Study clarifies role of midstream cultures in UTI

The use of voided midstream urine cultures in diagnosing cystitis has limited value but can be modified to avoid undertreatment and inappropriate treatment, according to a recently published study.

Diagnosis of urinary tract infection is often based on a voided midstream urine culture, but this methodology, while convenient and noninvasive, does not always accurately reflect the microbiology of the bladder. 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.

Previous studies suggested guidelines for quantifying bacteriuria with gram-negative bacteria, but no study has examined other organisms causing cystitis, such as gram-positive bacteria (enterococcus or group B streptococcus). To address this issue, researchers from the University of Miami Miller School of Medicine, Miami and the University of Washington School of Medicine, Seattle gathered midstream urine samples and catheter-derived urine from 202 women who presented with UTI symptoms. The goal was to compare microbial species and colony counts from both samples, with the catheter urine used as the reference.

In this study, which was published in the New England Journal of Medicine (2013; 369:1883-91), the authors found that even at very low counts of 10² colony-forming units (CFU) per milliliter, 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.

The study results reinforce the opinion of many that the voided urine culture for diagnosis of cystitis has limited value, said first author Thomas Hooton, MD, of the University of Miami. If, however, clinicians use a voided urine culture in evaluating a symptomatic patient, Dr. Hooton advised that the laboratory 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.

Because 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.

“Misinterpreting these cultures may result in undertreating those with low-level E. coli infections or inappropriately treating patients with enterococcus or group B streptococcus,” Dr. Hooton said.

Linda Brubaker, MD, MS, of Loyola University Chicago Stritch School of Medicine, told Medscape Medical News: “The significance of this paper is that it really updates some outdated clinical information and decision making for young premenopausal women with standard, garden-variety urinary tract infection. It's been more than 50 years since the original studies about urine culture were done, so it was a much-needed study.”

Dr. Hooton has received consulting fees from Merck and Pinnacle Pharmaceuticals and has an equity interest in Fimbrion Therapeutics.

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