The number needed to treat (NNT) is a simple method of looking at the benefit of a treatment intervention to prevent a condition or complication. It is the inverse of the absolute risk reduction for the treated versus untreated control populations. It can be used to extrapolate findings in the literature to a given patient at an arbitrary specified baseline risk when the relative risk reduction associated with treatment is constant for all levels of risk.
Variables:
(1) number of people in control group
(2) number of people in control group who develop condition of interest during the time interval
(3) number of people in active treatment group
(4) number of people in active treatment group who develop condition of interest during the time interval
event rate in control group =
= (number of people in control group with condition) / (number of people in control group)
event rate in active treatment group =
= (number of people in active treatment group with condition) / (number of people in active treatment group)
relative risk reduction =
= relative risk difference =
= ((event rate in control group) - (event rate in active treatment group)) / (event rate in control group)
absolute risk reduction =
= absolute risk difference =
= (event rate in control group) - (event rate in active treatment group)
number needed to treat =
= 1 / (absolute risk reduction) =
= 1 / ((event rate in control group) - (event rate in active treatment group))
Interpretation:
• The number needed to treat indicates the number of patients who need to be treated to prevent the condition of interest during the time interval.
• The smaller the number needed to treat, the greater the benefit of the treatment to prevent the condition.
• The number needed to treat should be considered together with other factors such as the seriousness of the condition to be prevented and the risk of adverse side effects from the treatment.
