What is the minimum number of bacteria required in a clean-catch urine specimen to diagnose a urinary tract infection (UTI)?

The minimum number of bacteria required in a clean-catch urine specimen to diagnose a urinary tract infection (UTI) is 10^5 bacteria/mL. This threshold is based on a combination of clinical experience and microbiological studies, which have demonstrated that a concentration of at least 10^5 colony-forming units (CFUs) per milliliter is typically indicative of a significant infection. This level helps distinguish between true infections and contamination, particularly in outpatient settings where other lower counts might be attributable to transient colonization or contamination during the collection process.

In clinical practice, a urine specimen showing less than 10^5 CFUs may not be indicative of a UTI, unless accompanied by specific symptoms or if the patient is immunocompromised or has other risk factors. This guideline is critical for both appropriate diagnosis and treatment to avoid unnecessary antibiotic use, which can contribute to resistance and other side effects. Understanding this cutoff is essential for correctly interpreting urinalysis results in the context of suspected urinary tract infections.