Numbers needed to treat derived from meta-analyses - sometimes informative, usually misleading

Numbers needed to treat are often used to summarise treatment effects in a clinically relevant way. They are derived from the baseline risk without treatment and the reduction in risk achieved with treatment. Numbers needed to treat are sensitive to factors that change the baseline risk such as the outcome considered, patients' characteristics, secular trends in incidence and case fatality, and the clinical setting. Pooled numbers needed to treat derived from meta-analyses can be seriously misleading because the baseline risk often varies appreciably between the trials. Applying the pooled relative risk reductions calculated from meta-analyses or individual trials to the baseline risk relevant to specific patient group produces a useful number needed to treat.