Sunday, April 17, 2005
In cardiovascular medicine, NNT remains a popular measure, most commonly referring to the number required to avoid one death. In the treatment of myocardial infarction, the NNT for systematic nitrate use is about 250. At the other extreme, NNT for 2-year beta-blocker use after myocardial infarction in patients with heart failure or moderate-to-severe left-ventricular dysfunction is about 33. Although the former is regarded as too high to meet the threshold for recommending generalised use, the latter is the basis for a firm endorsement.
NNT puts the emphasis on the positive side of therapeutic intervention. “To save one life” is a powerful driving force in clinical medicine. However, it tends to obscure the reality that, too often, very large numbers of patients are being treated without benefit.
We suggest a new index that is complementary to NNT. It could be called “numbers treated needlessly” (NTN). If NNT is 250, NTN would be 249; the higher the number, the greater the treatment burden. An NNT of 33 gives an NTN of 32, certainly better but a substantial burden nonetheless. Alternatively, NTN could be rendered as a percentage: if NNT is 250, 249 of 250 individuals or 99·6% are treated without benefit. This proportion could be called the “index of therapeutic impotence” (ITI). The ITI for an NNT of 33 would be 97·0%.
Here's a look at numbers needed to treat, or numbers treated needlessly, as it relates to cholesterol lowering:
In studies of treatment of hypercholesterolemia in patients with known coronary artery disease (secondary prevention), NNTs have varied from 12 to 33. In other words, 12 to 33 patients with known coronary artery disease need to be treated for five years to prevent one fatal or nonfatal heart attack. For patients without coronary artery disease, studies have shown 42 to 55 patients need to be treated for five years to prevent one event. In contrast, a very effective treatment such as triple-drug therapy to eradicate H. pylori has an NNT in the 2 to 4 range.
So, 98% of people without known coronary artery disease who are taking cholesterol medication will never benefit from it. That's a lot of people taking expensive medication to no benefit to themselves. The beauty of it, from a drug marketing stand point, is that no one can predict which patients will fall in the lucky 2%. And that's why the commercials are so effective. But we physicians could certainly play a more active role in helping our patients understand the truth behind the numbers.
UPDATE: From a reader:
NNT needs to be balanced against cost and metrics of side effects in
order to determine the probable increase in lifespan compared to cost
and quality of life. By itself, a NNT of 250 (or a NTN of 249, as you
put it) doesn't mean that a treatment is undesirable/not worthwhile.
For instance, about 14,000 patient-years of screening with mammography
is required to prevent one breast cancer death; still, we tend to
think mammography is valuable due to the large probable increase in
lifespan for each case detected.
Beta-blockers weren't my example. They were the example of the authors of the article, who acknowledged they may be worth the cost. My example was statins for lowering cholesterol, which may arguably not be worth the cost.
posted by Sydney on 4/17/2005 07:35:00 AM 1 comments
By 3:46 PM, at