Ultrasound as a Screening Test for Genitourinary Anomalies in Children With UTI (2024)

Abstract

Keywords: urinary tract infection, imaging, vesicoureteral reflux, pediatrics

Recommendations regarding appropriate evaluation of infants and young children who have a first febrile urinary tract infection (UTI) continue to evolve. The 1999 American Academy of Pediatrics (AAP) clinical practice guidelines recommended both voiding cystourethrogram (VCUG) and renal and bladder ultrasound (RBUS) in this situation.¹ The most recent AAP guidelines, however, state that after an initial febrile UTI in an infant age 2 to 24 months, only RBUS should be performed and that VCUG should only be performed after a second febrile UTI, or if the RBUS “reveals hydronephrosis, scarring, or other findings that would suggest either high-grade vesicoureteral reflux (VUR) or obstructive uropathy.”²

Although the new guidelines do not explicitly frame RBUS as a screening test, the guidelines do suggest that the decision to perform VCUG after a first febrile UTI should be based in part on RBUS findings. The implicit assumption is that RBUS is a useful tool to identify patients likely to have abnormalities on VCUG, and that a normal RBUS effectively rules out clinically significant genitourinary (GU) abnormalities. However, most published studies evaluating RBUS as a screening tool in this context have significant limitations.

The purpose of this study was to assess the test characteristics of RBUS as a screening test for VUR and other GU conditions, and to determine the positive and negative predictive value of RBUS for these conditions, particularly among children age 2 to 24 months who have a history of first febrile UTI.

Methods

Results

We identified 3995 clinical encounters during which patients underwent RBUS and VCUG studies on the same date between January 1, 2006 and December 31, 2010. We excluded 930 patients who had previous postnatal GU imaging, leaving 3065 subjects. Of these, 198 were age ≥60 months and were also excluded, leaving 2867 children. Among this group, the indications for imaging were UTI in 2259 (78.8%), prenatally identified abnormalities in 509 (17.8%), and other indications in 99 (3.5%). The 2259 patients who underwent initial GU imaging for UTI are described in Table 3. A total of 79.0% were female, 75.3% were aged 2 to 24 months, and 43% (975/2259) were seen clinically in the Department of Urology at our institution. Among the boys, most were uncircumcised. Among the group aged 2 to 24 months, we confirmed that this was an initial, febrile UTI episode in 1203 patients.

RBUS findings among both the whole cohort and “initial febrile UTI, age 2 to 24 months” group are shown in Table 4, along with the proportions in each group meeting thresholds for a “positive” RBUS screening test. Depending on the threshold, RBUS was “positive” (abnormal) in 4% (RBUS-A), 17% (RBUS-B), 23% (RBUS-C), or 25% (RBUS-D) of children who had “any UTI.” Positive rates were similar for the “initial febrile UTI” group. Overall, the RBUS findings were notable for the small number of patients who had higher grades of hydronephrosis; only approximately 1.5% of children had hydronephrosis greater than “mild” on either side. Similarly, <5% of children had ureteral dilation noted, in any degree. The most common renal parenchymal findings were size discrepancy and duplication; more discrete renal scarring or other cortical pathology was seen in ∼1% of the group.

VCUG findings among both patient groups are shown in Table 5, along with the proportions in each group meeting each threshold for a “positive” VCUG. Abnormalities of any kind were identified in 43.9% of studies (49.2% of the initial febrile UTI group), and VUR was identified in 41.7% (47.5%). Significant numbers of children had dilating VUR, with VUR grade >II seen in 20.9% (26.9%). However, high-grade VUR was uncommon: VUR grade >III was present in just 2.7% (2.6%). As with RBUS, the proportion of children who had a positive VCUG depended on the threshold used: VCUG was “positive” (abnormal) in 7% (VCUG-A), 23% (VCUG-B), 39% (VCUG-C), or 43% (VCUG-D) of the “any UTI, age <60 months” group. Positive rates were slightly higher for the “initial febrile UTI, age 2 to 24 months” group.

Test characteristics and predictive values for RBUS as a screening test for VCUG findings are shown in Table 6. Receiver Operating Characteristic curves are shown in Fig 1 A–D. As expected, there was little difference in test characteristics (sensitivity and specificity) between the “any UTI” and “initial febrile UTI” groups. Predictive values differed somewhat more, particularly for negative predictive value. RBUS is not a sensitive test regardless of the threshold used, with a maximum sensitivity of 55% (RBUS-D for the VCUG-A outcome). Specificity did reach high levels (maximum 97%) but only at extremely low levels of sensitivity (<10%). Positive predictive values were also low, suggesting that only a fraction of those who have a positive RBUS have VCUG findings at any “positivity” level. Finally, negative predictive values were high, but only for the highest grades of VUR.

Discussion

The 2011 AAP guidelines regarding the evaluation of infants who have a first febrile UTI represent significant evolution in management, with the most significant change being the recommendation that VCUG be deferred until after a second febrile UTI.² The discussion regarding VCUG timing has been vigorous⁵ and is beyond the scope of this paper. Much less attention has been paid to the recommendation that VCUG be performed if the RBUS is abnormal. Although RBUS is not explicitly characterized in the guidelines as a screening test, and although the guidelines acknowledge that RBUS is insensitive for VUR, the practical result of this recommendation is to sort children into groups who should or should not proceed to VCUG after first febrile UTI, based on RBUS results. Use of RBUS in this manner bears many features of a screening test, yet there has been very little discussion regarding the value of RBUS as a screening test in such circ*mstances.

Although the AAP guidelines committee recommended RBUS, they graded the supporting evidence with a “C” grade. They felt that, although the RBUS would only identify “abnormalities that would lead to action” in 1% to 2% of cases, the potential benefit outweighed the potential harm. However, few primary references are cited.

Before this investigation, several large studies have reported test characteristics of RBUS as a screening test for GU abnormalities, and although findings have varied widely, none have found RBUS to be an accurate screening test in this setting. Sensitivity has ranged from 18% to 79% and specificity from 41% to 99%, depending on how a “positive” RBUS was defined and what VUR outcome was assessed (eg, any VUR, “dilating VUR,” “high-grade VUR”).^6–11 Many other groups have reported GU imaging findings among children who have a history of UTI. However, most of these papers have limitations that make it impossible to determine the test characteristics of RBUS; most common is that many studies do not provide sufficient data to directly compare RBUS findings with VCUG findings in individual patients.^12–19 Other studies focus on the value of RBUS to predict renal scintigraphic findings (scarring).^13,20–22

Verification bias is a common weakness in the published literature. For example, Foresman et al assessed the correlation between RBUS and VCUG among patients hospitalized for acute pyelonephritis.²³ RBUS was performed in all patients during the hospitalization; however, not all patients subsequently underwent VCUG, and performance of the VCUG varied depending on RBUS findings, with 67% of patients who had normal RBUS having VCUG, but 87% of patients who had abnormal RBUS having VCUG. Such differential assessment introduces an inherent bias into the data. Many studies have similar limitations.^6,7,9,19,24 Other studies have limited generalizability owing to narrow^10,25 or broad^26–28 age ranges, or small sample size.^8,26–28 One study looked specifically at the predictive value of ureteral dilation as an isolated finding on RBUS.²⁹

Specific features of GU imaging practice at our institution during the study period address some previous studies’ weaknesses. First, routine practice in our region (before 2011) was to obtain both RBUS and VCUG in children who had febrile UTI. Such universal assessment is a key characteristic of evaluation of any screening test: both the screening test and the gold standard test must be performed in all subjects. Differential ascertainment can result in verification bias, because patients who have certain (usually negative) findings will be systematically excluded from the sample of patients undergoing both tests. In the current study we have limited this bias by including only patients scheduled a priori for RBUS and VCUG on the same day; performance of either test was not dependent on the findings of the other. Second, the sample includes a large number of patients followed at community practices, and seen at our institution only for imaging (only 43% [975/2259] were ever seen in our Urology Department). Our sample therefore is more representative of the population of children who had UTI, and does not only contain patients treated at a tertiary center. Such patients can be expected to have a higher disease severity than the overall study population; inclusion of community patients therefore reduces the selection bias that often compromises clinical studies at tertiary centers.

Additionally, the imaging studies were almost entirely performed after the acute pyelonephritis episode. (Among a 10% random sample, mean time from UTI to imaging was 41 days, and only 7% were imaged within 5 days of UTI). Thus, the RBUS test characteristics we observed may differ from RBUS performed during the acute infection. Although the AAP guidelines state that RBUS may be useful during the first 2 days of treatment to identify complications (eg, abscess), the guidelines are also clear that acute-phase imaging may actually be misleading.²

The low overall incidence of significant GU anomalies in our sample warrants comment. Previous studies have reported a similar phenomenon. Hoberman et al examined imaging results among 309 children who had febrile UTI, and noted that patients who had high-grade VUR were under-represented in their sample, and stated that “the validity of renal ultrasonography in identifying such children [with high-grade VUR] warrants further study.”³⁰ A relatively low incidence of high-grade VUR has been seen in other series as well.⁸ Although our sample was much larger than these studies, we too noted that <3% of our sample had high-grade VUR (grade >III). Similarly, <1.5% of children had higher-grade hydronephrosis on RBUS. The low prevalence of severe abnormalities may be attributable to the effect of prenatal screening. We excluded children who had a history of prenatally diagnosed abnormalities because (1) the AAP imaging guidelines can be read as being applicable to children who have no history of prenatal abnormalities, and (2) such children usually undergo GU imaging as newborns, and we excluded children who had a history of previous GU imaging. Many, although not all, cases of high-grade disease are likely detected prenatally, reducing the incidence of such anomalies among children presenting with de novo UTI postnatally. Fifty years ago, we presumably would have observed much higher rates of such anomalies, because many of the children who are now diagnosed in utero would have presented postnatally with clinical UTI.

The results of our investigation should be interpreted in light of its limitations. This study was retrospective, subject to the limitations of this design. For example, study subjects all had history of UTI, but many of these diagnoses were made elsewhere and could not be independently verified. As some patients may have been misdiagnosed, the results may not reflect those that would be seen among children who had strictly defined and confirmed UTI. However, our radiologists take a detailed history before VCUG, to verify the history. Furthermore, irrespective of the diagnostic details, these are the patients being referred for GU imaging, and so reflect the “real-world” screening population seen in practice. With respect to imaging findings, we relied on the final interpretations of the imaging studies as dictated by the clinical radiologist; independent confirmatory review of images was not performed. The findings therefore are subject to the relative variability of interpretation (eg, grading of VUR) that occurs in all clinical care. Furthermore, the radiologists reading each study were not systematically blinded to the findings of the other test, so it is possible that interpretation of 1 study could have been influenced by knowledge of the findings on the other test. However, the radiologist interpreting the VCUG was usually a different individual than that radiologist interpreting the RBUS, and in most patients the RBUS was performed (and usually read) before the VCUG, which would minimize the impact of such unblinded interpretation. Furthermore, most findings on both RBUS and VCUG are relatively objective. As noted, verification bias is a concern in any evaluation of a screening test. To minimize this, we included only patients who underwent both VCUG and RBUS on the same day. However, it is likely that some patients underwent RBUS or VCUG separately, or underwent 1 test but not the other; such patients would be excluded from our sample. If such patients were numerous, and if the decision not to obtain the second test was based on the results of the first test, then bias could have been introduced into our sample. As we also noted, however, several features of our practice environment during the study period make this less likely, including the widespread adherence within our institution and community to the 1999 AAP guidelines, the routine practice of scheduling both RBUS and VCUG on the same day for patients who have a history of UTI, and the routine completion of both tests, regardless of findings.

Conclusions

Among young children who have a history of UTI, RBUS is a poor screening test for GU abnormalities, with low sensitivity/specificity. A negative RBUS does not rule out significant GU pathology (particularly VUR grades III and higher), whereas a positive RBUS is a poor predictor. In such children, RBUS and VCUG should be considered complementary as they provide important, but different, information.

Glossary

Footnotes

References