Elsevier

The Lancet

Volume 356, Issue 9243, 18 November 2000, Pages 1757-1759
The Lancet

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Implications of trial results: the potentially misleading notions of number needed to treat and average duration of life gained

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Summary

Consider a physician advising a patient to start antihypertensive treatment. He has the unenviable task of presenting often conflicting trial data in a comprehensible manner. He must justly represent the known risks and benefits such that his patient can make a reasonably informed choice. Should the physician cite the number needed to treat or the average duration of life gained to clarify his explanations?

Section snippets

Number needed to treat

For acute conditions, let us consider thrombolysis for acute myocardial infarction, or antibiotics for pneumonia. Such treatments last for a fixed time. Mortality tends to cluster in time, such that trials can have a follow-up long enough to observe the outcome of the acute episode. NNTs can be calculated by taking the reciprocal of the difference in risk of death between the untreated and treated arm at some time point after randomisation1, 3 (see appendix I).* For example, in ISIS-2 the

Average duration of life gained

If we imagine a placebo-controlled trial in which all patients were followed until death while using assigned treatment, the ADLG would be the difference in mean survival between treated and untreated patients. Survival curves for such a trial would appear as shown in figure 2. As the area under each curve is equal to the mean survival time for each treatment arm, the surface area between the two curves is equal to the ADLG.

Sometimes the ADLG can be obtained directly from published data without

Discussion

No uniformity presently exists in the calculation and presentation of NNTs. For acute conditions with a fixed duration of treatment, NNTs based on differences in risk and expressed as numbers of patients do not materially depend on the duration of follow-up as long as follow-up exceeds the acute phase. For lifelong treatments of chronic conditions, NNTs should not depend on duration of follow-up either. This can be achieved by basing NNTs on differences in hazard and expressing them as

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