Group exposed to a treatment (left) has reduced risk of an adverse outcome (grey) compared to the unexposed group (right). 4 individuals need to be treated to prevent 1 adverse outcome (NNT = 4).
The number needed to treat (NNT) or number needed to treat for an additional beneficial outcome (NNTB) is an epidemiological measure used in communicating the effectiveness of a health-care intervention, typically a treatment with medication. The NNT is the average number of patients who need to be treated to prevent one additional bad outcome (e.g. the number of patients that need to be treated for one of them to benefit compared with a control in a clinical trial). It is defined as the inverse of the absolute risk reduction, and computed as , where is the incidence in the control (unexposed) group, and is the incidence in the treated (exposed) group.[1][2] This calculation implicitly assumes monotonicity, that is, no individual can be harmed by treatment. The modern approach, based on counterfactual conditionals, relaxes this assumption and yields bounds on NNT.
A type of effect size, the NNT was described in 1988 by McMaster University's Laupacis, Sackett and Roberts.[3] While theoretically, the ideal NNT is 1, where everyone improves with treatment and no one improves with control, in practice, NNT is always rounded up to the nearest round number[4]
and so even a NNT of 1.1 becomes a NNT of 2[5]
. A higher NNT indicates that treatment is less effective.[6]
NNT is similar to number needed to harm (NNH), where NNT usually refers to a therapeutic intervention and NNH to a detrimental effect or risk factor. A combined measure, the number needed to treat for an additional beneficial or harmful outcome (NNTB/H), is also used.
^Porta M, ed. (2016-07-21). "A Dictionary of Epidemiology". Dictionary of Epidemiology - Oxford Reference. Oxford University Press. doi:10.1093/acref/9780199976720.001.0001. ISBN 9780199976720. Retrieved 2018-05-09.
^Vancak, V., Goldberg, Y., Levine, S. Z. (2020). "Systematic analysis of the number needed to treat". Statistical Methods in Medical Research. 29 (9): 2393–2410. doi:10.1177/0962280219890635. PMID 31906795. S2CID 210041962.{{cite journal}}: CS1 maint: multiple names: authors list (link)
^Laupacis A, Sackett DL, Roberts RS (1988). "An assessment of clinically useful measures of the consequences of treatment". N. Engl. J. Med. 318 (26): 1728–33. doi:10.1056/NEJM198806303182605. PMID 3374545.
^Richard T, Vanhaeverbeek M, Van Meerhaeghe A (September–October 2011). "The number needed to treat (NNT)". Revue Médicale de Bruxelles. 32 (5): 453–458. PMID 22165523.
^Citrome L (2011). "Number Needed to Treat: What It Is and What It Isn't, and Why Every Clinician Should Know How to Calculate It". The Journal of Clinical Psychiatry. 72 (3): 412–413. doi:10.4088/JCP.11ac06874. PMID 21450157.
^"Number Needed to Treat". Bandolier. Archived from the original on 2020-10-19. Retrieved 2017-04-21.
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