Feature Antibiotics

Defining antibiotic effectiveness and resistance: how a private party may soon rule judgments over susceptibility testing

BMJ 2016; 352 doi: https://doi.org/10.1136/bmj.h6849 (Published 06 January 2016) Cite this as: BMJ 2016;352:h6849
  1. Peter Doshi, associate editor, The BMJ
  1. pdoshi{at}bmj.com

Will outsourcing determination of antibiotic effectiveness allow drug companies to influence prescribing? Peter Doshi reports

When treating infections physicians routinely rely on laboratory tests to guide the selection of an effective antibiotic. These reports indicate which drugs the patient’s bacteria are susceptible to and those to which they are resistant, with implications for morbidity and survival.

Traditionally, the Food and Drug Administration has set the thresholds at which antibiotics are deemed effective based on information submitted by the manufacturer. But the US Congress is considering legislation to shift the decision making to private organizations that use other methods. This opens the potential for vested interests to adjust the laboratory thresholds (“breakpoints”) used to distinguish between sensitive and resistant for established drugs and inappropriately influence physicians to select new antibiotics over old ones without compelling evidence of superiority on patient centered outcomes. These new powers could also ultimately mislead the public and governments about the threat of resistant organisms.

Susceptibility testing explained

Bacterial isolates are generally tested for susceptibility to antibiotics using an in vitro test. One method places the specimen from the patient in multiple tubes or plates of growth media and exposes each to increasing concentrations of a given drug. The lowest concentration of drug that inhibits bacterial growth is deemed the isolate’s minimum inhibitory concentration (MIC). When the MIC falls below a predetermined breakpoint established for that organism and drug combination, the isolate is deemed susceptible to the drug. A second predetermined breakpoint differentiates between results interpreted as “intermediate” or “resistant.” The same process is repeated for each antibiotic, and a final laboratory report generally lists all relevant antibiotics to which the organism is susceptible, intermediate, and resistant.

“It’s so fundamental,” says Lisa Plymate, an internist and member of a new task force fighting the proposed law. As a clinician “you …

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