Veronica Hackethal, MD
February 10, 2014
Renal and bladder ultrasound (RBUS) is a poor screening tool for genitourinary (GU) abnormalities identifiable on voiding cystourethrogram (VCUG) after febrile urinary tract infections (UTIs) in children, according to a study published online February 10 in Pediatrics.
"Neither positive nor negative ultrasounds reliably identify or rule out [genitourinary] abnormalities," write Caleb P. Nelson, MD, MPH, from the Department of Urology, Boston Children’s Hospital, Harvard Medical School, Massachusetts, and colleagues. "Ultrasound and VCUG provide different, but complementary, information."
American Academy of Pediatrics (AAP) 2011 guidelines advise RBUS to screen for GU abnormalities in children aged 2 to 24 months with index cases of febrile UTIs, according to the authors, with VCUG used for confirming abnormalities suggestive of high-grade vesicoureteral reflux (VUR) or obstructive uropathy or after a second febrile UTI. The implication of these guidelines, the authors point out, is that a normal RBUS rules out clinically significant GU pathology.
The researchers looked at 3995 medical records between January 1, 2006, and December 31, 2010, in which VCUG and RBUS were conducted on the same day and then selected only those with UTI as an indication for imaging. They excluded records with postnatal GU imaging or prenatal GU abnormalities. They then placed both RBUS and VCUG into 4 categories each, based on GU abnormality type, with VUR graded on the 5-point international grading system. They also assigned diagnostic criteria thresholds for positive tests, ranging in severity from relaxed to stringent.
The researchers identified 2259 children younger than 60 months whose indication for imaging was UTI. RBUS was normal in 75%, but VCUG indicated 41.7% had evidence for any VUR, 20.9% had VUR greater than grade 2, and 2.7% had VUR higher than grade 3. Among those with a first febrile UTI, these percentages were 47.5%, 26.9%, and 2.6%, respectively. Depending on threshold, RBUS had a sensitivity ranging from 5% (specificity, 97%) to 28% (specificity, 77%), with the sensitivity of VUR higher than grade 3 ranging from 18% (specificity, 97%) to 55% (specificity, 77%).
There were 1203 children aged 2 to 24 months who received imaging after an initial febrile UTI. The positive predictive value of RBUS in this group was 37% to 47% for VUR higher than grade 2 and 13% to 24% for VUR higher than grade 3, with negative predictive values ranging from 72% to 74% (VUR higher than grade 2) and 95% to 96% (VUR higher than grade 3).
Limitations include possible misdiagnosis relating to the study's retrospective nature. In addition, the initial radiologist review was unblinded and not confirmed by independent review. To minimize verification bias, only patients with RBUS and VCUG on the same day were selected, which could have introduced selection bias if foregoing a second test was based on the first test's results.
"A negative RBUS does not rule out significant GU pathology (particularly VUR grades III and higher)," the authors conclude, "whereas a positive RBUS is a poor predictor."
In an independent commentary, Stephen M. Downs, MD, from Children’s Health Services Research, Indiana University School of Medicine, Indianapolis, commends this study for being the largest and most well-conducted of its kind.
However, although noting that this study's results are valid and consistent with past studies, and agreeing that RBUS is a "lousy screen for VUR," Dr. Downs ultimately supports the AAP guidelines. He points out that the guidelines actually suggest watchful waiting, rather than RBUS, to screen for high-grade VUR, with VCUG recommended only after the second UTI. Early RBUS is recommended for identifying parenchymal damage and obstructive uropathy associated with infection, he explains. Children with VUR are likely to have another UTI, he continues, and most (about 90%) will never have another UTI and would be unnecessarily exposed to the discomfort, cost, and radiation of VCUG.
"Readers should know that the RBUS recommended by the AAP guideline does serve a critical role in the evaluation of young children who have a first febrile UTI," Dr. Downs argues, "but it is watchful waiting that screens for VUR."
The authors and Dr. Downs have disclosed no relevant financial relationships.
Pediatrics. Published online February 10, 2014. Abstract