EL NUMERO NECESARIO A TRATAR

The "number needed to treat" turns 20 — and continues to be used and misused

CMAJ • September 9, 2008; 179 (6). doi:10.1503/cmaj.080484.© 2008 Canadian Medical Association or its licensors

In the 20 years since the initial description of the number needed to treat,1 this method of expressing the efficacy of an intervention has become widely used. Indeed, the Consolidated Standards of Reporting Trials statement recommends that the number needed to treat be reported in randomized trial publications,2 and journals of secondary publication (e.g., American College of Physicians Journal Club) routinely calculate and report the number needed to treat for studies of therapy. As well, there have been increasing calls for health care policy makers to use numbers needed to treat to inform their recommendations;3 and league tables comparing numbers needed to treat have appeared in the literature4–7 and on the internet (See www.cebm.utoronto.ca/glossary/nnts.htm#table and www.jr2.ox.ac.uk/bandolier/band50/b50-8.html for examples from different branches of medicine).
Having attended hundreds of journal clubs as well as departmental and divisional rounds over the past 2 decades, I am consistently impressed by the frequency with which audience members display skepticism about a therapy if its efficacy is presented only in relative terms such as odds ratios or relative risk reductions. Not infrequently, this skepticism is healthy — the dangers of misinterpreting the importance of a therapy when relying solely on relative effect estimates are well known.1 However, I have also been struck by the extent to which discussions of a therapy's number needed to treat, and even comparisons between therapies on this basis, are accepted at face value. A review of the literature and their experiences in journal club and critical appraisal settings led Chong and colleagues to also express concern that many clinicians appear to hold "the impression that NNT [number needed to treat] values in and of themselves are broadly comparable" and display "an implicit belief that an unadjusted NNT value adequately captures the overall worth of a treatment."8
In this article, I explore the factors (beyond the efficacy of a therapy) that influence the number needed to treat and that must be taken into account when comparing these values between therapies.
What is the number needed to treat?
The number needed to treat is an aggregate measure of clinical benefit that represents the number of patients who would need to be treated to prevent 1 additional adverse event. It is calculated by taking the reciprocal of the absolute risk reduction between 2 treatment options. This number is a useful way to summarize the potential impact of a therapy when discussing treatment options with patients. A detailed discussion of how to personalize this number to each patient's situation, including means to incorporate potential harms as well as patient values and preferences, has been published.9..................

http://www.cmaj.ca/cgi/content/full/179/6/549