Absolute risk reduction

From Wikipedia, the free encyclopedia

The absolute risk reduction is the decrease in risk of a given activity or treatment in relation to a control activity or treatment. It is the inverse of the number needed to treat.^[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 absolute risk reduction for a 5-year treatment with the drug is therefore 1/6,000, as 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, absolute risk reduction is usually 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 treatment with placebo, i.e. 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 absolute risk reduction is computed as p_B-p_A.

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

1 Worked example
2 Reference
3 See also
4 External links

[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.