What is the correct formula for the number needed to treat (NNT) in most clinical contexts?

What is the correct formula for the number needed to treat (NNT) in most clinical contexts?

• A. NNT equals the absolute risk increase (not typically used).
• B. NNT equals the relative risk.
• C. NNT equals the absolute difference in risk (ARR).
• D. NNT equals 1 divided by ARR.

Answer: (D)

NNT shows how many patients need to be treated to prevent one additional adverse outcome. In most clinical contexts it is the reciprocal of the absolute risk reduction. ARR is the difference between event rates in the control and treated groups (control event rate minus treatment event rate). Therefore, NNT = 1 / ARR. For example, if a treatment lowers the event rate from 20% to 10%, the ARR is 0.10, and the NNT is 1 / 0.10 = 10. This means you’d need to treat about 10 patients to prevent one event. Relative risk is a ratio, not the NNT, and ARR alone is a difference, not its reciprocal. If the treatment increases risk (negative ARR), the NNT isn’t used in the same way (we’d use NHN for harms).

NNT shows how many patients need to be treated to prevent one additional adverse outcome. In most clinical contexts it is the reciprocal of the absolute risk reduction. ARR is the difference between event rates in the control and treated groups (control event rate minus treatment event rate). Therefore, NNT = 1 / ARR.

For example, if a treatment lowers the event rate from 20% to 10%, the ARR is 0.10, and the NNT is 1 / 0.10 = 10. This means you’d need to treat about 10 patients to prevent one event. Relative risk is a ratio, not the NNT, and ARR alone is a difference, not its reciprocal. If the treatment increases risk (negative ARR), the NNT isn’t used in the same way (we’d use NHN for harms).