Editor’s Note: Dr. Ella Perrin (she/her/hers) is a resident physician in pediatrics at Naval Medical Center San Diego. Her interests include disordered eating, obesity, decreasing weight stigma and bias, and the field of hospital medicine. The views expressed in this blog are those of the author and do not necessarily reflect the official policy or position of the Department of the Navy, Department of Defense, or the US Government. Dr. Perrin declares no conflicts of interest. -Rachel Y. Moon, MD, Associate Editor, Digital Media, Pediatrics
Urinary tract infections (UTIs) are a common cause of fever in infants and are diagnosed via urine culture. Since it can take 1–2 days to get urine culture results, urine dipstick and microscopy are tests that help clinicians determine whether antibiotics should be started while awaiting culture results. Findings suggesting a UTI include white blood cells (WBC) seen on urine microscopy or presence of leukocyte esterase or nitrite on urine dipstick. Importantly, urine microscopy must be done by a lab, whereas urine dipstick can be done in the ambulatory setting, making it a cheaper, quicker, and more accessible alternative.
However, infants cannot yet concentrate their urine, which may affect the accuracy of these tests. In a study being early released in Pediatrics this week, entitled “Urine Dipstick for the Diagnosis of Urinary Tract Infection in Febrile Infants 2-6 Months Old,” Dr. Kathryn Hunt from Boston Children’s Hospital and colleagues from around the country set out to determine the accuracy of urine dipstick versus microscopy in diagnosing UTI in young infants (10.1542/peds.2024-068671).
This was a secondary analysis of retrospective cross-sectional data from a study that included 5 pediatric medical centers. All infants were aged 2–6 months, presented to the emergency department (ED) for a fever, had a fever documented during their ED encounter, and had a urine culture collected. They did not include infants with complex medical histories (including urogenital abnormalities).
The authors compared test characteristics of urine dipstick and microscopy in detecting UTI (defined as urine culture growing ≥50,000 colony-forming units (cfu)/mL of a single bacterial pathogen):
- Sensitivity: how well the test could correctly tell if the baby had a UTI
- Specificity: how well the test could correctly tell if the baby did not have a UTI
- Negative predictive value (NPV): the probability that a negative test result is accurate in ruling out UTI
- Positive predictive value (PPV): the probability that a positive test result is accurate in diagnosing a UTI
The authors also used receiver operator characteristic curve analysis to determine the optimal urine microscopy WBC cutoff to detect UTI.
The authors found that the optimal cutoff point for urine microscopy was ≥7 WBC/hpf. Using this cutoff, they found:
- Urine dipstick had a higher sensitivity and specificity than microscopy (90.2% and 92.6% versus 83.9% and 87% respectively).
- Urine dipstick also had a better NPV than microscopy (98.5% versus 96.3%).
- These findings persisted at a urine culture cutoff of ≥10,000 cfu/mL.
I think this was such a cool study. The biggest limitation, which the authors point out, is that they were only able to analyze samples that had a urine culture sent, which may inflate sensitivity (as providers often don’t send a culture if initial testing does not look infectious). The high NPV (which is not affected by disease prevalence) indicates that providers can likely rest easy not prescribing antibiotics in the setting of a negative dipstick in this age group.
Overall, this study provides a strong argument for the utility of urine dipstick as a diagnostic test for UTI in this age group, which is great news both for those practicing in resource-limited settings and in busy EDs.
