Number needed to treat
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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 pA and pB of this endpoint under treatments A and B, respectively, are known, then the NNT is computed as 1/(pB-pA).
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.
Contents |
[edit] 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 |
[edit] Reference
- ^ 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.
[edit] See also
- Number needed to harm - the converse for side-effects