The One Medical Statistic You Need to Know

Dear Readers,

Every single recommendation you will ever get for health care — at least if it’s grounded in science, not fantasy — depends on numbers. How many people live, and how many people die, if they get Treatment A, and what difference if they get Treatment B?  That’s what most medical researchers do: Count human lives and compare groups to see what works and what doesn’t work.

The trouble is that medical news is chock-a-block with intimidating numbers that are hard for a lay person to sort out.  Worse, sometimes the sellers of new medical “breakthroughs” are actually trying to mislead and confuse you.

But now, we ride to the rescue with this: The One and Only Medical Statistic You’ll Ever Need.  It’s called the Number Needed to Treat. This is a statistical idea that is simple and eye-opening. It will not only help you make truly intelligent health care decisions for yourself and your loved ones.  It will also give you an idea of why American health care is so ruinously expensive.  Read on for more.

“Number Needed to Treat:” The Simple Way to Understand What Works  

Powerful ideas are usually simple.  So with the medical statistic called Number Needed to Treat (NNT).  It asks the question: How many people need to get this particular drug/test/treatment in order for ONE person to benefit?
The lower the number, the better.  If the NNT of a treatment is one, that means everyone treated is helped. One person treated equals one person’s life made better. But that’s true only for imminently life-threatening conditions when everyone dies who is not treated: like an appendix about to burst or a heart that has stopped beating and needs to be shocked back into rhythm. For every other medical condition, the NNT is higher than one, sometimes a lot higher.
Screening tests for early detection of cancer frequently have NNT’s in the thousands: one person’s life saved for every few thousand tested.  That can be worthwhile, as long as there is little harm inflicted on the thousands tested. But the reason the PSA test for prostate cancer was recently nixed by the US Preventive Services Task Force is that it not only had a very high NNT — 5,000 or even higher — but also because the test inflicts a lot of harm in the downstream consequences when a man learns he may have early prostate cancer. For every life that may be saved, dozens of men are killed or maimed by the surgery.
Many drug treatments have NNT numbers that show they’re great in some circumstances, not so great in others.  (Aspirin for heart attack is one such drug. See the next story for more on that.)
This story is often repeated in American medicine, especially for lucrative drugs that are still patent-protected from generic competitors. A drug gets tested and proven to work for one condition, and then it gets used for many more conditions without good evidence of usefulness.
The Real NNT for Statins
 One current example, that shows off the usefulness of the NNT, is prescribing statins (cholesterol-lowering drugs) for people who don’t have high cholesterol but do have high levels of an inflammatory blood marker called C-Reactive Protein.  As reported in Patrick Malone’s patient safety blog in 2010, the statin drug makers like to tout a 50% reduction in heart attacks in the research group that took the statins.  But careful examination of the numbers showed that what they meant was a reduction in heart attacks from four in 1,000 patients to two in 1,000 (a 50% drop in what’s called the relative risk rate), but that’s only a real drop of two in 1,000 total, or a Number Needed to Treat of 500.  The point: if you look past the vague and deceptive boasts of “50% improvement” to the real NNT, you find that it takes 500 patients getting the drug every day for years to save one life, and that’s not so impressive after all.

How to Find the Number Needed to Treat for Just About Any Condition

To borrow a line from the late Steve Jobs, here is one “insanely great” website:

It’s all about the Number Needed to Treat.  The site explains the NNT concept and gives dozens of examples in many fields of medicine and disease, with new ones added regularly.

Aspirin’s NNT: It All Depends on the What-For

Take a look at the different NNT’s for using aspirin to treat or prevent heart attack.  You can see the value all depends on how sick the patient is to start with, and what the drug is used for.

  • Aspirin to treat heart attack: NNT = 42.  A very good NNT, especially since we’re talking about administering a drug with a unit cost of maybe ten cents.
  • Aspirin to prevent a second heart attack: NNT = 50. Another excellent NNT.
  • Aspirin to prevent a first heart attack or stroke: NNT = 1,667.  You see how much less bang for the buck you get when healthy people use drugs, even cheap ones like aspirin, to try to prevent disease.  This is still a decent NNT for the price, as long as you’re not someone whose stomach bleeds from aspirin.

And here are a few more examples of NNT’s crunched by the physicians on this website:

  • CT scans to screen high-risk smokers for lung cancer: NNT = 217
  • Defibrillation for cardiac arrest: NNT = 2.5 (one of the best NNT’s out there, highly effective if done in time for an otherwise fatal condition)
  • Coumadin to prevent stroke in someone with atrial fibrillation: NNT = 25 

Some Eye-Popping NNT Numbers for Breast Cancer Screening

Pop quiz: What do you suppose is the Number Needed to Treat for mammograms for women in their 40s? In other words, how many 40-ish women do you think have to get mammograms to save one life? Is it:


A. Eight?

B. Eighty-eight?

C. 297?

D. 5,130?


The answer is “D” — more than 5,000 women in their 40s have to undergo mammograms to prevent a single breast cancer death.   And the reason mammograms are controversial in this age group is not just the slim pickings in lives saved, but the many more women in this age group who are actively harmed by mammograms.  They are subjected to biopsies, mastectomies, other cancer treatments and fear and anxiety because they receive a “false positive” test result that they may have cancer when they really don’t.

If a woman waits to age 50 to get breast cancer screening, the odds of saving a life improve — somewhat — to a NNT of 2,970.  That’s because the older a woman gets, the greater the chances of getting breast cancer, and therefore the higher chance that the screening will catch something bad.  The NNT is still not that great; that’s because breast cancer screening is a lot less beneficial than its advocates claim.

If you want to read the technicalities of how these NNT numbers for mammograms were calculated, click on this link.  Table 2 is where you will find the key numbers.

Special caveat: These figures only apply to average risk women.  Those with a family history of breast cancer or tested positive for the  BRCA gene abnormality have a lot higher risk and therefore a lot more benefit from regular screening.

And that raises a key general point for disease screening tests: High risk people should worry and should get tested. Low risk folks — not so much. The numbers prove that the lower your risk is from the start, the less your chance of being helped by a screening test.


To your continued health!

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Patrick Malone
Patrick Malone & Associates