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Relative and absolute risk

The results of therapeutic trials are commonly expressed as % reduction of an unfavourable (or % increase in a favourable) outcome, i.e. as relative risk, and this can be very impressive indeed until the figures are presented as the number of individuals actually affected per 100 people treated, i.e. as risk.

Where a baseline risk is low, a statement of relative risk alone is particularly misleading as it implies big benefit where actual benefit is small. Thus a reduction of risk from 2% to 1% is 50% relative risk reduction, but it saves only one patient for every 100 patients treated. But where the baseline is high, say 40%, a 50% reduction in relative risk saves 20 patients for every 100 treated.

Relative risk reductions can remain high (and thus make treatments seem attractive) even when susceptibility to the events being prevented is low (and the corresponding numbers needed to be treated are large). As a result, restricting the reporting of efficacy to just relative risk reductions can lead to great — and at times excessive — zeal in decisions about treatment for patients with low susceptibilities.30

A real-life example follows:

Antiplatelet drugs reduce the risk of future nonfatal myocardial infarction by 30% [relative risk] in trials of both primary and secondary prevention. But when the results are presented as the number of patients who need to be treated for one nonfatal

To make clinical decisions, readers of therapeutic studies need to know: how many patients must be treated1' (and for how long) to obtain one desired result (number needed to treat)- This is the inverse (or reciprocal) of absolute risk reduction.

29 See Cooke R J, Sackett D L1995 The number needed to treat: a clinically useful treatment effect. British Medical Journal 310: 452.

30 Sackett D L, Cooke R J 1994 Understanding clinical trials:

What measures of efficacy should journal articles provide busy clinicians? British Medical Journal 309: 755.

myocardial infarction to be avoided [absolute risk] they look very different.

In secondary prevention of myocardial infarction, 50 patients need to be treated for 2 years, while in primary prevention 200 patients need to be treated for 5 years, for one nonfatal myocardial infarction to be prevented. In other words, it takes 100 patient-years of treatment in primary prevention to produce the same beneficial outcome of one fewer nonfatal myocardial infarction.31

In the context of absolute risk, the question whether a low incidence of adverse drug effects is acceptable becomes a serious one.31

Nonspecialist, primary care doctors particularly, need and deserve clear and informative presentation of therapeutic trial results that measure the overall impact of a treatment on the patient's life, i.e. on clinically important outcomes such as morbidity, mortality, quality of life, working capacity, fewer days in hospital, etc. Without it, they cannot adequately advise patients.

Statistical significance and its clinical importance Confidence intervals

Number needed to treat, or absolute risk

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