Number needed to treat (NNT) is defined as which of the following?

• A. NNT equals the absolute risk reduction.
• B. NNT equals the relative risk.
• C. NNT equals 1 divided by the absolute risk reduction; a lower NNT indicates greater treatment effectiveness.
• D. NNT measures the cost per prevented event.

Answer: (C)

Number needed to treat shows how many patients must receive the treatment to prevent one additional adverse outcome over a defined time period. It is calculated as the inverse of the absolute risk reduction: NNT = 1 / ARR. The absolute risk reduction is the difference in event rates between the control group and the treated group (ARR = control event rate − treated event rate). For example, if the control group has a 10% event rate and the treated group has a 5% event rate, ARR = 0.10 − 0.05 = 0.05, so NNT = 1 / 0.05 = 20. This means about 20 people would need to be treated to prevent one additional event, with a smaller NNT indicating a more effective treatment. Keep in mind that NNT depends on the time horizon and the population being studied. NNT is not the same as relative risk, and it does not measure cost per prevented event.