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Systematic Review Series

Series Editors: Cynthia Mulrow, MD, MSc Deborah Cook, MD, MSc

From the University of Oxford, Oxford, United Kingdom

For definitions of terms used in this article, see glossary at end of text.

Using Numerical Results from Systematic Reviews in Clinical Practice

Henry J. McQuay, DM; and R. Andrew Moore, DSc
Ann Intern Med. 1997;126(9):712-720. doi:10.7326/0003-4819-126-9-199705010-00007
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Systematic reviews summarize large amounts of information and are more likely than individual trials to describe the true clinical effect of an intervention.Traditional statistical outputs from systematic reviews cannot immediately be applied to clinical practice. The number needed to treat (NNT) has that clinical immediacy. This number can be calculated easily from raw data or from statistical outputs, and the principle involved in its calculation can be applied to different outcomes: treatment efficacy, adverse events (harm), or other end points. The NNT defines the treatment-specific effect of an intervention, and we suggest it as a currency for making decisions about individual patients. Knowing the NNT for different interventions that have the same outcome for the same disorder can help shape individual and institutional practice. Knowing or estimating the number needed to harm is also an important part of the equation. Knowing or estimating an individual patient's risk can, with the NNT, be a guide to the overall or net value of a prophylactic intervention. We advocate an approach to systematic reviews that distills information into, in effect, one number: the NNT. This is simple to remember and directly supports efforts to work with patients to make the best possible clinical decisions for their care.


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Figure 1.
Issues involved in making choices in prophylaxis.

NNT = number needed to treat.

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Figure 2.
Calculation of the number needed to treat (NNT) from odds ratios.[28]

Table for estimating the NNT when the odds ratio (OR) and control event rate (CER) are known, published for preventive interventions in . The formula for determining the NNT for preventive interventions is {1 −[CER x (1 −OR)]}/[(1 −CER) x CER x (1 −OR)]. For treatment, the formula is [CER (OR −1) + 1]/[CER (OR −1) x (1 −CER)].

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