose causes a false elevation in USG, it has been recommended to interpret USG in relation to the presence and amount of urine glucose.8 The addition of 1 g of glucose/dL is expected to change the specific gravity of water by 0.003 to 0.005.1 For example, a canine urine sample with a USG of 1.025 without glucose would have a USG of approximately 1.035 if 2,000 mg of glucose/dL was present, and interpretation of renal concentrating ability would differ substantially between these 2 results.
However, to our knowledge, expected changes in USG attributable to glucose in dog and cat urine have not been evaluated. Glucosuria has no effect on linearity of the relationship between urine osmolality and USG in dogs9 and humans.10 Therefore, USG may be an accurate method for assessment of urine concentrating ability despite the presence of glucosuria. The purpose of the study reported here was to evaluate the effect on USG of the addition of glucose to urine samples of dogs and cats on USG and determine whether glucosuria affected the assessment of renal concentrating ability. We hypothesized that USG measured via refractometry would not be altered by the presence of glucose, and, accordingly, glucosuria would not affect the assessment of renal concentrating ability.
The authors declare that there were no conflicts of interest.
Urine specific gravity
Thermo Fisher Scientific, Waltham, Mass.
TS-400, Reichert Technologies, Buffalo, NY.
Hitachi 911, Boehringer Mannheim, Indianapolis, Ind.
Roche/Hitachi reagents, Roche Diagnostics, Indianapolis, Ind.
GraphPad Prism, version 8, GraphPad Software Inc, San Diego, Calif.
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