How to assess the evidence for the safety of medical interventions

Luigi Naldi


As for any other human activities, medical interventions may carry a risk of unintended adverse events. Whenever a physician prescribes a drug, there is the potential for an adverse reaction connected with drug use. Despite limited accurate data, a widely cited meta-analysis of 39 prospective studies performed in US hospitals from 1966 to 1996 found that the incidence of severe adverse drug reactions (i.e. life-threatening reactions and reactions that prolonged hospitalisation) among inpatients was 6-7%, with 0-32% being fatal.1 Despite these impressive figures, the rate of severe adverse reactions for any given drug is usually very low. However, the system works in such a way that even a small increase in the incidence of a clinically severe reaction may prompt the withdrawal of the implicated drug from the market.

It is commonly stated that clinical decisions should balance the benefit of the available options with the risk. A difficulty stems from the fact that data on benefits and risks of medical interventions are usually derived from different study designs and information sources. A large part of our discussion will be focused on the safety of drug use. While systems to survey the safety of medications are well established, they are not for other medical interventions such as surgical procedures and invasive diagnostic tests. It is well accepted that no in vitro or animal models can accurately predict adverse events associated with drug use before the drug is employed in humans. Advances in understanding the causes of adverse reactions (for example pharmacogenomics) may, in the future, enable the risk in individual patients to be predicted in a more reliable way.2-4

Data sources for determining the safety of medical interventions The limitation of randomised controlled trials (RCTs)

The great strength of RCTs is the ability to provide an unbiased estimate of treatment effect by controlling not only for determinants of outcome we know about, but also for those we do not know about. If RCTs demonstrate an important relationship between an agent and an adverse event, then we can be confident of the results. However, RCTs are usually designed to document frequent events, that is, those associated with the intended effect of a treatment. With the usual sample size, which rarely exceeds a few thousand people, RCTs are not suited to accurate documentation of the safety of medical intervention for uncommon events.5,6 Besides the issue of statistical power, additional limitations include the usual short duration of most clinical trials and the careful selection of the eligible population (restriction in patient selection according to age, comorbidity, etc.). All in all, when an intervention has been proved to be effective in an RCT, the safety issue still remains to be well established. Pharmaceutical companies may strive to work out the adverse effect profile of a drug before licensing, but because only a limited number of selected individuals can be exposed to the drug before it is released, only common adverse events can be accurately documented and the complete range of adverse events remains to be elucidated in the post-marketing phase. This limitation is particularly true for delayed reactions and rare but severe acute events.

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