Many have experienced multiple occurrences of this bacterial infection that leads to tens of millions of courses of antimicrobial therapy in the U.S. A new study from the University of Miami Miller School of Medicine sheds light on the accuracy of midstream urine cultures used to determine treatment.
In a study published in the November 14 issue of The New England Journal of Medicine, Thomas M. Hooton, M.D., professor of medicine and Clinical Director of the Division of Infectious Diseases, led a team of researchers who more accurately determined the diagnostic value of voided midstream urine cultures. The study, “Voided Midstream Urine Culture and Acute Cystitis in Premenopausal Women,” involved researchers from the University of Washington School of Medicine in addition to the Miller School.
In 2007, UTI accounted for 8.6 million outpatient visits, and while complications are rare, acute cystitis can be extremely uncomfortable and results in lost productivity for afflicted women. The majority of urinary tract infections are caused by bacteria that colonize and induce inflammation in the bladder (cystitis).
Diagnosis is often based on a voided midstream urine culture, but this methodology, while convenient and non-invasive, 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, Hooton and his team 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, the researchers found that even at very low counts of 102 colony-forming units (CFU) per milliliter, the presence of E. 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.
Hooton, who is also Associate Chief of Staff of Medical Service at the Miami VA Healthcare System, 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, Hooton advises that the laboratory be asked to quantify gram- negative bacteria, especially E. coli, down to 102 CFU per milliliter, 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 per milliliter, 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 per milliliter or higher, but these organisms appear to rarely cause cystitis in young women. “Misinterpreting these cultures,” said Hooton, “may result in under treating those with low-level E. coli infections or inappropriately treating patients with enterococcus or group B streptococcus.”
The study was supported by grants from the National Institute of Diabetes and Digestive and Kidney Diseases.
University of Miami