Refractometers
A Closer Look:
In veterinary medicine, refractometers are indispensable for assessing urine specific gravity (USG), a key indicator of an animal’s renal concentration ability and overall hydration. Accurate USG measurements are vital for diagnosing and monitoring various health conditions.
Selection and Calibration: Choosing a refractometer designed for veterinary use is essential, as these devices are calibrated to account for species-specific urine characteristics. Before each use, calibrate the refractometer with distilled water, which should yield a USG reading of 1.000. If the reading deviates, adjust the calibration screw accordingly.
Sample Handling: Ensure urine samples are at room temperature before analysis, as temperature variations can affect USG readings. Refrigerated samples should be allowed to warm to room temperature for approximately 30 minutes prior to testing. For turbid or cloudy samples, centrifuge them to obtain a clear supernatant for accurate measurement.
Measurement Technique: Place a drop of the urine sample on the refractometer’s prism, close the cover plate to spread the liquid evenly, and hold the device horizontally towards a good light source. Look through the eyepiece to read the USG value at the boundary between the light and dark areas on the scale.
Maintenance: After each use, clean the optical surfaces of the refractometer with distilled water and a soft, lint-free cloth to prevent residue buildup, which can lead to inaccurate readings. Regularly check and recalibrate the device as needed to maintain its precision.
Considerations: Be aware that certain substances, such as glucose or protein in the urine, can artificially elevate USG readings. For instance, glucosuria can increase USG by 0.004–0.005 units per 1 g/dL of glucose. Therefore, interpret results in conjunction with other diagnostic information and the animal’s clinical signs.
By adhering to these guidelines, veterinary professionals can ensure reliable USG measurements, leading to more accurate assessments of animal health.
There are notable differences between dog and cat urine, particularly concerning urine specific gravity (USG) and the influence of substances like glucose and protein. A study published in the American Journal of Veterinary Research evaluated the effect of adding glucose to urine samples from dogs and cats. The results indicated that while the addition of glucose increased USG in both species, the impact was minimal and did not significantly affect the assessment of renal concentrating ability.
Rubini, M. E. and Wolf, A. V. 1956. Refractometric determination of total solids and water of serum and urine. J. Biol. Chem. 225:869-876