Bladder debris on renal and bladder ultrasound: a significant predictor of positive urine culture (2024)

Summary

Introduction

Renal and bladder ultrasound (RBUS) is recommended in the evaluation of children after an initial, febrile urinary tract infection (UTI) [1]. As a part of RBUS, the American College of Radiology recommends that the bladder walls and lumen be evaluated [2,3]. Findings may include stones, masses, or mobile debris in the lumen (Fig. 1). Bladder debris in particular is observed commonly, and is often attributed to infection. Small studies in symptomatic adult women have found a relatively high prevalence of debris (61.6–72.0%) [4,5], but data regarding children and asymptomatic adults are lacking. Thus, while debris is commonly considered to be abnormal, the clinical significance of this finding, and in particular its relationship with urinary tract infection, remains uncertain.

In some cases, intraluminal echoes on sonography may be technical artifacts. These can result from scanning technique or settings, or from physical limitations of the imaging modality itself [6]. Of greater concern is the scenario in which echogenic debris reflects a true pathologic process. Patients with bladder mucus, gross hematuria, inflammation of the genitourinary system, or sloughing of urothelium can demonstrate debris and linear stranding on ultrasound [7]. Such echogenic debris also has been reported in the setting of acute UTI with associated pyuria [8].

The management of patients in whom bladder debris is seen on RBUS is also uncertain and may be based on many clinical factors (e.g. ongoing fever, urinary symptoms, etc.). One recent study in adults concluded that bladder debris is not significantly associated with an abnormal urinalysis [3]. In that study, the imaging and urine sample were obtained asynchronously – within 1 week - and in a non-sterile fashion. Furthermore, active symptomatology at the time of study was not considered. Two other adult studies of symptomatic individuals suggested no association between sonographic debris and positive urine culture [4,5]. To date, the pediatric literature lacks any investigation of a potential association between bladder debris on RBUS and UTI (as indicated by positive urine culture). This is of particular importance for children undergoing initial work-up of a urinary tract infection (UTI), for whom the risk of UTI may be elevated, and in whom bladder debris may be an indicator of persistent or recurrent infection.

At our institution, it is standard practice to send a catheterized urine specimen for culture on all patients undergoing voiding cystourethrogram (VCUG), and many children undergo VCUG and RBUS on the same day as part of their work-up for a prior UTI. The aim of this study was to evaluate the association of bladder debris noted at the time of RBUS with positive urine culture results from catheterized VCUG specimens, among patients undergoing both studies on the same day. We hypothesized that the presence of sonographic debris would be significantly associated with a positive synchronous urine culture in otherwise asymptomatic children undergoing evaluation for a prior UTI.

Methods

We performed a retrospective, cross-sectional study of all patients presenting for same-day RBUS and VCUG, after obtaining institutional review board approval with a waiver of consent.

Results

A total of 3,995 patients were identified who had a RBUS and VCUG completed on the same day during the study period. Of these, 1,736 were initially excluded who met any of the following criteria: ≥60 months, prior postnatal imaging, prior diagnosis of prenatal GU abnormalities, or a non-UTI study indication. The 2259 remaining patients were further reviewed, and of these, 36 (1.6%) were identified with debris on RBUS. Of these, two patients with bladder debris were excluded because of lack of a catheterized urine specimen. A total of 34 patients with bladder debris were therefore successfully matched to 155 controls for the final analysis (Fig. 2).

The median age of the cohort was 18 months (IQR 5–35), and 128 (67.7%) of the included patients were female (Table 1). Of the 61 males, 41 (67.2%) were uncircumcised, five (8.2%) were circumcised, and 15 (24.6%) were of unknown circumcision status. Among all included patients, eight (4.2%) of the collected urine specimens met the AAP threshold of 50,000 CFU/mL to qualify as a positive culture (summary Table). Of patients with debris, 5/34 (14.7%) demonstrated a positive urine culture, with 3/155 (1.9%) patients without debris demonstrating a positive urine culture. Escherichia coli (2/8 (25%)), Enterococcus (2/8 (25%)), and Proteus mirabilis (2/8 (25%)) were the most frequent organisms, while Enterobacter Cloacae (1/8 (12.5%)) and Proteus vulgaris (1/8 (12.5%)) populated the remaining cultures (summary Table).

In conditional logistic regression stratifying by matching age, gender, circumcision status, and presence of vesicoureteral reflux, there was a statistically significant association between bladder debris on RBUS and positive urine culture result collected on the same day during VCUG (OR 7.88, 95% CI 1.88–33.04, p=0.0048). This corresponds to a 688% increase in odds of positive urine culture for patients with debris.

Discussion

Despite the prevalence of bladder debris seen in clinical practice, surprisingly little has been written on its significance, including its potential association with positive urine cultures. A recent study in the radiologic literature failed to correlate intraluminal debris with “abnormal” urinalysis results; however, urine specimens were collected asynchronously with the bladder ultrasound [3]. Our ability to review a large number of catheterized urine specimens, collected nearly simultaneously with the RBUS, is a unique strength. We conclude that among a select pediatric population (<60 months, presenting for initial work-up of a prior UTI, and without prior postnatal imaging or prenatal GU abnormalities) there exists a significant association between bladder debris and a positive urine culture. In conditional logistic regression stratifying by matching age, gender, circumcision status, and presence of vesicoureteral reflux, children with debris demonstrate a 688% increased odds of a positive urine culture compared with those children without debris.

These results support our hypothesis that there is an association between bladder debris and positive urine culture. Previous authors have commented on the lack of sensitivity and specificity of echogenic debris in the renal collecting system for the diagnosis of pyonephrosis [12]. Just as sonographic echoes can be seen in uninfected hydronephrosis, it seems logical to assume that not all cases of pyuria can be differentiated by such echogenic features on ultrasound. Scanning technique or machine settings can lead to artifact in some cases [6]. Even in scenarios in which there is true particulate matter in the setting of a recent infection, like many of the patients in our cohort, it is attractive to dismiss debris as a product of inflammation: sloughed urothelium that is otherwise sterile. However, the results of our study suggest that these patients are at increased risk of having a positive culture, if a specimen is sent.

At our institution, all children undergoing VCUG have a culture routinely sent from the initial catheterization, if urine is obtained. We do not, however, routinely culture children based on RBUS findings alone. The overall positive culture rate for children undergoing VCUG and RBUS on the same day is approximately 4% (unpublished data), which is much lower than that seen in children with bladder debris on RBUS. Thus, one might argue that the routine ordering of cultures for all subjects undergoing this imaging should be abandoned, while culture should be actively sought in those patients with findings predictive of positive culture, such as debris. It would clearly be helpful to identify specific populations of children undergoing cystography in whom a culture is most likely to be positive, as well as children in whom a culture may be safely omitted. Sonographic debris may be one helpful predictor in appropriately identifying such “at risk” patients.

The results of this study should be interpreted in light of its limitations. Methodologically, this study represents a secondary analysis of data collected for another study [9,10]. Given the specific inclusion and exclusion criteria, this sample is not representative of all pediatric patients evaluated at our institution, or even of all pediatric patients undergoing RBUS. Strictly speaking, these findings are applicable only to patients undergoing RBUS and VCUG for evaluation of prior UTI, and may not be generalizable to other populations. Extrapolation of these findings to older children, children with no UTI history, those who are undergoing initial imaging for prenatal hydronephrosis, or children with known GU anomalies or conditions undergoing follow-up imaging, should be approached with caution. We further recognize that such a secondary analysis limits the covariates available for consideration in our model. Nonetheless, we were able to control for several factors which have been associated with UTIs. Circumcision status in particular has a well-established relationship to febrile UTIs, with uncircumcised boys <1 years of age having an increased risk [1].

It is uncertain whether the association of debris with positive culture would be different among children undergoing imaging when they have active signs and symptoms of infection. It is also important to note that these data are based on the radiology report as dictated; we did not independently review the images to determine the presence of bladder debris. These findings were somewhat subjective, and we did not establish a priori criteria for identifying debris. It is possible that radiologists at other institutions might interpret these same images differently, with resulting impact on the analysis.

Despite these limitations, this remains the only study to consider the association of bladder debris on RBUS with synchronous positive culture among children, and we hope that this serves as a useful starting point for future investigations. Specifically, there is further work to be done to determine how the finding of bladder debris on ultrasound should inform clinical decision-making with respect to urine cultures and other testing.

Acknowledgments

Funding: None.

Footnotes

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