Absolute risk reduction

In epidemiology, 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. [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.]

For example, consider a hypothetical drug which reduces the relative 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 rate of colon cancer 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.

Worked example

References

please RRR IS (EER-CER) /CER

ee also

* Absolute risk increase
* Number needed to harm

External links

* [http://gim.unmc.edu/dxtests/Effect1.htm Measures of effect size of an intervention] - unmc.edu.