How is a positive urine culture defined?

A positive urine culture is defined as having greater than 10^5 colony-forming units (CFUs) of bacteria per milliliter. This threshold is established based on the need for a reliable indicator of urinary tract infection (UTI). When the bacterial count exceeds this level, it suggests that the bacteria are present in significant numbers, which is necessary to establish a diagnosis of infection rather than just contamination during sample collection.

Lower thresholds, such as 10^4 CFUs/mL or 10^3 CFUs/mL, may not reliably indicate infection because these levels could also result from contamination of the urine sample. A count of greater than 10^6 CFUs/mL is typically indicative of a severe infection but is not the standard definition for a positive culture in the routine assessment of UTIs. Hence, the 10^5 CFUs/mL benchmark strikes a balance, allowing for accurate diagnosis while minimizing false positives from contamination.