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New UTI Culture Cutoff in Children

October 26, 2023

Written by Laura Murphy

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A new cutoff of 10,000 colony-forming units per milliliter (CFU/mL) on urine culture to diagnose urinary tract infection (UTI) from catheterized specimens in infants and children <3 years improved sensitivity and specificity.

Ur in(e) for a culture shift…
Prior studies have suggested the use of the accepted cutoff of 50,000 CFU/mL to diagnose UTI in catheterized specimens in children younger than 2 years of age. However, these studies did not utilize a culture-independent reference standard to determine this cutoff.

This was a single-center study that enrolled 341 febrile children between 1 month and 2 years and 11 months who had a catheterized urine specimen to rule out UTI. Children who had previously received antibiotics or corticosteroids within 3 days, had concurrent bacterial infection, immunodeficiency, or genitourinary abnormalities were excluded. Samples were sent for conventional urine culture but also had 16S sequencing (a non-culture based method to identify bacteria in urine) to assess diagnostic accuracy of urine culture at various colony count cutoffs.

UTI was defined as ≥80% relative abundance of any organism on 16S sequencing and elevated urinary markers of inflammation (≥10 WBC/mm3, ≥5 WBC per Hpf, ≥trace leukocyte esterase, or neutrophil gelatinase-associated lipocalin (NGAL) above 39.9 ng/mL). Using a cutoff of 10,000 CFU/mL, the sensitivity and specificity of urine culture were 98% (95%CI 93-100%) and 99% (95%CI 97-100%), respectively. Using an cutoff of 50,000 CFU/mL and 100,000 CFU/mL decreased sensitivity to 80% (95%CI 68-93%) and 70% (95%CI 55-84%), respectively, without impacting specificity.

Further analyses demonstrated that the cutoff of 10,000 CFU/mL had a lower number of false positives compared to the number of identified UTIs that would have been missed with prior cutoff. One limitation was that the majority of the UTIs in this sample were caused by E. coli, so additional studies may be needed to determine cutoffs for less common causative pathogens.  It also focused on catheterized urine specimens, and the results would not likely be applicable to alternative methods of specimen collection.

How will this change my practice?
While I do not routinely wait on urine culture results to make decisions about treating young children with suspected UTI in the emergency department, this is still an important cutoff for emergency physicians to be aware of, as it is likely to change practice patterns in the community.

Editor’s note: In addition, I am often asked by our case manager about culture results that come back later and frequently need to interpret prior culture data on patients I see in the ED. This new cutoff is important for us to know. ~Clay Smith

Support for the Use of a New Cutoff to Define a Positive Urine Culture in Young Children. Pediatrics. 2023 Oct 1;152(4):e2023061931. doi: 10.1542/peds.2023-061931.

What are your thoughts?