How specific is urine specific gravity? - DVM
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How specific is urine specific gravity?
An in-depth look at which urinary concentrating ability test might be best in your patients. (Part one of a four-part series.)


DVM360 MAGAZINE


Specific gravity reagent strips to measure urine specific gravity

The reagent strip specific gravity test is an indirect calorimetric method of assessing ionic urine specific gravity. Indirectly measuring urine specific gravity with test strips is based on the change in pK (dissociation constant) of a polyelectrolyte in the test pad in relation to the concentration of ionic (charged) solutes in urine. The test pad contains a polyelectrolyte and a pH indicator that are maintained at an alkaline pH.

When the test pad is immersed in urine, the polyelectrolyte reagent ionizes and releases protons (hydrogen ions) in proportion to the concentration of urine ionic analytes. The protons released from the polyelectrolyte decrease the pH of the test pad, causing a color change in the indicator dye from dark blue-green (specific gravity = 1.000) to yellow-green (specific gravity = 1.030). The greater the number of ionic solutes in urine, the greater the release of hydrogen ions from the test pad, and the greater the color change in the test pad indicator dye.

This method is not influenced by nonionic urine analytes such as urea or glucose. Therefore, specific gravity values determined by this method do not need to be corrected for significant quantities of urine glucose. However, the total specific gravity of urine is dependent on both ionic and nonionic solutes. Also, urine pH values of 6.5 or greater could influence test results because the indicator dye is active in this range.

Because the highest value that these reagent strips can detect is approximately 1.025 to 1.030, this test is unsatisfactory for detection of adequate renal concentrating capacity in dogs and cats. We have also found this test to be unreliable in less concentrated urine samples.

Urine refractive index and refractometers

Aqueous solutions such as urine contain substances that absorb various wavelengths of light. This may be measured by determining the refractive index of the light, which is defined as the ratio (or comparison) of the velocities of light in two different media. In the context of urine, the refractive index is the unitless ratio of the refraction of light in air compared with the refraction of light in urine. As the solute concentration of urine increases, the velocity of light passing from air through urine decreases and the light beam is refracted.

As a consequence, the light rays bend (i.e., the angle of light refraction changes). The index of refraction may be measured by an instrument called a refractometer. Refractometry provides an indirect assessment of osmolality and specific gravity. Small handheld refractometers calibrated to determine urine specific gravity are commonly used.

Measurement of the refractive index of urine is affected by the concentration of all solutes (ionic and nonionic) in urine. In addition to urine solute concentration, temperature affects the density (or specific gravity) of urine. Therefore, refractometers are calibrated at a reference temperature.

There is considerable variability in the quality and cost of refractometers. Ideally, refractometers should be calibrated by the manufacturer for the species studied by using urine samples of known specific gravity. Therefore, dog and cat urine require different scales. Those designed for physicians are calibrated for human urine.

Summary

Measurements made by osmometers, urinometers, reagent strips and refractometers are related but not interchangeable. Measurement of urine osmolality provides information that is more closely related to renal concentrating capacity than does specific gravity or refractive index. Osmometers provide more accurate assessment of osmolality of individual urine samples than refractometers or urinometers.

Urine specific gravity is a direct but not proportional function of the number of solute particles in urine. Urine specific gravity varies with the kind of solute present, whereas urine osmolality is independent of the type of solute present. Therefore, urine specific gravity provides only an estimation of osmolality. In this regard, specific gravity is not specific. Indirect measurement of urine specific gravity by refractometry is useful as a screening test of renal function. However, urine osmolality measured with osmometers should be used for patients with undiagnosed persistent polyuria when errors in assessment of renal function are of significant consequence.

Evaluation of specific gravity is essential when interpreting test results of the complete urinalysis. Refractometers are recommended over urinometers for determination of urine specific gravity because they provide more reproducible results, require a small sample size, are temperature-compensated and are technically easy to use.

Still have your answer to the question posed in the beginning of this article? Remember it for next month's Diagnote in which we will discuss the interpretation and misinterpretation of urine specific gravity.

Dr. Carl A. Osborne is the director of the Minnesota Urolith Center and a professor at the College of Veterinary Medicine at the University of Minnesota. Dr. Eugene Nwaokorie is pursuing a PhD at the University of Minnesota.


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