Personal perils: are numbers needed to treat misleading us as to the scope for personalised medicine?
A common misinterpretation of Numbers Needed to Treat is causing confusion about the scope for personalised medicine.
Thirty years ago, Laupacis et al1 proposed an intuitively appealing way that physicians could decide how to prioritise health care interventions: they could consider how many patients would need to be switched from an inferior treatment to a superior one in order for one to have an improved outcome. They called this the number needed to be treated. It is now more usually referred to as the number needed to treat (NNT).
Within fifteen years, NNTs were so well established that the then editor of the British Medical Journal, Richard Smith could write: ‘Anybody familiar with the notion of “number needed to treat” (NNT) knows that it’s usually necessary to treat many patients in order for one to benefit’2. Fifteen years further on, bringing us up to date, Wikipedia makes a similar point ‘The NNT is the average number of patients who need to be treated to prevent one additional bad outcome (e.g. the number of patients that need to be treated for one of them to benefit compared with a control in a clinical trial).’3
This common interpretation is false, as I have pointed out previously in two blogs on this site: Responder Despondency and Painful Dichotomies. Nevertheless, it seems to me the point is worth making again and the thirty-year anniversary of NNTs provides a good excuse. Continue reading