SAN FRANCISCO – Vancomycin and glycopeptide resistance detection e-tests are better than the automated microbiology systems used in many hospitals when it comes to detecting vancomycin-resistant Staphylococcus aureus infections, according to researcher Meghan Jeffres, Pharm.D
An associate professor of pharmacy practice at Roseman University of Health Sciences in Henderson, Nev., Dr. Jeffres reached her conclusion after testing 63 blood, 115 respiratory, and 29 cystic fibrosis sputum S. aureus isolates for vancomycin susceptibility using all three methods.
Dr. Meghan Jeffres
She reported that a GRD (glycopeptide resistance detection) e-test vancomycin MIC (minimum inhibitory concentration) of 8 mcg/mL or more, in conjunction with a vancomycin e-test MIC of 4 mcg/mL or less, is diagnostic of heteroresistant vancomycin-intermediate S. aureus (hVISA). By itself, a vancomycin e-test MIC of 1.5 mcg/mL or greater is associated with poor outcomes, and suggests the need for an alternative antibiotic, Dr. Jeffres said at the annual Interscience Conference on Antimicrobial Agents and Chemotherapy.
The combined GRD and vancomycin e-tests found 10 hVISA isolates, all respiratory.
Ninety-three (45%) of the isolates, however, had vancomycin e-test MICs of 1.5 mcg/mL or more. Of those 93 isolates that vancomycin e-testing suggested needed a vancomycin alternative, the hospital’s BD Phoenix Automated Microbiology System identified only one as being resistant to vancomycin, with an MIC of 2 mcg/mL; it reported the rest as having vancomycin MICs of 0.5 or 1 mcg/mL, suggesting susceptibility to vancomycin.
"Our automated system accurately identified one out of 93 isolates" as needing something other than vancomycin, she said. "As a clinician, when your isolates are 1 and 0.5, you treat it as a susceptible isolate." Ultimately, if those results come from automated testing, "it’s a false sense of security. None of the [automated testing systems] are very good at reporting vancomycin MICs," Dr. Jeffres said.
Because of that, many larger academic institutions have moved to e-testing, but other hospitals – such as the large community hospital in Las Vegas where Dr. Jeffres did her research – still rely heavily on automated vancomycin resistance testing.
That can have serious consequences. Dr. Jeffres’ e-test results were not reported in patients’ medical records, so clinicians at the hospital used the automated results to help make treatment decisions. All 10 of the hVISA isolates reported out of automated testing had vancomycin MICs of 0.5 or 1 mcg/mL, indicating vancomycin susceptibility.
Three of those patients were treated with vancomycin; they died. The rest were treated with linezolid – a vancomycin alternative – probably based on clinical hunches. Those patients lived, and were able to be sent home or to rehab.
"There is no statistical analysis" here, "but you can’t help but to at least pause," given the results. Had even the [vancomycin] e-test alone been done by clinicians, "there’s no way" the three patients would have gotten vancomycin, Dr. Jeffres said.
E-testing adds a bit to the cost of assessing vancomycin resistance; the GRD e-test costs about $5.39, the vancomycin e-test about $2.90, and the necessary blood agar about 79 cents. Alternative antibiotics are more expensive than vancomycin, as well.
"However, dead people cost way more, from both economic and humanistic points of view," she said. E-testing will increase a hospital’s pharmacy budget, especially if it leads to alternative antibiotics, but overall, the hospital budget should stay the same if not decrease, Dr. Jeffres said.
The conference was sponsored by the American Society for Microbiology. Dr. Jeffres disclosed research funding from Pfizer, the maker of linezolid.