Revised and reviewed 26 August 2015
OVERVIEW
- Risk is a nebulous term in evidence-based medicine, as it may refer to either relative or absolute risks
- Overall, absolute risks are more meaningful and can be converted in ‘numbers need to treat’ (NNT), which are useful in the clinical setting
RISK
Absolute Risk
- the actual event rate in the placebo or treatment group
Relative Risk
- the ratio of the incidence of disease among exposed to the incidence among non-exposed
- a measure of the strength of an association between groups
- prospective studies (RCTs and cohort studies)
- also called the incidence risk
RR = risk of disease in the exposed (a/a+b) / risk of disease in the non-exposed (c/c+d)
- RR of 3 -> there is three times the risk
- RR of 0.5 -> the risk is halved
- RR of 1 -> there is no association
- if the RR is reported with a CI that includes 1 -> then the RR is not significant.
Relative Risk Reduction
- expressed as a percentage reduction in events in treated vs untreated groups
- = 1 – (incidence in exposed / incidence in unexposed)
Attributable Risk
- a measure of association that provides information about the absolute effect of the exposure or excess risk of disease of those exposed compared to unexposed
Absolute Risk Reduction (ARR)
- incidence in exposed – incidence in unexposed
- a measure of treatment effect
- reverse of attributable risk
NUMBER NEEDED TO TREAT
- NNT is the number of patients who need to be treated in order to avoid one adverse event, which is the reciprocal of the absolute risk reduction
- NNT gives the RR some relevance in terms of the magnitude of clinical effect
- ie. if the incidence of an adverse event = 0.06% & relative risk reduction = 0.33 the absolute risk reduction -> 0.02% and thus numbers needed to treat = 1/0.0002 = 5000
- however, if the incidence of an adverse event was 6% the absolute risk reduction -> 2% and thus the NNT = 1/0.02 = 50
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