The crucial health stat you’ve never heard of

Tuesday, September 26th, 2006

Darshak Sanghavi explains that the crucial health stat you’ve never heard of is something called NNT, or number needed to treat:

Researchers in Scotland reported a 31-percent reduction in the risk of heart attacks among men taking the statin pravastatin, sold by Bristol-Myers Squibb under the brand name Pravachol. Due in part to this study, Pravachol became one of Bristol-Myers’ most profitable drugs and now grosses more than $2 billion in sales per year.

A 31 percent reduction in heart attacks, after all, seems impressive. Yet this pervasive way of describing clinical trials in medical journals — focusing on the “relative risk,” in this case of heart attack — powerfully exaggerates the benefits of drugs and other invasive therapies. What, after all, does a 31 percent relative reduction in heart attacks mean? In the case of the 1995 study, it meant that taking Pravachol every day for five years reduced the incidence of heart attacks from 7.5 percent to 5.3 percent. This indeed means that there were 31 percent fewer heart attacks in patients taking the drug. But it also means that the “absolute risk” of a heart attack for any given person dropped by only 2.2 percent (from 7.5 percent to 5.3 percent). The benefit of Pravachol can be summarized as a 31 percent relative reduction in heart attacks — or a 2.2 percent absolute reduction.
In the end, 100 people needed to be treated to avoid two heart attacks during the study period — so, the number of people who must get the treatment for a single person to benefit is 50. This is known as the “number needed to treat.”

Instead of a 31-percent relative reduction in heart attacks, we get this:

To a savvy, healthy person with high cholesterol that didn’t decrease with diet and exercise, a doctor could say, “A statin might help you, or it might not. Out of every 50 people who take them, one avoids getting a heart attack. On the other hand, that means 49 out of 50 people don’t get much benefit.”

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