Medicine's Secret Stat

Researchers can gather all the hard-nosed evidence they want about the effectiveness of a particular drug or treatment. But there's one figure doctors don't much talk about despite its importance. It's called number needed to treat, or NNT, a new measure developed in the past 20 years that's one of the best-kept statistical secrets in medicine.

The idea of NNT is simple enough. Most clinical trials look at how much better people do on a particular medicine compared with how they would do without it or whether they should be on a different medicine. Take statins, drugs that aim to reduce bad cholesterol. A typical trial might give one group of men a statin for, say, five years and give a second group a placebo, or fake pill.

Generally, you will see fewer heart attacks in the statin group (about 30% fewer in one real-world trial). Reducing the risk by a third sounds like a lot, which is one reason many hundreds of thousands of men with no sign of heart disease take statins. 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.