Ryane E. Englar Chemical evaluation of urine involves the use of commercially available, colorimetric, reagent test strips to provide qualitative and/or semiquantitative measurements of the following chemical properties of urine [1–6]: Test strips for the chemical evaluation of urine are commonly called dipsticks. By design, dipsticks have one or more pads that are impregnated with reagents [4]. These pads change color in the presence of the substance of interest [4]. How much of that substance is present in the urine sample will determine the degree of color change [4]. Color change is also time dependent. Manufacturers will provide specific instructions concerning how long to wait after soaking the pad with urine to interpret the test results [5]. Results are typically read manually by the members of the veterinary team who are tasked with performing urinalysis. However, automated urine chemistry test strip analyzers are available that will interpret and print results in lieu of personnel [4]. Many different brands of dipsticks are commercially available within human healthcare and veterinary practices. Dipsticks may be manufactured to test one or more chemical properties of urine. Dipsticks that test for only one substance in the urine include: Dipsticks that test for two substances include: Dipsticks that test for more than two substances include: Human urine dipstick tests are also available over the counter to screen for the presence of drugs that are commonly abused. These commercially available test kits have been used off‐label by veterinarians to successfully identify barbiturates, opiates, benzodiazepines, and amphetamines/methamphetamines within canine urine in emergency settings [7]. However, these are beyond the scope of this chapter, which will focus on routine rather than toxicological screening of urine. Figure 14.1 Example of reagent test strip that exclusively tests for urine ketones. Source: Courtesy of Ryane E. Englar, DVM, DABVP (Canine and Feline Practice). Figure 14.2 Example of reagent test strip that exclusively tests for urine glucose. Source: Courtesy of Ryane E. Englar, DVM, DABVP (Canine and Feline Practice). Figure 14.3 Example of a reagent test strip that assesses multiple chemical parameters within a urine sample. Source: Courtesy of Ryane E. Englar, DVM, DABVP (Canine and Feline Practice). For cost efficiency and the sake of completeness, most veterinary practices incorporate a multiple‐test reagent strip into routine urinalysis. Because strips such as Multistix® were developed as a diagnostic tool for human health care, it is important to note that not all tests that are provided on a multiple‐test reagent strip are useful or reliable in veterinary species [4]. Specifically, the following test pads on the Multistix® are not advised for use in veterinary practice [1–4]: USG is a measure of how concentrated or dilute the urine is. This is determined by the patient’s renal tubular function and hydration status. The USG pad on commercially available dipsticks only measures up to 1.030. This is not an appropriate cutoff point for assessing the canine or feline patient’s ability to concentrate urine [5, 10–12]. In addition, dilute urine is not always accurately identified by the USG pad [6]. Therefore, the gold standard method by which to measure USG is via refractometry. See Chapter 13 for additional details. Urobilinogen is a metabolic byproduct of bilirubin. It is normal to excrete some urobilinogen in the urine. In people, an increased amount of urobilinogen can indicate elevated levels of serum bilirubin. Elevated bilirubin is supportive of hepatobiliary disease in people. However, in dogs and cats, there is not necessarily an association between increased levels of urobilinogen and hepatobiliary disease or hemolysis. Therefore, the urobilinogen pad is not diagnostically useful in veterinary species [6, 11–13]. The nitrite and leukocyte esterase pads are not diagnostically accurate for veterinary species [5, 10, 13]. The nitrite pad aims to detect certain Gram‐negative bacteria that convert nitrate to nitrite [6]. These bacteria are commonly implicated in UTIs in people, hence the growing popularity of this test among over‐the‐counter products that screen people with lower urinary tract signs. However, detection is unpredictable among veterinary patients, even if the bacterial population is abundant [6]. The leukocyte esterase pad tends to produce false positive results for cats and false‐negative results for dogs [3, 6]. Evaluating urine sediment is therefore a more appropriate method by which to accurately identify leukocytes in veterinary patients. See Chapter 15 for additional details. Urine is an aqueous solution. Ninety‐five percent of urine is water; however, knowledge of the chemical composition of its remaining 5% is essential because this recipe reflects the body’s water and electrolyte balance and renal tubular function. In order of decreasing concentration, urine contains: The presence/absence of these constituents and the relative abundance of each have implications for patient health. Being able to qualitatively and/or quantitatively identify chemical features of a urine sample contributes essential data to the diagnostic process. An understanding of chemical features of the urine can also facilitate case management by monitoring how a disease progresses and/or how the patient is responding to therapy. Examination of the chemical properties of a urine sample requires: Start this process with urine in a collection container (e.g. a clear, conical centrifuge tube). This allows you to submerge the entire test strip in urine at once. After complete, but brief immersion, remove the dipstick. Tap the dipstick against the urine collection container or on an absorbent pad to remove excess urine (see Figure 14.7). Start the clock. Hold the strip against the test chart to manually read results. The manufacturer will provide recommendations concerning the timetable for reading results. Figure 14.4 Gathering supplies for urine dipstick analysis. Courtesy of Jeremy Bessett, Inaugural Class of 2023, University of Arizona College of Veterinary Medicine. Source: Courtesy of Jeremy Bessett, Inaugural Class of 2023, University of Arizona College of Veterinary Medicine. Figure 14.5 One individual Multistix® test strip. Source: Courtesy of Ryane E. Englar, DVM, DABVP (Canine and Feline Practice). Figure 14.6 Immersing the dipstick within the patient’s urine sample so as to saturate all reagent pads simultaneously. Source: Courtesy of Ryane E. Englar, DVM, DABVP (Canine and Feline Practice). Figure 14.7 Removing excess sample from the reagent test pads immediately following immersion. Source: Courtesy of Jeremy Bessett, Inaugural Class of 2023, University of Arizona College of Veterinary Medicine. Figure 14.8 The label of the product contains the recommended times to read each reagent pad. This photograph displays the recommended time to report out the results for both glucose and bilirubin. Source: Courtesy of Ryane E. Englar, DVM, DABVP (Canine and Feline Practice). Read each pad at the time shown on the label, beginning with the shortest time. For example, Multistix® reports the following schedule for manual interpretation of test pad color changes (see Figure 14.8): Manually compare the color on your strip for each individual test pad to the corresponding row of color blocks on the test chart (see Figures 14.9 and 14.10). See which color on the test chart is the closest match to the color on your strip (see Figures 14.11–14.14). Document each result in the patient’s medical record. Sometimes a test pad produces an erroneous result: the color of the test pad does not match up against the test chart. In this case, it is reasonable to question the accuracy of the results and retest the urine sample using a new dipstick (see Figure 14.15). Place a paper towel or a lab bench absorbent pad on the countertop where you will be performing chemical evaluation of the urine. Lay a dipstick on top of the paper towel or absorbent pad. Using a pipette, place one drop of urine on each pad of the dipstick (see Figures 14.16–14.18). Figure 14.9 Manually reading the urine dipstick. Source: Courtesy of Jeremy Bessett, Inaugural Class of 2023, University of Arizona College of Veterinary Medicine. Figure 14.10 Manually reading the urine dipstick. Source: Courtesy of Jeremy Bessett, Inaugural Class of 2023, University of Arizona College of Veterinary Medicine. Figure 14.11 This photograph demonstrates how you would read the test pad for glucose. This patient’s sample is negative for glucosuria. Source: Courtesy of Ryane E. Englar, DVM, DABVP (Canine and Feline Practice). Figure 14.12 This photograph demonstrates how you would read the test pad for glucose. This patient’s sample is positive for glucosuria. Source: Courtesy of Ryane E. Englar, DVM, DABVP (Canine and Feline Practice). Figure 14.13 This photograph demonstrates how you would read the test pad for urine pH. This patient has aciduria. Source: Courtesy of Ryane E. Englar, DVM, DABVP (Canine and Feline Practice). Figure 14.14 This photograph demonstrates how you would read the test pad for urine pH. This patient has alkalinuria. Source: Courtesy of Ryane E. Englar, DVM, DABVP (Canine and Feline Practice). Figure 14.15 Manually reading the urine dipstick. Note that the bilirubin test pad has produced a vibrant orange color. This is not a viable option if we compare the test pad color against the test chart. In this case, we need to consider retesting the sample to get a valid result concerning the presence or absence of bilirubinuria. Source: Courtesy of Jeremy Bessett, Inaugural Class of 2023, University of Arizona College of Veterinary Medicine. Allow each droplet to fully coat the test pad. Tap the dipstick against the urine collection container or on the paper towel or absorbent pad to remove excess urine. Start the clock. Hold the strip against the test chart to manually read results. The manufacturer will provide recommendations concerning the timetable for reading results. Read each pad at the time shown on the label, beginning with the shortest time. Figure 14.16 Pipetting urine droplets onto each test pad. Source: Courtesy of Jeremy Bessett, Inaugural Class of 2023, University of Arizona College of Veterinary Medicine. Figure 14.17 Urine has been successfully applied to all reagent test pads. Source: Courtesy of Jeremy Bessett, Inaugural Class of 2023, University of Arizona College of Veterinary Medicine. Figure 14.18 Close‐up of test strip after urine has been applied to all reagent pads. Source: Courtesy of Jeremy Bessett, Inaugural Class of 2023, University of Arizona College of Veterinary Medicine. Manually compare the color on your strip for each individual test pad to the corresponding row of color blocks on the test chart. See which color on the test chart is the closest match to the color on your strip. Document each result in the patient’s medical record. Sometimes a test pad produces an erroneous result: the color of the test pad does not match up against the test chart. In this case, it is reasonable to question the accuracy of the results and consider retesting the urine sample using a new dipstick. Less than five minutes.
14
Chemical Evaluation of Urine: Urine Dipstick Analysis
14.1 Procedural Definition: What Is This Test About?
14.2 Procedural Purpose: Why Should I Perform This Test?
14.3 Equipment
14.4 Procedural Steps
14.5 Time Estimate to Perform Test
14.6 Procedural Tips and Troubleshooting
14.6.1 Regarding the Urine Sample
14.6.2 Regarding the Test Strips
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