How is a quantitative urine culture interpreted?
Isolation of ≥10 5 colony-forming unit (CFU)/mL of an organism generally distinguishes bacteria causing infection from contaminants. However, 25% to 30% of young women with acute cystitis have organisms isolated in quantitative counts <10 5 CFU/mL; any gram-negative organism isolated in counts ≥10 2 CFU/mL is considered relevant for this presentation.
Lower quantitative counts are also occasionally isolated from patients with other clinical presentations of urinary infection. When this occurs, the diagnosis should be critically reassessed, considering the specimen collection method (i.e., the likelihood of contamination) and the number and species of organisms grown. Isolation of multiple organisms or gram-positive organisms from voided specimens is more likely to be contaminants. Any quantitative count ≥10 2 CFU/mL is considered diagnostic of infection for specimens obtained by in-and-out catheter, including intermittent catheterization, as these collection methods are less subject to contamination.
