The evaluation of the very young infant with a fever is complex, with multiple competing factors including the rarity of serious illness, the severity of serious illness, and the cost of the intensive evaluation frequently required. The most commonly identified bacterial source for fever is a urinary tract infection, and our bedside test in the Emergency Department is the urinalysis.
So, how reliable and accurate is that test?
This is an analysis of prospectively collected data from the PECARN network, looking at the evaluation of febrile infants fewer than 60 days of age. Of 4,147 patients enrolled, 289 patients had UTIs by a 50,000 CFUs/mL definition on the subsequent urine culture. Only 27 patients had bacteremia and a UTI. The news is generally mixed: using the 50,000 CFUs/mL cut-off, any abnormality on the UA was 94% sensitive for UTI and 91% specific, but was 100% sensitive for a UTI associated with bacteremia.
The authors also do analyses including different cut-offs for UTI based down to 10,000 CFUs/mL and, as you might expect, the sensitivity for any UTI diminishes. While the interpretation of the urine culture result is less applicable to the initial Emergency Department evaluation, the subsequent threshold for diagnosis is relevant to the ongoing follow-up care for the febrile infant, particularly if an initial decision involved observation without antibiotics and the infant remains symptomatic without another source.
Overall, it is reasonable to suggest – if the UA is negative, a serious bacterial illness is unlikely to be present. Some consideration should be made to the duration of illness, and natural course of delayed onset of development of cystitis or pyuria in the urine. A positive UA, however, despite the apparent high specificity, does not reliably indicate a true positive for UTI, owing to the low prevalence. This should also be taken into consideration regarding whether additional invasive evaluation is indicated.
“Accuracy of the Urinalysis for Urinary Tract Infections in Febrile Infants 60 Days and Younger”
http://pediatrics.aappublications.org/content/early/2018/01/12/peds.2017-3068