MS, LVT, RLATG, VTS (Clinical Pathology)
Barbie teaches clinical pathology and laboratory animal courses in the veterinary technology program at Murray State University and is a continuing education instructor for the Veterinary Support Personnel Network. Before full-time teaching, she worked at the Breathitt Veterinary Center diagnostic laboratory. She currently serves as secretary of the Academy of Veterinary Clinical Pathology Technicians and the Kentucky Veterinary Technician Association. She resides in western Kentucky, near Land Between the Lakes, where she loves to hike with her 2 sons and husband. She shares her home with 3 dogs, 4 cats, and an assortment of feathered and scaled animals.Read Articles Written by Barbie Papajeski
Clinical refractometers are common in-house screening tools for measuring urine specific gravity (USG) with a minute volume of urine.1 Veterinarians use USG to determine the concentrating ability of the kidneys, pinpoint the anatomic location of any abnormalities, and assess hydration status.1-3 This article describes how refractometers work, refractometer types and maintenance, and the steps to follow when measuring USG.
How Refractometers Work
Refractometers compare the gravity of urine with that of distilled water, measuring solid particles in the sample as the refractive index.3 The refractive index correlates with increasing solids within a sample; as light waves pass through the solution, the light is bent depending on the amount of solute within the sample.3 As light is bent through a prism, the optical density is measured on a scale designed for urine gravity. This scale increases with increased urine concentration and provides a visual indication of the urine:distilled water ratio (FIGURE 1).3
Types of Refractometers
There are many types of refractometers on the market today, including analog and digital models (FIGURE 2). Results are typically accurate, although they may be altered by certain factors (BOX 1).
Some models offer separate scales for samples from different species (multispecies refractometers) (FIGURES 3 AND 4), but the advantage over single-scale models is questionable.5-7 Multispecies refractometers will have one scale for dogs/large animals and a separate scale for cats.6,7 Compared with multispecies scales, single-scale refractometers tend to provide higher specific gravity values for cats.3
Both analog and digital refractometer specific gravity readings correlate well with urine osmolality, the gold standard for evaluating dissolved particles in a sample.2,5,8 To get the most reliable and accurate measurements, always refer to calibration recommendations set forth in the instruction manual for the specific refractometer used. Some refractometers are temperature compensated and do not require frequent calibration.7 Patient factors that may affect USG results include glucosuria and marked proteinuria (BOX 1, FIGURE 5).2,3,5
Steps for Measuring Urine Specific Gravity
Step 1: Calibrate
For analog models, before applying the sample to the prism, bring the scale into clearer view by turning the eyepiece while holding the refractometer in the direction of a light source.9 Environmental temperatures outside the range of 16 °C to 38 °C (60 °F to 100 °F) may affect the accuracy of readings.1,2,4,10 If the environmental (workplace) temperature fluctuates more than -15 °C (5 °F), some models may require recalibration4; however, sample temperature rarely affects USG readings.4,6,7 The author’s facility calibrates refractometers daily to adjust for any room temperature changes.
To calibrate the refractometer, first gather the necessary supplies (BOX 2, FIGURE 6). Then place a sample-equivalent volume of distilled water on the prism and close the cover before checking the reading.
On an analog model, if the reading is not on the 1.000 line of specific gravity (FIGURE 7), use a screwdriver on the calibration screw to set the line exactly on the mark while viewing the scale (FIGURE 8).4,7,9 Digital models are calibrated with the press of a button.6,10 After calibration is complete, wipe the prism dry with a soft cloth or optical tissue paper.6,7,9,10
Step 2: Reading Urine Specific Gravity
When taking a sample reading, place 1 to 5 drops of urine on the refractometer prism, similar to the distilled water in the calibration step. For accurate readings, be sure to add an adequate amount of sample to the prism (FIGURE 9).4 Urine samples may be well mixed by gently swirling or inverting the sample, or the supernatant of a centrifuged sample may be used for measurement.5,8 To read analog models, close the sample cover and read the interface between the blue and white color; for digital models, read the numbers displayed.4,6,7,9,10 The advantage of digital models is minimization of recording errors.8 For samples that exceed the USG scale, dilute the sample by adding 1 drop of distilled water to 1 drop of urine, mix well, and then multiply the results past the decimal point by 2. Commonly, USG is merely reported as greater than the scale limit (i.e., >1.060).3 The cutoff value will vary by model. Most models have a USG range from 1.000 to 1.060, and on some digital models, USG on the feline setting can be read up to 1.120.4,6,7,9,10
After each use, clean the refractometer with a soft cloth or laboratory optical tissue and distilled water. Do not use abrasive cleaners or cloths on the prism as doing so will damage the surface and impede readings.4,6,7,9,10
Step 3: Reporting and Interpreting Urine Specific Gravity Results
Because USG is a ratio of urine to distilled water, no units of measure are used. USG is described both in terms of concentrating ability compared with the expected specific gravity of plasma or glomerular ultrafiltrate. USG references ranges vary widely. Categories of USG are as follows (TABLE 1)1-3:
- Hyposthenuric indicates dilute urine, often resulting from renal tubule inability to concentrate or from excessive water consumption by the patient.
- Isosthenuric (FIGURE 10) may result from renal dysfunction in dehydrated and azotemic animals, indicating an inability to either dilute or concentrate urine.
- Hypersthenuric (FIGURE 11) indicates concentrated urine, usually resulting from dehydration.
Refractometers are valuable tools for accurately determining USG readings for veterinary patients. Small volumes of urine can yield valuable information regarding an animal’s health status. Veterinary nurses increase the value of results by properly calibrating, reading, and maintaining refractometers.
- Thrall M, Weiser G, Allison RW, Campbell TW. Laboratory evaluation of renal function. In: Veterinary Hematology and Clinical Chemistry. Wiley Blackwell; 2012:333-346,366.
- Latimer KS. Urinary system. In: Duncan & Prasse’s Veterinary Laboratory Medicine Clinical Pathology. 5th ed. Wiley Blackwell; 2011:255-257.
- Sink CA, Weinstein NM. Routine urinalysis: physical properties. In: Practical Veterinary Urinalysis. Wiley Blackwell; 2012:19-25.
- Westover. RHC-200 Refractometer for Clinical Use Operation Manual; 2001.
- Mösch M, Reese S, Weber K, Hartmann K, Dorsch R. Influence of preanalytic and analytic variables in canine and feline urine specific gravity measurement by refractometer. J Vet Diagn Invest. 2020;32(1):36–43. doi:10.1177/1040638719896785
- Misco. Misco Veterinary Refractometer – Veterinary Urine Scales – Dog & Cat Urine Specific Gravity, Total Protein, & Refractive Index – MISCO VETMED01 Refractometer. Accessed July 14, 2022. https://www.misco.com/product/veterinary-refractometer-cat-dog-urine-specific-gravity-protein-refractive-index-vetmed01
- Reichert Technologies Analytical Instruments. Reichert Vet 360 Instruction Manual; 2003.
- Bennett AD, McKnight GE, Dodkin SJ, Simpson KE, Schwartz AM, Gunn-Moore, DA. Comparison of digital and optical hand-held refractometers for the measurement of feline urine specific gravity. J Feline Med Surg. 2011;3(2):152-154. doi:10.1016/j.jfms.2010.09.012
- Jorgenson Laboratories. Instruction Manual J-351 Clinical Refractometer. Accessed July 14, 2022. http://jorvet.com/wp-content/uploads/2012/01/J351_Refractometer.pdf
- Fisher Scientific. Fisherbrand Digital Handheld Refractometer User Manual; 2017.