Potential Shortcoming of Antibiotic Lab Tests Determined

As multidrug-resistant organisms continue to emerge, specific tests, called antibiotic susceptibility assays, are increasingly critical. Clinicians depend on reliable results when choosing the right drug to treat patients.

To determine which antibiotics reliably treat which bacterial infections, diagnostic laboratories that focus on clinical microbiology test pathogens isolated from patients for antibiotic sensitivity. However, a recent study reveals that one aspect of these tests may fall short and not be stringent enough.


Microbiologists at the Beth Israel Deaconess Medical Center (Boston MA, USA) and their colleagues examined pathogens cited by the US Centers for Disease Control and Prevention (Atlanta GA, USA) and the World Health Organization (Geneva, Switzerland) as urgent and concerning drug resistance threats.

To obtain consistently reliable results, scientists conducting antibiotic susceptibility assays follow national guidelines with standardized methods, including the use of a specific number of organisms, or "inoculum" that is added to each assay. There is a target inoculum, and then a range of an allowable inoculum, or acceptable upper and lower bounds around the target inoculum.

The team found that the susceptibility determination against two broad spectrum drugs, meropenem and cefepime, were dramatically affected by inoculum differences within the allowable range of inoculum. Although they have no idea about how often clinical laboratories deviate even beyond the allowable range, they expect this happens with some frequency and would further skew results. The findings indicate that clinical microbiology laboratories must hit the target inoculum as accurately as possible to obtain reliable testing results for multidrug-resistant pathogens.

James Kirby, MD, the Director of the Clinical Microbiology Laboratory and senior co-author, said, “Our question was whether this wiggle room impacts results. Our findings were clear: inoculum matters.” The study was published on May 21, 2018, in the journal Antimicrobial Agents and Chemotherapy.

Related Links:
Beth Israel Deaconess Medical Center
US Centers for Disease Control and Prevention
World Health Organization