Saturday, October 18, 2014

Measuring the effectiveness of medication

Sarah Fallon at Wired has an informative story on putting a number, called the number needed to treat (NNT), on the effectiveness of medication and procedures.

Developed by a trio of epidemiologists back in the ’80s, the NNT describes how many people would need to take a drug for one person to benefit. [...] If your kid is throwing up and you take her to the hospital, she might get a drug called Zofran. The NNT for that is 5, meaning that only five kids need to take Zofran for one of them to stop throwing up.

The story goes on to talk about a site called with the tagline, "Quick summaries of evidence-based medicine."

It’s unfortunate ... that the NNT is not a statistic that’s routinely conveyed to either doctors or patients. But you can look it up on a site that you’ve probably never heard of: Started by David Newman, a director of clinical research at Icahn School of Medicine at Mount Sinai hospital, the site’s dozens of contributors analyze the available studies, crunch the numbers on benefits and harms, and then post the results.

Here's a bit more on NNT and how it is assigned to a medication or a treatment procedure:

As statistical tools go, the idea of the number needed to treat is relatively new. It was first described in 1988 by epidemiologists Andreas Laupacis, David Sackett, and Robin Roberts in a New England Journal of Medicine article titled “An Assessment of Clinically Useful Measures of the Consequences of Treatment.” They start by sketching out the problems with a number called the relative risk reduction. That’s the measure you often see hyped in media reports of scientific studies. Imagine, for example, a study of heart disease that finds that a new drug reduces the risk of death by an astonishing 50 percent. The reality behind that number is that the risk of death over a 10-year period for, say, a healthy 45-year-old man weighing 200 pounds went from 5 percent to 2.5 percent—50 percent! Such a finding is clinically significant, yes. Worthy of publication, maybe. But not quite as astonishing.

It would be better, the authors write, to look at a number called the absolute risk reduction—the 2.5 percent reduction that resulted from the new drug. But working with that measure can be hard to understand, because it is actually a percent of a percent. To make it more intuitive and apprehendable, the authors explain, you can use the inverse of absolute risk reduction: Divide 1 by 2.5 percent, or .025, to get 40. And that’s the number needed to treat. Forty people have to take the drug for one person to benefit. So is it worth taking? That depends. The NNT isn’t crazy high, so you might go for it, especially since a heart attack can kill you. But if the drug has terrible side effects, you might not.