February 23, 2007
NNT – Number Needed to Treat
TIME magazine this week has an article on what it calls “medicine’s secret stat,” NNT or number needed to treat. The magazine uses the example of a study that might show that statin drugs lower the risk of heart attacks by 30%.
“But that number is meaningless unless you take into account the percentage of men in both groups who have heart attacks in the first place. If those people represent only a tiny fraction of the two populations, an improvement of 30% isn’t much–maybe one heart attack fewer in a group of thousands.
The effort to handicap those odds is where NNT comes in. It answers the question, How many people have to take this drug to avoid one heart attack? The same principle can be applied to avoiding one recurrence of cancer or stroke or whatever end point you choose to measure. In healthy men, the NNT for statins is about 50 (depending on which of dozens of statins is taken, age, family history, lifestyle and so on). So 50 men have to take these drugs in order to prevent a single–not necessarily fatal, heart attack.
Presented that way, taking statins sounds like less of a no-brainer–especially given that the drugs cost hundreds of dollars a year, side effects could include liver and muscle damage and you have to take twice-yearly blood tests just in case. Still, factored out over the entire U.S. population, even a 1-in-50 figure means many thousands of heart attacks are avoided every year.
Since public-health officials want to save lives, they focus on the thousands and avoid the NNT. Since pharmaceutical companies are in business to sell drugs, they do the same. Those two forces have kept NNT from being a big part of medical education. We could all help change that by doing nothing more than asking for the number up front the next time we’re handed a prescription.”
But the TIME article didn’t discuss how journalists could be and should be using NNT in their reporting.
That’s why we put a little primer on the topic on our HealthNewsReview.org website. More consumers would understand the topic, and ask their doctors about it, if more journalists put the concept in more stories about research studies.
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