Number needed to treat

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Overview

The number needed to treat (NNT) is an epidemiological measure that indicates how many patients would require treatment with a form of medication to reduce the expected number of cases of a defined endpoint by one. It is defined as the inverse of the absolute risk reduction. It was described in 1988.^[1]

For example, consider a hypothetical drug which reduces the risk of colon cancer by 50%. Even without the drug, colon cancer is fairly rare, maybe 1 in 3,000 in every 5 year period. The NNT for a 5-year treatment with the drug is therefore 6,000: by treating 6,000 people with the drug, one can expect to reduce the number of colon cancer cases from 2 to 1.

In general, NNT is always computed with respect to two treatments A and B, with A typically a drug and B a placebo (in our example above, A is a 5-year treatment with the hypothetical drug, and B is no treatment). A defined endpoint has to be specified (in our example: the appearance of colon cancer in the 5 year period). If the probabilities p_A and p_B of this endpoint under treatments A and B, respectively, are known, then the NNT is computed as 1/(p_B-p_A).

The NNT is an important measure in pharmacoeconomics. If a clinical endpoint is devastating enough (e.g. death, heart attack), drugs with a high NNT may still be indicated in particular situations. If the endpoint is minor, health insurers may decline to reimburse drugs with a high NNT.

Worked example

Abbreviation	Variable	Equation	Value
-	subjects in control group	-	250
-	subjects in experimental group	-	150
-	events in control group	-	100
-	events in experimental group	-	15
CER	control event rate	= events / subjects in control group	0.4, or 40%
EER	experimental event rate	= events / subjects in experimental group	0.1, or 10%
ARR	absolute risk reduction (or increase)	= CER - EER	0.3, or 30%
RRR	relative risk reduction (or increase)	= (CER - EER) / CER	0.75
NNT	number needed to treat/number needed to harm	= 1 / ARR	3.33
OR, RR	odds ratio, relative risk (not really identical, but similar -- see articles for details)	= CER / EER	4

See also

• Number needed to harm - the converse for side-effects

Reference

1. ↑ Laupacis A, Sackett DL, Roberts RS. An assessment of clinically useful measures of the consequences of treatment. N Engl J Med 1988;318:1728-33. PMID 3374545.

External links

• Bandolier article on NNT
• Number needed to treat (Slate)
• What is an NNT? (Hayward Medical Communications)
• Number Needed to Treat (Centre for Evidence Based Medicine)
• Online Calculator for NNT (GraphPad Software)