Urine Culture Updates Approach to Diagnosing Infection

The cause of acute uncomplicated cystitis is determined based on cultures of voided midstream urine, but few data guide the interpretation of such results, especially when gram-positive bacteria grow.

Interpreting a midstream urine culture is complicated by the potential for the specimen to become contaminated by microorganisms in the periurethra, making it difficult to distinguish whether the bacteria originated in the bladder or the periurethra.

Scientists at the Miller School of Medicine (University of Miami; FL, USA) collected midstream urine samples and catheter-derived urine from 202 women who presented with urinary tract infections (UTI) symptoms. The goal was to compare microbial species and colony counts from both samples, with the catheter urine used as the reference. Women were eligible if they were between the ages of 18 and 49 years, were in good general health, and had had typical symptoms of cystitis, dysuria and urinary frequency or urgency for seven days or less.

In catheter urine cultures, all microorganisms were fully identified and quantified to 10 colony forming units (CFU)/mL. In midstream urine cultures, all uropathogens, gram-negative rods, enterococci, group B streptococci, and Staphylococcus saprophyticus, were identified and quantified to 10 CFU/mL. Pyuria was defined as eight or more leukocytes/cu mm. The scientists found that found that even at very low counts of 102 CFU/mL, the presence of Escherichia coli in midstream urine had a high rate of predicting the presence of bacteriuria in the bladder. In contrast, the presence of enterococci and group B streptococci in midstream urine was not predictive of bladder bacteriuria in any amount.

Many commercial laboratories routinely report growth only for counts of 104 CFU/mL, cultures with lower E. coli counts may be falsely reported as negative. Conversely, midstream urine cultures often reveal enterococci and group B streptococci in amounts of 104 CFU/mL or higher, but these organisms appear to rarely cause cystitis in young women.

Thomas M. Hooton, MD, the lead author and a professor of medicine, said, “The study results reinforce the opinion of many that the voided urine culture for diagnosis of cystitis has limited value. If, however, clinicians use a voided urine culture in evaluating a symptomatic patient, the laboratory should be asked to quantify gram-negative bacteria, especially E. coli, down to 102 CFU/mL, and that low counts, even when in mixed growth with other organisms, should not be ignored. The study was published on November 14, 2013, in the New England Journal of Medicine.

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