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Bullish or bearish? The new index could track resistance in Salmonella and many other bacterial pathogens.


A Dow Jones for Drug Resistance

ATLANTA—Antibiotic resistance is a huge, worldwide problem, but making sense of the ever-shifting resistance patterns can be hard, especially for policymakers unschooled in microbiology. Now, there's a new index, developed by health economist Ramanan Laxminarayan, that simplifies matters by expressing the resistance problems for any given microbe in a single number that goes up or down from year to year. The index, not unlike the Dow Jones or the cost-of-living index, could help drive home the importance of smart drug policies and make it easier to compare resistance in different countries.

"It's a fascinating idea," says Keith Klugman, who studies antimicrobial resistance at Emory University here. Klugman chaired a symposium here yesterday at the annual meeting of the American Society of Tropical Medicine and Hygiene where Laxminarayan, who heads the Center for Disease Dynamics, Economics & Policy in Washington, D.C., presented his as-yet-unpublished Drug Resistance Index. (He's welcoming suggestions for a catchier name.)

Reading antimicrobial resistance data can be confusing because there are so many antibiotics. Take a bacterium like Escherichia coli. A whopping 80% of the samples from a given country might be resistant to drug A, 20% to drug B, and 4% to drug C, whereas there may be no resistance problems at all with drugs D and E. But just how bad is that? And if next year, one of these numbers goes up, another down, and the rest stays the same, have things gotten better or worse? A single number that tells the story would make it much easier for policymakers and the general public to understand the issue, says Laxminarayan.

His index—there would be one for every microbe in every country—uses a "basket" of resistance data for different drugs, comparable to the basket of groceries and other expenses that makes up the cost-of-living index. These numbers are weighted according to the amount of each that is used. For instance, if doctors use cephalosporin twice as often as they do penicillin to treat a certain infection, cephalospirin's resistance percentage carries twice as much weight. This way, the index reflects how serious the resistance problem really is, says Laxminarayan. If a microbe develops resistance to a drug that is hardly ever used anyway—because better or cheaper ones exist—than the index will go up only slightly. But if resistance occurs against a drug prescribed to almost every patient, the index will skyrocket.

The new index would make also make it easier to compare resistance data across countries, says Laxminarayan. For instance, he presented the index for Streptococcus pneumoniae, one of the bacteria that can cause pneumonia, in four South American countries over the past 5 years. During that period, the index rose in Peru and Brazil, but it was more or less stable in Argentina and Venezuela. You could also produce an aggregate index for an entire continent, says Laxminarayan, or for all the microbes that can cause pneumonia together.

"It's a very elegant tool because it encompasses both resistance and use," says Heiman Wertheim, who studies drug resistance in Vietnam at the Oxford University Clinical Research Unit in Hanoi. "I think it can really help get the problem of resistance on policymakers' agendas."

One problem with the index is that you need detailed information, updated frequently, about resistance as well as antibiotic consumption, preferably broken down by the bug for which a drug was prescribed. In many countries—especially the poor ones—such data are hard if not impossible to come by. But Laxminarayan says that widespread adoption of the index would provide an impetus to start collecting more data on antibiotics, which he says is a good idea anyway.