Higher Colony-Count Threshold for Urinary Pathogens Cuts Antimicrobial Treatment

By Will Boggs MD

May 02, 2019

NEW YORK (Reuters Health) - Raising the colony-count threshold for treatment of urinary pathogens in hospitalized patients is associated with reduced antimicrobial use, researchers report.

"Raising the threshold for identifying potential urinary pathogens was well accepted by physicians at our hospital who were generally happy to ignore low-colony-count urine cultures," Dr. Jerome A. Leis from the University of Toronto, Canada, told Reuters Health by email. "This simple change led to an immediate and sustained change in antimicrobial prescribing."

Treatment of asymptomatic bacteriuria and candiduria (ASB/C) is a leading reason for unnecessary antimicrobial use among hospitalized patients. But the optimal colony-count threshold for identifying and reporting growth from inpatient urine cultures is unknown.

Dr. Leis and colleagues investigated changes in antimicrobial prescribing at their acute care hospital after the threshold for identifying potential uropathogens from urine cultures submitted from inpatient units was increased from 10K CFU/mL or greater to 100K CFU/mL or greater.

After the change, urine cultures with low colony counts (between 10K and 100K CFU/mL) were automatically issued a report stating that these organisms usually represent ASB/C.

Over a period of two years, 608 patients with a low colony count and 690 patients with a high colony count were included in the study.

After implementation of the change, the rate of antimicrobial prescribing for ASB/C was 86% lower in the low-colony-count group than in the high-colony-count group (P=0.01), the researchers reported in JAMA Internal Medicine, online April 29.

When clinicians opted to have low-colony-count urine cultures worked up, patients were more likely to have a urinary-tract infection (35% incidence vs. 7% incidence among those not worked up).

Clinical outcomes did not differ between the low-colony-count and high-colony-count groups.

The researchers estimate that the higher threshold was associated with 70 fewer treatment courses per year, which could translate into 14 fewer adverse drug events annually.

"Positive urine cultures introduce clinical bias that leads to over-diagnosis and treatment of urinary tract infection (UTI)," Dr. Leis said. "The way the microbiology laboratory processes specimens can help address these biases and improve antimicrobial prescribing practices."

"Despite popular belief, urine is not sterile," he explained. "Reporting all organisms isolated from urine cultures is unhelpful to clinicians, and we need more research into understanding the optimal threshold for identifying and reporting potential urinary pathogens. These future studies should continue to weigh the risks of over-treatment against the risk of missing clinically important results."

Dr. Linda Brubaker of the University of California, San Diego, in La Jolla, who recently addressed the limitations of current standards for UTI diagnosis, told Reuters Health by email, "There is still a great deal of uncertainty about what exactly 'asymptomatic' means - which symptoms - what about chronic urinary symptoms, such as overactive bladder. It is virtually impossible to find over 600 adult women who have no urinary symptoms."

"We have been pretty casual in our assessment of this for many years - and we know that the 'acute dysuria, urgency, and frequency' of college-age women isn't the typical symptom set for postmenopausal women," she said. "So I have two concerns - 1) how are symptoms assessed, especially in those with a urinary catheter! and 2) I don't like the idea of a single threshold for all uropathogens - I suspect (but cannot prove) that we should consider variable thresholds as the microbes likely have different roles."

"Only get a urine culture when you would act on it (as in treat) or you need to document the negative for an important clinical reason (i.e., documenting prior therapeutic efficacy in highly select situations)," advised Dr. Brubaker, who was not involved in the study.

SOURCE: https://bit.ly/2VyHmEn

JAMA Intern Med 2019.