"The Numbers Needed to Treat"

Robert W. Griffith, MD

When the doctor advises you to take a particular form of treatment you may want to ask him or her: "Will this cure me, doctor?", or "Will this stop me getting XYZ, doctor?" Chances are the doctor doesn't know the answer to this. But a new tool to improve such communications between patient and doctor exists, and is being examined in use. It's termed "the numbers needed to treat", or NNT, and it's commonly reported along with the results of large clinical trials.

Specifically, the NNT is the number of patients who need to be treated to produce one specified outcome; it should include the treatment duration, and some information on the treatment (drug dose or procedure).

A study of the reaction of patients to information on NNT has been reported in the Annals of Internal Medicine. The patients were presented with theoretical information on a drug to reduce the risk of heart disease and one to reduce the risk of hip fracture. The drugs had to be taken for 5 years. To prevent a heart attack, for drug A the possibilities were: postponement for 2 months for all patients, postponement by 8 months for 1 of 4 patients, or treating 13 patients to prevent one attack over the 5-year period (the NNT option). For preventing hip fractures, the choices were: postponement for 16 days for all patients, postponement for 16 months for 3 of 100 patients, or an NNT of 57 patients to prevent one fracture. It must be emphasized that in each case the 3 options presented represented the same actual chances of outcome, but expressed differently.

What do you suppose the patients chose? For preventing a heart attack, 93% chose the NNT, 82% the long postponement, and 69% the short postponement. For preventing a hip fracture, the same order: NNT 74%, 56% long postponement, and 34% the short postponement.

The conclusions drawn by the authors of the article were that treatment outcomes expressed in terms of NNT produced higher consent-to-treatment rates than those presented as equivalent postponements. Clearly the NNT concept is one which finds favor with patients, if they have had it clearly explained to them. And they don't seem to be phased, in at least two examples, by the relatively high numbers needed to treat. We hope that the NNT becomes more widely used by physicians in discussion with their patients. And if it's not readily available for a specific outcome, the calculation needed to obtain it is not rocket science.

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