By Juliette Bouillon, DMV, MVetSc, DACVIM (small animal internal medicine)

What to expect from urinalysis?

Urinalysis is a useful tool to investigate systemic illnesses and urinary tract disorders. It can provide information about renal function, acid-base balance, diabetes mellitus, liver disease, hemolysis, or muscle damage.^12,48 Urinalysis may allow differentiation between inflammation, infection, and/or neoplasia.^12,48 It also enables identification of possible predisposing factors for UTIs, such as low urine‐specific gravity (USG) or glucosuria.¹²

A complete urinalysis includes recording of the collection method, visual inspection (volume, color, odor, transparency), determination of USG, biochemical analysis with a urine dipstick, and evaluation of the sediment.^10,49

Recording of the collection should be systematic, as it guides interpretation of the findings. For example, cystocentesis and urinary catheterization may induce low level blood contamination.^12,48 Voided samples will reflect urogenital tract health status and, although first-stream free catch is indicated to investigate urethral pathology, midstream free catch is best to limit contamination.^12,48 Spermatozoa and blood may be identified depending on the patient's sexual status.^12,48

Normal urine should be yellow to amber in color and transparent.⁴⁹ Color intensity depends on urine volume and concentration.⁴⁹ There are many causes of urine coloration changes that are described elsewhere.⁴⁹ Discoloration of urine is most commonly associated with hematuria, hemoglobinuria, myoglobinuria, and bilirubinuria, which cause red, brown, or black urine coloration.⁴⁹ Hematuria can be confirmed by identification of erythrocytes on urine sediment. Absence of hematuria indicates the color change results from pigmenturia. Examination of the color of plasma and review of blood work can help distinguish whether hemoglobin, myoglobin, or bilirubin are present and is described elsewhere.⁴⁹

Urine specific gravity should be determined with a refractometer, as the reagent pad of dipstick analysis is not reliable.^7,10,12,49 If urine is discolored or turbid, the specific gravity can be measured on the supernatant after centrifugation.⁴⁹ Urine specific gravity is used to assess the capacity of renal tubules to concentrate and dilute urine to maintain fluid homeostasis.⁵⁰ Therefore, a USG <1.035 in cats and <1.030 in dogs may reflect inappropriate concentrating ability, whereas hyposthenuria (USG <1.006) indicates active urinary dilution, and therefore, adequate kidney function.⁵¹ Interpreting USG should take into consideration the patient's hydration status and serum biochemical findings.⁴⁹ Indeed, while urine concentration can vary in healthy patients,^49,50 minimally concentrated urine in a dehydrated patient always reflects poor urine concentrating ability and should prompt further investigation. Highly concentrated urine in an azotemic patient is indicative of prerenal azotemia (e.g. dehydration, hypovolemia, etc.), while isosthenuria (USG = 1.008 to 1.012) in a well-hydrated azotemic or non-azotemic patient indicates the kidneys do not modify the concentration of urine from the concentration of plasma. Glucosuria invalidates the measurement of urine specific gravity as a reflection of concentration, as glucose particles increase the refractive index and, therefore, in the presence of glucosuria, USG is inaccurate.⁴⁹

A decreased USG may reflect upper urinary tract involvement or concurrent disorders predisposing to UTI or SB (e.g. endocrinopathy).^10,12 Concentrated urine is associated with a high osmolality due to high concentrations of urea, organic acids, and antimicrobial peptides, which give urine a bacteriostatic effect.²² While some studies have suggested a decreased urine specific gravity is a risk factor for UTIs or SB,^{11,19,21,30,32} other studies did not confirm this finding,^31,33,39,52 which suggests further mechanisms may be involved in the predisposition to UTIs. One study demonstrated neutral to decreased canine urine pH and decreased canine urine concentration positively influenced Escherichia coli growth in vitro.⁵³

Urine dipstick analysis allows testing for the presence of proteins, glucose, ketones, hemoglobin, bilirubin, urobilinogen, acetone, nitrite, leukocytes, pH, and USG. The chemical pads for detection of leukocytes, nitrite, urobilinogen, and specific gravity are inaccurate in animals and should be ignored.^{7,10,12,21,49} Alkaluria may be suggestive of UTI associated with urease-producing bacteria or can be associated with diet, medication, or aging urine specimen.⁴⁹ A visible colorimetric reaction for proteinuria has a low specificity and should be interpreted with pH, USG, and urine sediment examination.^49,54 False positive results occur frequently with highly concentrated or alkaline (pH>7.5) urine.⁵⁴ Therefore, trace amounts of protein or 1+ protein in a patient with well concentrated urine is unlikely to be clinically significant, whereas it should not be ignored in a patient with dilute urine.⁴⁹ Proteinuria with an active sediment is likely associated with renal (glomerular or tubular) or post-renal inflammation, hemorrhage, and/or infection.⁵⁴ Glucosuria may reflect diabetes mellitus or renal tubulopathy and has been shown to predispose dogs to emphysematous cystitis.^7,55

Microscopic examination of urine sediment allows evaluation of urine cellular components (leukocytes, erythrocytes, epithelial cells), crystals, and bacteria. Healthy animals have less than three to five leukocytes per high-power field. In urine collected by catheterization or voiding, higher numbers may be observed, as additional leukocytes may originate from the urethra or genital area.^48,49 Significant pyuria (greater than three to five leukocytes per high-power field) reflects inflammation and/or infection. The former may result from infection, urolithiasis, irritation, and/or neoplasia.^48,49

Similarly, few erythrocytes may be present in urine of healthy individuals (zero to five under high-power field). Significant hematuria may reflect iatrogenic hemorrhage following cystocentesis or catheterization. Pathological causes of hematuria include hemorrhage in the urinary system secondary to trauma, coagulopathies, inflammation, infection, necrosis, or neoplasia.^48,49

The transitional epithelial cells constitute the urothelium from the renal pelvis to the urethra.⁵⁶ Natural desquamation of the epithelial cells occurs with normal cell turnover and, as a result, few transitional epithelial cells may be present in the urine of healthy animals.⁴⁸ High numbers of dysplastic
cells with greater or lesser criteria of malignancy may be seen with irritation, inflammation, or neoplasia.⁴⁸ Renal epithelial cellular casts or white blood cells casts develop from cells' entrapment in mucoproteins within the renal tubules. These casts respectively indicate renal tubular damage and renal inflammation.^48,49

Struvite crystalluria is frequent in dogs and cats.⁴⁹ Crystalluria without urolith formation is not considered pathologic because crystals do not damage the urothelium.^49,57 In dogs, however, the presence of struvite crystals concomitant with a UTI deserves particular attention, as struvite crystals and uroliths can form following urine alkalization in the presence
of urease-producing bacteria (e.g. Staphylococcus spp.).^57,58 In cats, struvite crystalluria is suspected to be metabolic in origin.⁵⁷ Therapeutic diet trial may not be indicated unless the cat presents severe crystalluria and signs of LUTD.⁵⁹

Bacteria can be detected on sediment examination. Bacterial contamination may be associated with sample collection (catheterization, free catch) or surface contamination.¹² Intracellular bacteria confirm infection, rather than contamination. However, bacteria (particularly cocci) may be difficult to differentiate from particles of debris.^48,49 Evaluation of dried urinary sediment with a modified Wright-stain significantly improves sensitivity and specificity (Sens 82.8 percent; Spec 98.7 percent) compared with the wet-unstained method (Sens 75.9 percent; Spec 56.7 percent).⁶⁰ Romanowsky-stained urine sediment may also improve visualization of bacteria.⁴⁸ The modified Sternheimer-Malbin urinary stain (Sedi-Stain, Becton Dickinson) is water-based and can be added to the sediment prior to resuspension. It may help to visualize erythrocytes, leukocytes, and casts. However, it only stains dead bacteria and may contain an overgrowth of bacteria or fungal elements, leading to misinterpretation.^7,48

Sediment interpretation and UTI

Although not synonymous with a UTI, pyuria may increase the suspicion for infection.⁵⁷ Clinicians should interpret this finding in light of the patient's clinical signs and complete sediment analysis, as pyuria is also frequently observed in patients with SB,^4,33,35 while pyuria alone is frequently observed in cats with feline idiopathic cystitis.⁶¹ The finding of hematuria in the absence of pyuria or bacteriuria on sediment examination does not support the presence of bacterial growth in urine in dogs with proteinuria and hyperthyroid cats with SB.^62,63 In contrast, one study determined any amount of hematuria increases the odds ratio for bacterial UTI in cats with CKD, DM, HT, or LUTD.³⁹

One study found pyuria and bacteriuria were neither sensitive (52.9 percent and 72.9 percent, respectively) nor specific (85.5 percent and 67.7 percent, respectively) for detection of UTIs.³² Conversely, the finding of pyuria and bacteriuria or pyuria, bacteriuria, and hematuria were found to be predictive of positive urine culture in several studies^{25,31,35,39,62,63} Regardless, a QUC is required for definitive diagnosis.^4,12

When to do a quantitative urine culture

Quantitative urine culture should be performed in patients with signs of LUTD even when the urine sediment is inactive. Indeed, one study found three percent of dogs with signs of LUTD and an inactive urine sediment had a positive urine culture.⁶⁴ In this same study, another three percent of dogs with an inactive sediment had a positive urine culture, but no clinical signs of LUTD. In accordance with the current guidelines for SB management, the author concluded urine culture in canine patients without clinical signs of LUTD and with an inactive urine sediment is not indicated.⁶⁴ In patients without clinical signs of LUTD, indications for screening are patients suspected with pyelonephritis, investigation of the bladder as a source of bacteremia/septicemia, dogs with suspected struvite urolithiasis, or patients who will undergo a surgical or minimally invasive procedure involving penetration or transection of the urinary tract.⁴

Recent studies have questioned whether animals with known risk factors of UTIs, such as CKD or endocrinopathies, should be routinely screened for SB.^33,63 One study showed culturing the urine in feline patients with CKD, DM, HT, or LUTD with inactive sediment is of low diagnostic yield.³⁹ Another study confirmed hyperthyroid cats are not at increased risk for SB.⁶³ Similarly, no association was detected between SB and azotemia or renal disease at necropsy in cats with CKD.³³ Although pyelonephritis may aggravate a preexisting CKD, the number of patients diagnosed with chronic pyelonephritis among human patients with renal failure in several studies is low (10 to 26 percent).^65-69 Moreover, a retrospective case controlled study of 99 dogs with acute renal failure showed only two dogs developed pyelonephritis secondary to the development of acute renal failure.⁷⁰ The current guidelines for the diagnosis and management of bacterial urinary tract infections in dogs and cats⁴ discourage the antimicrobial treatment of SB. They also state that, although treatment should be considered in patients suspected with high risk of complications, it should be used parsimoniously.

Another recent study investigated whether a QUC should be routinely performed to rule out a UTI as a post-renal cause of proteinuria in dogs with proteinuria and an inactive urine sediment.⁶² The authors found only 2.7 percent of dogs with proteinuria and an inactive sediment had a positive urine culture. The majority of these dogs had a history of LUTD, which would have prompt a QUC, leaving only 1.8 percent of dogs with an undiagnosed bacterial growth in urine if QUC was not routinely performed in these patients. This study concluded a QUC might not be necessary in the evaluation of all proteinuric dogs and that it should be performed in proteinuric dogs with an active urine sediment or evidence of LUTD.

What is the best time to collect urine for culture?

Ideally, urine should be sampled prior to the start of any antimicrobial therapy. If the patient is already receiving antibiotics, the treatment should be discontinued for three to five days before performing urine culture to minimize inhibition of in vivo and in vitro bacterial growth.^10,71

What are acceptable methods of urine collection for culture?

Guidelines for the diagnosis and management of bacterial UTIs in dogs and cats⁴ state urine for culture should be collected by cystocentesis (ultrasound-guided or with manual palpation and immobilization) unless there is a contraindication. Main contraindications to cystocentesis are summarized in Table 2 (below).^48,72-74 It may be challenging to collect urine by cystocentesis in morbidly obese patients or in patients with pollakiuria, as the urinary bladder may be too small to allow proper collection.^4,10,12 In such cases, voided samples or sterile urinary catheterization can be used for culture; however, careful interpretation is required, as the risk of contamination is higher.^4,38 Indeed, one study in cats with clinical signs of acute feline LUTD found no bacteria were cultured from 79 percent of the samples taken by cystocentesis, 55 percent of the samples obtained by catheterization, and 17 percent of the samples obtained at voiding.⁷⁵ On the other hand, one study in cats with signs of LUTD did not show differences between the three sampling methods with regard to bacteriuria,²⁸ suggesting urinary catheterization can be suitable when performed in a strictly sterile manner. Nevertheless, interpretation of urinalysis and QUC results should always take urine sampling method into consideration.^4,12,48

What are appropriate handling and storage of urine for culture after collection?

Variation in handling and storage of urine samples may alter urinalysis and QUC interpretation.

Studies showed urine samples stored at room temperature provided inaccurate results for diagnostic QUC after 24 hours,^76,77 with 50 percent of the results being false positive and four percent being false negative.⁷⁶ Indeed, both multiplication or destruction of bacteria may occur within an hour of urine collection.^10,71 One study did not show significant difference in median bacterial concentration between urine samples stored for zero hours, and those stored for 24 hours at 4 C (39.2 F).⁷⁷ Therefore, when urine samples cannot be processed immediately following collection, they should be stored in a sealed plain sterile container, refrigerated right away following collection, and processed in a microbiology laboratory for QUC within 24 hours.^4,71,77

Quantitative urine culture interpretation

Urine should be submitted to the microbiology laboratory for aerobic and quantitative culture, with the method of collection documented/indicated on the lab request.⁴ When interpreting QUC results, clinicians should take into consideration the patient's clinical signs, urine cytology results, level of growth (colony count), bacterial species, and whether the infection is mono or polymicrobial.⁴ A level of growth >103 colony forming units (CFU)/ml is considered significant in urine collected by cystocentesis,^3,38 while a cutoff of 100,000 CFU/mL is significant in samples obtained by catheterization or voiding.⁴ Sample contamination should be suspected in patients with low colony count and bacterial species known to be skin commensals (i.e. coagulase negative staphylococci).⁴ Polymicrobial infections may also reflect contamination.¹⁰ Growth of common uropathogens (E. coli, coagulase-positive staphylococci, etc.) may be more consistent with UTIs, although a recent study showed bacterial species isolated in cats with SB do not differ from those in cats with UTIs,³³ hence the importance to consider the clinical presentation of the patient.

Can culture and susceptibility results help distinguish relapse from reinfection?

Genetic analysis of the bacteria is necessary to determine whether a recurrent UTI is a relapse or a reinfection.⁴ Indeed, one study in cats with recurrent UTIs and CKD showed antibiotic susceptibility profiles cannot be relied upon to distinguish between the two, as changes in antimicrobial susceptibility profile for the same bacteria were detected over time.⁷⁸ Unfortunately, genotyping is not commonly available. Therefore, in practice, clinicians compare pretreatment bacterial culture and susceptibility findings with follow-up culture results to determine whether the recurrent UTI is a relapse or a reinfection. However, they should bear in mind the limitations of this method.³ In part two of this series, we will look at treatment and prevention of bacterial UTI. To read on, click here.

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Juliette Bouillon, DMV, MVetSc, DACVIM (small animal internal medicine), is assistant professor of small animal internal medicine at Ross University School of Veterinary Medicine (RUSVM). Originally from Paris, France,
Dr. Bouillon completed her veterinary studies in Liège, Belgium, and graduated in 2011. After one year in a general small animal practice in Paris, she performed a rotating internship at the veterinary school in Nantes, France, followed by a specialized internship in private practice in Bordeaux, France. Following this, Bouillon spent a year in French Guiana in South America working as a small animal general practitioner before starting the residency and graduate program at the Western College of Veterinary Medicine (WCVM) in Saskatoon, Canada, where she completed a master of veterinary science degree for which she studied the effects of dexmedetomidine on glucose homeostasis in healthy cats. She completed her residency and became board certified as a Diplomate to the American College of Veterinary Internal Medicine (small animal internal medicine) in September 2019. Beyond veterinary medicine, Bouillon enjoys spending time with her family and practicing activities such as running, yoga, and snorkeling.