Perianal Swabs Detect 80% of Multidrug-Resistant Gram-Negative Infections

Alice Goodman

September 13, 2010 (Boston, Massachusetts) — Perianal swabs failed to detect 1 in 5 cases of multidrug-resistant Gram-negative (MDRGN) bacteria in hospitalized patients with a newly identified MDRGN culture.
Researchers were not able to identify patient factors or types of care that could predict failure of the swab to identify MDRGN bacteria in this study, which was conducted in hospitalized patients known to be infected with MDRGN bacteria. The results were presented here at the 50th Interscience Conference on Antimicrobial Agents and Chemotherapy. A reliable method of detecting MDRGN bacteria in infected but asymptomatic patients has the potential to prevent the spread of MDRGN infection, the authors pointed out.
"We used these swabs in patients who were positive for MDRGN bacteria and we anticipated that we would detect MDRGN bacteria in 100%. Our study suggests that the ability to detect MDRGN bacteria among the generalized hospitalized population may be limited," said lead author Graham M. Snyder, MD, a fellow at Beth Israel Deaconess Hospital in Boston, Massachusetts. Erika D'Agata, MD, and Lara Venkataraman, PhD, also from Beth Israel Deaconess Hospital, were coauthors.
"Future research should be aimed at more accurate methods of surveillance than the swab used in our study and investigate whether there are groups of patients for whom the swab may be more accurate," Dr. Snyder added.
This study used standard agar plates infiltrated with ciprofloxacin or ceftazidime to screen for MDRGN bacteria. Other methods include PCR-based tests for carbapenemase-producing organisms and CHROMagar, he said. Perianal swabs were used because they are easy to perform and more comfortable than full rectal swabs, Dr. Snyder said. In addition to the rectum, potential sites for swabs include the groin, stool, wound, and perineum.
Thirty-five patients agreed to participate in the study, 3 of whom harbored 2 different species of MDRNG bacteria, for a total of 38 distinct MDRNG-patient pairs. The surveillance swab correctly identified 30 of the 38 (78.9%) MDRGN bacteria from clinical isolates. The perianal skin swab identified 1 or both MDRNG clinical isolates in 29 (82.9%) of the 35 patients.
Dr. Snyder gave some potential explanations for the findings. "Failure to detect 20% of MDRNG bacteria could be related to a time lag between identification of the organism initially and using a perianal swab. Or perhaps other sites were colonized and we didn't use a swab on those sites. It is also possible that our methods [microbiological plates] may not have been sensitive enough," he suggested.
"In the big picture, we need to carefully consider the methods we use for active surveillance. We need to think about the site of the swab and the methods we use," Dr. Snyder told Medscape Medical News.
More Optimistic View of Study
"I was impressed by the sensitivity of Dr. Snyder's method, which was around 80% in this study. The study consisted of patients already on antibiotics, which would theoretically have decreased the sensitivity of surveillance swabs to detect the organisms," said Mary-Claire Roghmann, MD, from the University of Maryland in Baltimore.
Dr. Roghmann told Medscape Medical News that she thought that the method of perianal swab testing could have higher sensitivity if performed prior to treatment with antibiotics, as occurred in this study.
"Based on this study, I actually think that the use of perianal swabs is promising," she stated.
Dr. Snyder and Dr. Roghmann have disclosed no relevant financial relationships.
50th Interscience Conference on Antimicrobial Agents and Chemotherapy (ICAAC): Abstract K-317. Presented September 12, 2010.