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Determining the Cutoff for Number of Colony Forming Units Needed to Diagnose a UTI

September 11, 2023

One of the cornerstones of diagnosing a urinary tract infection (UTI) is the concentration colony-forming units (CFUs)/mL of the predominant organism. In a catheterized specimen, the cutoff of 50,000 CFUs/ml is the generally accepted cutoff in young febrile children. However, sometimes a lower threshold is appropriate. A new study by Nader Shaikh and colleagues from the Children’s Hospital of Pittsburgh being early released in Pediatrics this month suggests that 16S ribosomal ribonucleic acid (rRNA) gene sequencing can help determine the optimal threshold (10.1542/peds.2023-061931).

The authors of this study enrolled 341 children between 1 month and 3 years of age with normal urinary tract anatomy who were febrile with no other identified source of infection and had a catheterized urine specimen obtained. A UTI was diagnosed if rRNA gene sequencing identified any organism with at least 80% abundance and the child had urinary markers of inflammation (e.g., white blood cells, a positive leukocyte esterase test, or evidence of neutrophil gelatinase-associated lipocalin (NGAL)

Based on rRNA sequencing, a cutoff of 10,000 CFU/ml in the setting of inflammatory markers was 98% and the specificity 99% for the presence of a UTI.  At 50,000 CFUs, the sensitivity dropped to 80%, and at 100,000 CFUs/ml, the sensitivity dropped to 70%. The specificity at these other two cutoffs was similar.

Should we use a cutoff of 10,000 CFUs/ml in a catheterized sample for considering a UTI? While these results do a good job of convincing us that is the case, there are limitations to consider as pointed out by both the authors and in an accompanying commentary by Drs. Ellen Wald and Jens Eickhoff from the University of Wisconsin and the American Family Children’s Hospital in Madison (10.1542/peds.2023-062883). Lowering the threshold could lead to overtreatment. Both the authors of the study and commentary conclude that it is hard to ignore the fact that UTIs can occur below the commonly accepted value of 50,00CFUs/ml. Until rRNA sequencing is readily available, it could be appropriate to use this lower threshold. Of course, it is critically important to use your clinical judgment when deciding when to treat a patient with a suspected UTI. Link to these articles and learn more.

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