In today’s VETgirl online veterinary continuing education podcast, we review whether or not the urine dipstick paddle works to help identify urinary tract infections (UTIs) in dogs and cats. Being that 14% of dogs will develop a UTI at some point in their life, and that UTIs are more prevalent in older (versus younger cats), veterinarians should be well aware of how to treat UTIs. Keep in mind that most UTIs in dogs and cats involve a single bacterial species, with E.coli being the #1 isolated bacteria from the urine of dogs and cats (followed by Staph, Proteus, Klebsiella, Enterococcus, and Strep).
So, what’s the best way to confirm a UTI? Ideally, the patient should have a cystocentesis followed by a complete urinalysis and quantitative aerobic bacterial culture (what we’ll call a QABC, from now on). The QABC is an important modality to get an estimate of the number of bacteria and determine if the bacteria are clinically relevant to the UTI. Unfortunately, QABC and antimicrobial susceptibility testing often aren’t done due to cost; rather, a presumptive diagnosis of a UTI is often made based on lower urinary tract signs with antibiotics empirically chosen by pet owners. However, repeated empiric antibiotic treatment without QABC and antimicrobial susceptibility testing can lead to inappropriate antibiotic selection, potential unnecessary adverse drug reactions, and possible selection of resistant bacteria.
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More recently, the use of urine dipstick paddles (aka “dip slides” or “paddle testers”) have been used in veterinary medicine. These paddles have been used for decades in human medicine for detection of bacterial cystitis. But, do they work in dogs and cats? Previously, the reported sensitivity (e.g., ability to detect a true positive) of these paddles was 73-99%, while the reported specificity (e.g., ability to detect a true negative) was 94-99%. That said, there has been a wide range of reported success (ranging from 54-95%) when it comes to correctly identifying the bacterial species.
What comes with the paddle? It’s a culture paddle with each side embedded with a different standard culture media, which supports growth of most commonly isolated urinary bacterial pathogens in dogs and cats. According to the manufacturer, there is a 99% correlation of results with laboratory culture methods. However, no antimicrobial susceptibility information can be obtained from the urine dipstick paddles. The pros of the urine dipstick paddles? They are potentially an inexpensive screening tool for veterinarians before starting empiric antibiotic therapy. BTW, the manufacturer recommendations are to pour urine directly onto the urine dipstick paddles; unfortunately, there is often insufficient volume to do this. Preliminary tests were then done to compare 2 different methods for inoculating the urine dipstick paddles: one by dripping 0.25 mL of urine from a syringe onto each side, and the other way by using a 10 ul calibrated loop to inoculate each side. The results found that 0.25 mLs of urine was deemed sufficient and they got comparable colony counts with both of these methods.
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So, Ybarra et al. out of UC-Davis wanted to confirm if these urine dipstick paddles that are marketed for veterinary use actually work or not. In a prospective study, they collected urine samples via cystocentesis, urinary catheter, or midstream catch on multiple patients. The urine sediments that were suggestive of UTI (e.g., pyuria, hematuria, bacteriuria) were prioritized for inclusion into this study to increase the number of positive samples; randomly selected urine samples with inactive sediments were also used in this study.
If you’ve never seen one of these urine dipstick paddles, they have 2 types of media (1 on each side) and are attached to a screw top that’s inserted into a plastic container. These should be stored at room temperature. The presence of bacterial colonies is identified based on color change (e.g., green to yellow or blue, depending on the bacteria). One can then estimate the bacterial colony counts using a chart provided by the manufacturer. The urine dipstick paddles, once inoculated, should be incubated at 37C (approximately 100F), and monitored for growth (which is typically seen 16-24 hrs after inoculation).
In this study, a total of 207 urine specimens were evaluated (e.g., 149 from dogs; 58 from cats). Additional microbiology laboratory testing of urine samples was performed on 30.9% (n=64/207) which had bacterial growth; of these, 58 of these samples were obtained by cystocentesis and 6 were collected by midstream catch. The most common isolate identified was E. coli (n=38), followed by Enterococcus (n=19), Staph (n=10), then far fewer other types of bacteria. Overall, there was excellent agreement of the urine dipstick paddle with the microbiology laboratory results for the presence or absence of bacterial growth. Two investigators evaluated these results on the urine dipstick paddles. Both investigators were not aware of urinalysis and QABC results for each sample. The first investigator found growth on 30.4% (n= 63/207) of the urine dipstick paddles, with a reported sensitivity and specificity (when compared to microbiology laboratory results) of 95.3% and 98.6%, respectively. With the first investigator, the urine dipstick paddles had 3 false positives and 3 false negatives. The second investigator identified growth on the urine dipstick paddles of 28% (n=58/207), with a reported sensivity and specificity of 89.1% and 99.3%, respectively. With the second investigator, the urine dipstick paddles had 2 false positives and 6 false negatives. Both investigators found that growth was observed within 24 hours after inoculation for all urine dipstick paddles that had growth. They also found complete inter-investigator agreement with regard to identification of the E.coli, but far lower with identification of other types of bacteria (e.g., Staph and Enterococcus). 11 of the urine samples had growth of >1 uropathogen at the microbiology laboratory; however, this mixed bacterial growth was not identified on the urine dipstick paddle by either investigator.
Randomly selected 33 urine dipstick paddles with bacterial growth were sent to the microbiology laboratory for confirmation; 75.8% (25/33) of samples submitted were the same pathogen identified on the urine dipstick paddle as isolated on QABC, which was good correlation. Of these, 3/33 samples that were submitted to the microbiology laboratory could not culture any bacteria despite having a reported positive urine dipstick paddle result (e.g., a false positive). Of the 33 samples submitted, 5/33 also isolated a totally different bacteria as reported by the urine dipstick paddle. Lastly, the investigators were able to detect substantial bacterial growth (≥105 CFU/mL) on the urine dipstick paddles but accurate counts were not possible. Overall, the urine dipstick paddle system proved sensitive and specific with good inter-investigator agreement for detection of bacterial UTI in dogs and cats.
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So, what can we take from this VETgirl podcast? The urine dipstick paddles were good for determining if a UTI was present, but not for identification of bacteria (despite what the manufacturer says in their provided materials). Urine dipstick paddles are especially poor at identifying when multiple bacteria are present in a urine sample. That said, based on the results of this study, you can use the urine dipstick paddles as a screening tool, but not for bacteria identification (Plus, you don’t get antimicrobial susceptibility with this technique). Urine dipstick paddles could also be a cost effective manner for re-checking cultures in patients during and after antibiotic treatment (especially those with recurrent UTIs).
There are some limitations of this study. First, there were relatively low numbers of urine samples with bacterial growth (64) and even fewer w/ mixed bacterial growth (11). Also, the authors didn’t separate dog and cat results to determine any species differences. Another limitation of this study? Not all urine dipstick paddles with bacterial growth were sent to a microbiology laboratory for confirmation, limiting the diagnostic power of this study. Finally, only a few samples had the less common pathogens (e.g. Klebsiella, Pseudomonas, Proteus, yeast) or Gram positive bacteria, so it was hard to determine the accuracy due to these low numbers.
In conclusion, if you have positive growth on a urine dipstick paddle, ideally you should recommend submitting a fresh sterile urine sample (or a refrigerated urine sample stored for < 24 hours) to a micro lab for QABC and antimicrobial susceptibility testing. Yes, it’s likely less convenient for the pet owner to have to come back to get another urine sample to send out, but it’ll potentially save them some money!
References: 1) Ling, GV et al. Canine urinary tract infections: a comparison of in vitro antimicrobial susceptibility test results and response to oral therapy with ampicillin or with trimethoprim-sulfa. JAVMA. 1984; 185:277-281. 2) Winkens R et al. Validity of the urine dipslide under daily practice conditions. Fam Pract 2003; 20:410-412. 3) Scarparo C et al. Evaluation of the DipStreak, a new device with an original streaking mechanism for detection, counting, and presumptive identification of urinary tract pathogens. J Clin Microbiol 2002;40:2169-2175. 4) Ybarra WL, Sykes JE, Wang Y, et al. Performance of a veterinary urine dipstick paddle system for diagnosis and identification of urinary tract infections in dogs and cats. J Am Vet Med Assoc 2014;244(7):814-819.
Abbreviations: UTI = urinary tract infection UA = urinalysis QABC = quantitative aerobic bacterial culture UDP = urine dipstick paddle
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