NEW ORLEANS – With the trade-off of an extra 24 hours for results, an enhanced protocol to culture clinically relevant urinary pathogens detected significantly more unique pathogens associated with urinary tract infection, compared with standard cultures, in a study of 150 women.
“What we were able to see is that for about 90% of the samples that were called negative by standard [approach], we were able to detect bacteria through our protocol,” said Travis K. Price, a PhD candidate in the department of microbiology and immunology at Loyola University, Chicago.
Typically, when a urine sample is cultured for a UTI at Loyola University Medical Center, the standard protocol is for the lab to test 1 mcL of urine using agar plates incubated aerobically for 24 hours, Mr. Price said. “When we’re testing the urinary microbiome, we expand on that protocol. We use 100 times more urine, different plates, different environmental conditions, and we hold them for 48 hours instead of 24.”
The investigators prospectively recruited 150 women coming in to the urogynecology clinic – half who felt they had a UTI that day, half who did not. “We wanted to understand if using our enhanced protocol was beneficial and essentially leading to better patient outcomes,” Mr. Price said at the annual meeting of the American Society for Microbiology.
“Among the women who felt they had a UTI, standard culture only picked up 50% of the pathogens we were picking up with our protocol,” Mr. Price said. “And when we looked closer, we realized most of that was Escherichia coli.” Excluding samples positive for E. coli, standard culture detected only 12% of UTI pathogens, he added, compared with 77% detected with the expanded quantitative protocol.
The expanded protocol detected significantly more unique pathogen species, 95, compared with 11 with standard cultures. In addition, of all the uropathogens detected by the new protocol, the standard protocol missed 67%, or 122 of the total 182.
In terms of clinical practicality, Mr. Price and his colleagues looked at “different conditions, multiple volumes of urine, different plates, 24 versus 48 hours – and at the end tried to figure out what is the least amount of work you can do to get the most information.” They then developed a streamlined protocol that involves 100 mcL of urine, a CNA agar plate that selects for gram-positive organisms, a MacConkey agar using 5% CO2, and 48 hours of incubation. “It’s easy to implement,” he added. “The only issue is the longer incubation time could lead to delayed treatment, potentially.”
The streamlined protocol detected more uropathogens – 152 of the 182, for an 84% detection rate – compared with standard cultures, which detected 60 of the 182, or 33%.
The streamlined protocol markedly improved uropathogen detection, the authors wrote. “These findings support the necessity for an immediate change in urine culture procedures.”
Another aim of the study was to evaluate the optimal threshold for UTI colony counts. Traditionally, the cutoff is set at 105 colonies or greater for diagnosis of a UTI, Mr. Price said. “We found there were always higher pathogen colony counts in people who thought they had a UTI. But there wasn’t one threshold that would have caught all of these.”
Next, the investigators looked for a correlation between the colony count cutoff and clinical outcomes. “For people who had a colony count greater than 105 – typically, it was a gram-negative organism – most people were treated with an antibiotic, and a week later most people, 62%, reported feeling better,” Mr. Price said. “But people who didn’t have a pathogen greater than 105, some were not treated, and when we called them a week later, most reported they were not feeling better. ... This suggests this threshold is not actually appropriate.”
Going forward, the investigators just started a clinical trial using the enhanced culture to confirm whether or not their protocol leads to better outcomes for women with UTIs.
Mr. Price did not have any relevant disclosures.