Urine bacterial culture growth and association with urine sedimentation and clinical findings in cats with acute kidney injury

Abstract

Objectives

A urine culture is often pursued in cats with acute kidney injury (AKI) to screen for bacterial growth in the urine, but it can be cost prohibitive. The aim of the study was to determine the ability of a urinalysis and lower urinary tract signs (LUTS) to predict urine culture results in cats with AKI.

Methods

Ninety-seven cats with AKI were included in this study. This was a retrospective, observational study. Medical records from 2008 to 2018 were reviewed to identify cats with AKI that had a paired urinalysis and urine bacterial culture. The sensitivity, specificity, positive predictive value and negative predictive values of microscopic bacteriuria, pyuria, hematuria and the presence of LUTS for predicting urine culture results was calculated.

Results

Thirty-two percent of cats (n = 31) had a positive urine culture. Of these, 28 (90%) had bacteriuria, 21 (68%) had pyuria, 13 (42%) had hematuria and 10 (32%) had LUTS. Of the 42 cats without hematuria or pyuria, seven had a positive urine culture (17%). Bacteriuria had a high sensitivity (90%) and specificity (92%) for predicting urine culture bacterial growth. The absence of bacteriuria had a high negative predictive value for no bacterial growth (95%). The odds of a positive urine culture were increased with bacteriuria (odds ratio [OR] 114, 95% confidence interval [CI] 29–621; P <0.001), pyuria (OR 21, 95% CI 7–70; P <0.001) and LUTS (OR 5, 95% CI 1.7–16; P = 0.004). Hematuria was not associated with a positive culture (sensitivity 42%, specificity 52%).

Conclusions and relevance

Microscopic bacteriuria and pyuria on urine sediment evaluation and LUTS can be helpful for predicting bacterial culture results in cats with AKI and in settings where submitting a urine culture may not be financially feasible.

Keywords

Urinalysis urine culture bacteriuria pyuria hematuria acute kidney injury

Introduction

Acute kidney injury (AKI) encompasses a broad spectrum of diseases associated with a rapid onset of renal parenchymal injury resulting in fluid, electrolyte and acid–base derangements. AKI can occur in cats with normal kidneys and in those with pre-existing chronic kidney disease (CKD) as an acute exacerbation (acute-on-chronic kidney disease [AoCKD]).^1,2 AKI has a high mortality rate in dogs and cats with 50–60% survival to discharge, and pyelonephritis is a cause of AKI that has higher survival rates.^3–8 Careful screening for pyelonephritis using laboratory and imaging modalities is often pursued by veterinarians during the diagnostic evaluation of AKI.

The complete urinalysis with urine reagent dipstick evaluation, urine specific gravity (USG) by refractometry and microscopic examination of urine sediment can be performed in house, and results can be obtained within 1 h of collection. Complete urinalysis aides in the diagnosis of AKI and is a first-line screening test to evaluate for evidence of a bacterial infection, such as bacteriuria, pyuria or hematuria on microscopic examination. Veterinarians often perform a urine culture in cats with AKI, and, if positive for bacterial growth, susceptibility profiles can guide antimicrobial therapy.^9–12 However, owners may not be prepared financially for the sudden cost of veterinary care in an acute setting, which may cause veterinarians to prioritize diagnostics. In this situation, the results of the urinalysis can heavily influence whether a clinician will prescribe an antimicrobial or not.

Urinalysis has been shown to have predictive value in urine culture results in a stable setting, such as healthy cats or those with CKD; however, studies specifically on the predictive power of urinalysis for culture results in an acute setting are lacking. A recent study showed that in cats with variable USGs, microscopic bacteriuria and pyuria had high specificity (96% and 88%, respectively) and moderate sensitivity (64% and 71%, respectively) in predicting a positive urine culture (⩾1000 colony-forming unit [CFU]/ml).¹³ Studies have also evaluated the utility of urinalysis to predict the results of a urine culture in cats with disease, including CKD, and found that microscopic bacteriuria and pyuria were associated with a positive urine culture.^14,15 In healthy cats and cats with CKD, lower urinary tract signs (LUTS) suggests lower urinary tract disease and supports the screening for a urinary tract infection with a urinalysis, and, in some cases, a urine culture.⁹ The overall prevalence of positive cultures in CKD patients has been shown to be lower than in cats in an acute-on-chronic setting, supporting differences in pathogenesis, which could be reflected on urinalysis.^5,15–17 Given that urinalysis findings can be predictive of positive culture, higher value could potentially be placed on the negative predictive power of the urinalysis in acute settings where a veterinarian must prioritize diagnostics due to the financial constraints of the owner. Therefore, the objective of this study was to determine the association between bacterial growth in urine, urinalysis findings and the presence of LUTS in cats with AKI. We hypothesized that urine sediment results and LUTS may be of benefit to veterinarians in predicting a negative urine culture.

Materials and methods

Cat population

A retrospective medical record review of cats evaluated at two veterinary teaching hospitals (Colorado State University and Oregon State University) between 2008 and 2018 was performed to identify cats that had a urine culture performed. Cats were included if they presented with an acute (⩽7 days) onset of clinical signs compatible with uremia (eg, vomiting, inappetence and lethargy), had a serum creatinine concentration >1.6 mg/dl and USG <1.035, and at least one of the following findings to support AKI: (1) increase in serum creatinine >20% above the last serum creatinine measurement; (2) urinary markers consistent with acute injury (eg, glucosuria in face of normoglycemia, any type of casts >2 per high-powered field [HPF]); (3) at least one ultrasonographic finding consistent with AKI (nephromegaly, ureteral dilation, pyelectasia [⩾3 mm], increased renal echogenicity, retroperitoneal and peritoneal fluid, hypo- or hyperechoic echogenicity at the corticomedullary junction);¹⁸ or (4) renal histopathology consistent with AKI.¹⁰ Cats were excluded if a urinalysis was not performed at the time of urine collection for culture, if immunosuppressant medications were being administered at time of presentation, if urine was collected by a route other than cystocentesis or if they had received antibiotics within 7 days of presentation.

Etiology

The medical records of cats with AKI were evaluated to determine the presence of underlying CKD. AoCKD was defined as known history of CKD (in accordance with the International Renal Interest Society)² and at least one of the following: increase in serum creatinine >20% above the last creatinine measurement; or ⩾2 findings consistent with CKD on renal imaging in at least one kidney (reduced renal corticomedullary differentiation, decreased kidney size [<2.8 cm], presence of renal cysts, presence of hyperechoic renal infarcts, irregular renal contour, increased renal cortical echogenicity).¹⁹ The putative etiology of the AKI was classified as either pyelonephritis, ureteral obstruction, pancreatitis, other etiology or unknown. In cats with AKI, pyelonephritis was suspected based on a positive urine culture or confirmed based on culture of urine collected by pyelocentesis. Ureteral obstruction was suspected based on ultrasound findings of hydroureter or pyelectasia (⩾3 mm), or both, with imaging findings confirming intraluminal (ie, ureterolith and ureteral stricture) or extraluminal (ie, circumcaval ureter) obstruction. Etiology was classified as secondary to pancreatitis if other causes of renal injury were excluded (ie, pyelonephritis and ureteral obstruction) and the cat had abdominal imaging consistent with pancreatitis with a positive feline pancreatic lipase immunoreactivity test, or had histopathologic findings consistent with pancreatitis on ante- and/or post-mortem sampling. AKI etiology was classified as other in cases that did not fit any of the definitions listed above (eg, toxin ingestion, sepsis, trauma and low output cardiac failure). AKI etiology was classified as unknown if an etiology was not identified. If more than one etiology was suspected, both etiologies were recorded.

Data collected

Records were reviewed for patient signalment, presenting owner complaint, body weight, owner-reported LUTS, clinicopathologic data and, when available, post-mortem necropsy and renal histopathology results within 3 months of AKI diagnosis. Lower urinary tract signs were defined as one or the combination of the following: pollakiuria, stranguria, periuria or dysuria. Clinicopathologic data recorded included manual neutrophil count, serum blood urea nitrogen (BUN), creatinine and glucose, and urinalysis and urine culture results, including bacteria identification, semi-quantitative (ie, 1+, 2+, 3+ or 4+) or quantitative bacterial colonies (CFU/ml). Based on reference intervals from both institutions, neutrophilia was defined as a neutrophil count >12 k/µl, elevated creatinine concentration was defined as >2.4 mg/dl and elevated BUN concentration was defined as >35 mg/dl. Data recorded from the urinalysis included USG, urine dipstick protein, pH and glucose readings, red and white blood cells per HPF, presence of casts and morphology, bacterial presence and morphology of bacteria if noted (cocci vs rod). If an ultrasound of the urinary tract was performed at the time of evaluation, findings were recorded.

Urinalysis and urine culture methodology

Urine was collected by cystocentesis and stored with refrigeration until analysis at each in-house laboratory (Colorado State University Veterinary Diagnostic Laboratory, Fort Collins, CO, USA; Oregon Veterinary Diagnostic Laboratory, Corvallis, OR, USA). At both institutions, urinalysis was performed within 6 h of collection. A manual refractometer was used for USG and wet-mount microscopic examination of unstained sediment was performed by trained laboratory technicians. At Colorado State University, a urine analyzer (Cobas U411 with Roche Chemstrip 10UA; Roche Diagnostics) is used for urine dipstick analysis, and a standard dipstick (Multistix 10 SG; Siemens) is used at Oregon State University. Hematuria was defined as >5 red blood cells (RBCs) per HPF present in the urine sediment. Pyuria was defined as >5 white blood cells per HPF present in the urine sediment. An active urinary sediment was defined as either pyuria and/or hematuria (>5 RBC/HPF). Bacteriuria was defined as bacteria noted on microscopic evaluation of the urine sediment.

Urine cultures were performed within 24 h of urine collection. Lack of bacterial growth after 48 h was considered a negative urine culture. Bacterial growth on urine culture >100 CFU/ml was considered a positive urine culture. Details on urine culture methodology at each institution is found in the supplementary material.

Statistical analysis

Analyses were performed using GraphPad Prism 9.2.0 and SAS 9.4. The percentage of cats with pyuria, hematuria and bacteriuria on urinalysis, LUTS and positive urine culture was calculated. The normality of data was assessed by evaluation of Q-Q plots. An unpaired Student’s t-test was used to compare select clinicopathologic data between cats with positive urine culture and cats with a negative urine culture. Exact binomial confidence limits were calculated for positive urine culture percentages and sensitivities, specificities, positive predictive values (PPVs) and negative predictive values (NPVs). Logistic regressions were used to test for effects of the presence of active urine sediment, hematuria, pyuria, bacteriuria or LUTS on the odds of having a positive urine culture. Log-likelihood ratio test P values and profile likelihood confidence intervals (CIs) were reported. A P value <0.05 was considered significant.

Results

The database identified 1592 medical records for cats that had a urine culture collected between 2008 and 2018 during a visit to either academic institution. The medical records were screened based on criteria described above, which led to further review of 166 medical records by two authors (KS and SCS). After final review, 69 cats were excluded. Six were excluded owing to the unavailability of full urinalysis data. Eighteen cats were excluded because a urine sample was not obtained for culture. Forty cats were excluded for having a final diagnosis that was not AKI based on our study definitions. Four cats were excluded because they received antibiotics prior to presentation. One cat was excluded because it was receiving immunosuppressants on presentation. A total of 97 cats met the inclusion criteria and were enrolled. The mean ± SD age of the cohort was 10.6 ± 4.8 years. Fifty-six cats (58%) were spayed females, 36 (37%) were neutered males and five (5%) were intact females. The most common breeds were domestic shorthair (n = 62; 64%) and domestic longhair (n = 20; 21%). Other breeds represented included domestic mediumhair, Siamese, Ragdoll and British Shorthair (<5 each).

The most common presenting complaints were anorexia (55%), weakness or lethargy (42%), and vomiting (31%). The mean ± SD serum creatinine and BUN on presentation were 8.4 ± 4.8 mg/dl and 134.3 ± 65.4 mg/dl, respectively. Twelve cats (n = 12; 12%) had glucosuria, of which 11 (92%) had glucosuria in the absence of hyperglycemia (>180 mg/dl). Fifty-three cats (n = 53; 55%) had a urine dipstick qualitative protein concentration >1+. Putative AKI etiologies were ureteral obstruction in 34 cats (35%), pyelonephritis in 23 (24%), pancreatitis in four (4%) and other in 12 (12%). Eleven cats had multiple suspected etiologies. An etiology was not identified in 35 cats. Most cats (n = 69; 71%) fit the definition of AoCKD.

The proportion of cats with an inactive urine sediment, active urine sediment, hematuria, pyuria, bacteriuria, LUTS and a positive urine culture is reported in Table 1. Of the 97 cats with AKI, 45 cats had hematuria (46%), 33 had bacteriuria (34%) and 27 had pyuria (28%) on urine sediment evalulation. Fifty-five cats had an active urinary sediment (pyuria and/or hematuria; 57%) and 42 (43%) had an inactive urinary sediment. Sixteen cats (17%) cats had LUTS reported by the owner at presentation, of which a majority had an active urine sediment (n = 13/16; 81%) and most had a positive urine culture (n = 10/16; 63%). Thirty-one cats (32%) had bacterial growth on urine culture, of which 28 (90%) had bacteriuria, 21 (68%) had pyuria, 13 (42%) had hematuria and 10 (32%) had LUTS. Of the 42 cats with an inactive urine sediment (ie, without pyuria or hematuria), seven cats (17%) had a positive urine culture, of which two cats also lacked microscopic bacteriuria. Of the 69 cats with AoCKD, 22 (32%) had bacterial growth on culture.

Table 1

Acute kidney injury cats with hematuria, pyuria or bacteriuria, an inactive (absence of pyuria and hematuria) or active (presence of pyuria or hematuria) urine sediment, and the presence or absence of lower urinary tract signs and a positive urine culture

Population	Positive urine culture (n)	Percentage with positive urine culture (95% CI)
All cats	31/97	32 (23–42)
Hematuria (>5 RBC/HPF)	13/45	29 (16–44)
Pyuria (>5 WBC/HPF)	21/27	78 (58–91)
Bacteriuria	28/33	85 (68–95)
Inactive urine sediment	7/42	17 (7–31)
Active urine sediment	24/55	44 (30–58)
Absence of LUTS	20/80	25 (16–36)
Presence of LUTS	10/16	63 (35–85)

CI = confidence interval; RBC = red blood cell; HPF = high-powered field; WBC = white blood cell; LUTS, lower urinary tract signs

Of the 31 cats with a positive urine culture, Escherichia coli (n = 27; 87%) was the most common bacteria, of which four also had concurrent growth of Enterococcus species (n = 1), Staphylococcus pseudintermedius (n = 1) and Streptococcus species (n = 2). In the remaining four cats, urine culture was positive for Enterococcus species (n = 2), for S pseudinermedius and coagulase-negative Staphylococcus mixed culture (n = 1) and for S pseudintermedius (n = 1). Most cats had bacterial growth >10,000 CFU/ml (n = 26/31; 84%).

The sensitivity, specificity, PPV and NPV of microscopic bacteriuria, pyuria, hematuria, an active urinary sediment and LUTS to predict positive urine culture results is shown in Table 2. The odds of having a positive urine culture were increased with an active urine sediment (odds ratio [OR] 3.9, 95% CI 1.5–11; P = 0.004), bacteriuria (OR 114, 95% CI 29–621; P <0.001), pyuria (OR 21, 95% CI 7–70; P <0.001) and the presence of LUTS (OR 5, 95% CI 1.7–16; P = 0.004). Hematuria was not found to increase the odds of a positive urine culture (OR 0.8, 95% CI 0.33–1.8; P = 0.6).

Table 2

Sensitivity, specificity, positive predictive value (PPV) and negative predictive value (NPV) of urinalysis and the presence of lower urinary tract signs (LUTS) in predicting urine culture results in cats with acute kidney injury

Indicator	Sensitivity(95% CI)	Specificity(95% CI)	PPV (95% CI)	NPV (95% CI)
Active urine sediment	77 (59–90)	53 (40–65)	44 (30–58)	83 (69–93)
Bacteriuria	90 (74–98)	92 (83–97)	85 (68–95)	95 (87–99)
Hematuria (>5 RBC/HPF)	42 (25–61)	52 (39–64)	29 (16–44)	65 (51–78)
Pyuria (>6 WBC/HPF)	68 (49–83)	91 (81–97)	78 (58–91)	86 (75–93)
Presence of LUTS	33 (17–53)	91 (81–97)	63 (35–85)	75 (64–84)

CI = confidence interval; RBC= red blood cell; HPF = high-powered field; WBC = white blood cell

Table 3 describes select clinicopathologic parameters and urinalysis findings stratified by urine culture result. Of select clinicopathologic and urinalysis parameters, the neutrophil count (P <0.0001) and the serum creatinine concentration (P = 0.036) were found to be significantly different between the positive culture and negative culture groups (P <0.0001).

Table 3

Select clinicopathologic and urinalysis parameters in acute kidney injury cats with (n = 31) and without (n = 66) bacterial growth in the urine

Laboratory finding	Cats with bacterial growth in urine (n = 31)	Cats with no bacterial growth in urine (n = 66)	Mean difference ± SEM	P value
Body temperature (°F)	100.0 (99.0–101.0)	99.9 (99.4–100.3)	0.12 ± 0.46	0.8
Neutrophil (k/µl)	20.3 (15.9–24.8)	10.2 (8.4–11.9)	10.2 ± 2.0	<0.0001
Creatinine (mg/dL)	6.4 (5.3–7.4)	10.4 (7.9–12.9)	4.0 ± 1.9	0.036
BUN (mg/dl)	126.3 (101.3–151.3)	136.8 (120.5–153.1)	10.5 ± 14.7	0.5
USG	1.015 (1.012–1.017)	1.014 (1.012–1.015)	0.001 ± 0.001	0.3
Urine pH	5.7 (5.4–6.0)	5.7 (5.6–5.9)	0.003 ± 0.16	1.0

Body temperature, neutrophil count, creatinine, blood urea nitrogen (BUN), urine specific gravity (USG) and urine pH are reported as mean (95% confidence interval). Neutrophilia defined as a neutrophil count >12 k/µl, elevated creatinine defined as >2.4 mg/dl and elevated BUN defined as >35 mg/dl based on the laboratories’ reference intervals

Thirteen cats (13%) had a post-mortem examination performed with renal histopathology within 3 months of initial presentation. Of the 13 cats, one was euthanized within 24 h of hospitalization and had confirmed pyelonephritis on histopathology with observed intracellular bacteria. This cat had pyuria and hematuria on urine sediment microscopic evaluation and had bacterial growth on urine culture (E coli, 120 CFU/ml). No tissue cultures of renal tissue were obtained during post-mortem examination.

Discussion

Our study showed that bacteriuria and pyuria on microscopic urine sediment evaluation were predictive of urine culture results in cats with AKI. Hematuria on urine sediment evaluation was not found to be predictive of urine culture results, which may be due to blood contamination secondary to iatrogenic trauma during cystocentesis or other non-infectious etiologies (eg, urolithiasis and feline idiopathic cystitis). Previously published studies in cats report similar findings.^13–15,20 In particular, a recent study in cats with variable USGs showed that bacteriuria had high PPV (84%) and NPV (90%) for predicting urine culture results, similar to our study. Our study showed a higher PPV for pyuria (78%) compared with the previous study (64%), with both showing a similarly high NPV for pyuria.¹³ Our findings support that a microscopic evaluation of urine is a useful screening tool and can be used to predict urine culture results in cats with AKI. Cats with AKI and without pyuria and bacteriuria on urine sediment analysis are unlikely to have a positive urine culture. However, a clinical suspicion for a bacterial etiology beyond a urinalysis (eg, fever, neutrophilia and renal pain) should be considered when deciding whether to perform a urine culture in this patient population.

The prevalence of LUTS in cats with AKI is not well reported in the literature; however, a recent study in cats with AoCKD reported a prevalence of LUTS of 8%.⁵ LUTS support a urinary tract disease in cats, such as an infection. A minority of cats with AKI in our study had LUTS (n = 16/96; 17%), most of which had a positive urine culture (n = 10; 63%). These findings are similar to a previous study in cats with CKD, hyperthyroidism and diabetes mellitus, which reported that 55% of cats with LUTS had a positive urine culture.¹⁵ Although we found that the presence of LUTS increased the odds of a positive urine culture, some cats with AKI in our study without reported LUTS had a positive urine culture (25%). Therefore, the lack of LUTS in a cat with AKI should not lessen a veterinarian’s suspicion for a positive urine culture. The limitation of using LUTS to predict culture results is that veterinarians are dependent on cat owners appreciating and reporting the presence of LUTS in their cat. If cats are urinating in the litter box, some owners may not recognize that their cat is urinating inappropriately. Frequency of urination, stranguria, urine color and volume of urine may all go unnoticed. Litter boxes are often placed in a part of the house that is less frequented and some cats urinate outside if allowed access. Thus, the presence of LUTS in cats with AKI included in our study may be higher than reported by the owner.

The International Society for Companion Animal Infectious Disease describes a tentative diagnosis of pyelonephritis as a positive aerobic bacterial culture in context with a combination of systemic signs, including, but not limited to, fever, polyuria/polydipsia, azotemia and peripheral neutrophilia.⁹ In this study, 26 cats had a putative diagnosis of pyelonephritis that was based on the diagnosis of AKI in combination with the presence of a positive urine culture collected by cystocentesis. Although ascending infection from lower urinary tract infection is a cause of pyelonephritis, some cats could have pyelonephritis with a negative urine culture, a finding appreciated in a small study in cats.²¹ Culture-negative pyelonephritis occurs in people and is diagnosed based the presence of a fever, abnormal urinalysis and a 99m-Tc-dimercaptosuccinic acid renal scan used to detect pyelonephritis lesions.^22–24 Culture-negative pyelonephritis is not well reported in the veterinary literature given the lack of invasive reference standard testing (ie, histopathology) in the limited studies of AKI in cats,^3–5,7 but presumably could occur, especially in those cats that have been exposed to recent antimicrobials. Additionally, while a positive urine culture from a sample collected by cystocentesis result may increase suspicion for pyelonephritis in AKI cases, a positive urine culture in a cat with AKI does not confirm pyelonephritis. Pyelonephritis is definitively diagnosed by culture of urine obtained by pyelocentesis or a renal tissue culture with consistent histopathology. Therefore, it is plausible that a portion of these cats did not have pyelonephritis, but rather subclinical bacteriuria or a urinary tract infection isolated to the lower urinary tract.

In our study, one cat had confirmed pyelonephritis with intracellular bacilli on post-mortem renal histopathology. Prior to euthanasia, the cat was found to have low bacterial growth of E coli on urine culture (120 CFU/ml). Cut-off values for clinically significant urine culture results have been proposed for dogs and cats, with significant bacteriuria in voided samples being >100,000 CFU/ml vs proposed cut-offs of 100 to 1000 CFU/ml in cystocentesis samples.^25–28 Although low bacterial culture counts are found in scenarios of contamination, this finding cannot exclude a clinically relevant infection. In people, up to 90% of acute pyelonephritis cases have a positive urine culture (>10,000 CFU/ml), but it is also noted that lower urine culture CFUs can be of concern in some people, particularly in men and pregnant women.^29,30 Thus, in the presence of AKI, low bacterial urine culture counts, especially monocultures, from a sample obtained by cystocentesis in a cat with AKI should be interpreted within the clinical context. There are several causes of low bacterial counts in patients with urinary tract infections, including early infection, as is reported in women, and varied urine output between patients, with higher urine output leading to diluted colony counts or increased bacterial clearance.³¹ The relationship between positive culture and pyelonephritis, while established in human medicine, still requires more prospective studies when compared against reference standard diagnostics (eg, histopathology) to establish correlation.

Some cats had discordant results between the finding of bacteriuria on urinary sediment evaluation and urine culture, a finding noted in previous studies in cats.^15,20 There are several reasons for finding bacteriuria on urinary sediment evaluation with a negative urine culture. First, the bacteria detected on the urinary sediment evaluation are non-viable because of recent antibiotic therapy; this is unlikely in our study, given our exclusion criteria. Bacterial viability can also decrease if the sample is exposed to extreme temperatures or pH extremes (⩾9 or ⩽4).³² Secondly, cellular debris can also be mistaken for bacteria (known as pseudobacteria).³³ Third, Brownian motion (random motion of small colloidal particles) can also be mistaken for cocci bacteria, especially with unstained urine sediment examination. Other possibilities for this discordance include anaerobic bacteria which were visualized but could not grow on aerobic media, or bacterial contamination of the sediment stain in laboratories that use a stained sediment for microscopic evaluation. A few cats had a positive urine culture and no bacteriuria reported on urinalysis. We hypothesize that bacteriuria in these cases could have been missed on urinary sediment examination due to localized pyelonephritis or misrepresentative samples. Wright-Giemsa or Gram staining of urine samples can improve detection of bacteriuria compared with an unstained wet mount.^33,34

Our retrospective study had several limitations. Considering the difficulty of diagnosing non-azotemic AKI, we opted to include only those cats with serum creatinine >1.6 mg/dl. In studies of dogs and cats with histopathology-confirmed pyelonephritis, 29% of dogs and 65% of cats had azotemia.^21,35 Thus, our inclusion criteria likely introduced selection bias by excluding non-azotemic cats with AKI. There is additional inherent selection bias by only including cats with a urinalysis and urine culture performed, as all cats presented with a clinical indication for performing these diagnostics. Second, the reported etiologies were putative and based on information found upon the retrospective evaluation of medical records. Definitive diagnosis for some etiologies was not possible as confirmatory testing (eg, antegrade pyelogram for ureteral obstruction or renal tissue culture for pyelonephritis) was not available for most cases. Last, while this paper demonstrates that urinary microscopic findings can largely predict culture results in cats with AKI, assumptions cannot be made at this time as to the clinical importance of not performing a urine culture in the individual cat with AKI that lacks bacteriuria, an active urine sediment or LUTS. Further prospective studies are recommended to evaluate the effect of a positive culture on outcome and survival in cats with AKI.

Conclusions

Bacteriuria and pyuria on microscopic evaluation of the urine sediment and the presence of LUTS can help veterinarians in determining the likelihood of bacterial culture results in cats with AKI. The absence of bacteriuria, pyuria and LUTS does not exclude an infection, so all cats must be considered in clinical context as to whether a urine culture should or should not be submitted. This consideration is a common conundrum encountered by veterinarians dealing with cats with AKI when finances are a major limitation. Future prospective studies evaluating the clinical significance of a positive urine culture and subsequent treatment paths on outcome and survival in cats with AKI are strongly recommended to provide context to screening tools, such as the urinalysis, in these cases.

Supplemental Material

Urine culture protocol at Oregon State University (Corvallis, OR) and Colorado State University (Fort Collins, CO) Veterinary Diagnostic Laboratories.

Footnotes

Acknowledgements

Results were presented in abstract form at the 2021 Annual Forum of American College of Veterinary Internal Medicine On-Demand.

Accepted: 13 May 2022

Supplementary material

The following file is available online:

Urine culture protocol at Oregon State University (Corvallis, OR) and Colorado State University (Fort Collins, CO) Veterinary Diagnostic Laboratories.

Conflict of interest

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

Ethical approval

The work described in this manuscript involved the use of non-experimental (owned or unowned) animals. Established internationally recognized high standards (‘best practice’) of veterinary clinical care for the individual patient were always followed and/or this work involved the use of cadavers. Ethical approval from a committee was therefore not specifically required for publication in JFMS.

Informed consent

Informed consent (verbal or written) was obtained from the owner or legal custodian of all animal(s) described in this work (experimental or nonexperimental animals, including cadavers) for all procedure(s) undertaken (prospective or retrospective studies). No animals or people are identifiable within this publication, and therefore additional informed consent for publication was not required.

ORCID iD

Stacie C Summers

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