The Diagnosis of UTI: Colony Count Criteria Revisited

Urinalysis (UA) is valuable in allowing clinicians to make a presumptive diagnosis of urinary tract infection (UTI) and initiate appropriate empirical antimicrobial treatment.1 The American Academy of Pediatrics (AAP) UTI Clinical Practice Guideline for infants and children 2 months to 2 years of age advocates a definition of significant bacteriuria as ≥50 000 colony-forming units (CFU) per milliliter and endorses the finding of inflammation on UA as a necessary component of the diagnosis of UTI.2,3 However, reports of the accuracy and application of UA in the diagnosis of UTI in infants <2 months of age have been somewhat variable. In this issue of Pediatrics, Tzimenatos et al4 (for the Pediatric Emergency Care Applied Research Network) substantiate the excellent sensitivity and high specificity of the aggregate UA for diagnosing UTIs in febrile infants 60 days and younger with and without concurrent bacteremia, extending the use of this laboratory test to those younger than the age addressed in the AAP guideline. Along with UA, the authors applied the criterion of ≥50 000 CFU/mL for the diagnosis of UTI but also evaluated ≥10 000 CFU/mL, raising the important issue of what threshold should be used to define significant bacteriuria.Before the 1950s, the number of bacteria associated with UTIs was described in vague terms, such as “numerous.”5 In his 1956 landmark study of adult women, Kass6 proposed that 100 000 CFU/mL be considered the diagnostic threshold, a criterion that was accepted quickly and widely. Fifteen years later, Pryles and Lustik,7 while acknowledging that data in children were still limited, considered the studies in adults sufficient to conclude the following: “urine specimens containing <1000 CFU/mL are indicative of contamination; specimens containing between 1000 and 100 000 CFU/mL are to be suspected of infection and the studies repeated; and urine specimens containing >100 000 CFU/mL are indicative of infection.” This view went unchallenged until 1994, when Hoberman et al8 proposed the colony count criterion be decreased to ≥50 000 CFU/mL in catheterized specimens, a view that was supported by Hellerstein9 but with the following proviso: