What does NNT represent?

• A. The number of patients to treat to prevent one adverse outcome
• B. The number of patients who experience the adverse outcome despite treatment
• C. The threshold probability for treatment decision
• D. The total cost to prevent one event

Answer: (A)

The main idea here is understanding what NNT measures in clinical practice. NNT tells you how many patients you would need to treat to prevent one adverse outcome, on average, over a specified time period. It’s the reciprocal of the absolute risk reduction (ARR), where ARR is the difference between the risk of the adverse outcome without treatment and the risk with treatment.

For example, if 10% of patients have the adverse outcome without the therapy and 7% have it with the therapy, the ARR is 0.10 − 0.07 = 0.03. The NNT is 1 divided by 0.03, which is about 33. This means you’d need to treat roughly 33 patients to prevent one adverse event during the study period.

This concept is specific to a particular outcome and time frame. It’s different from simply counting how many patients experience the event despite treatment, which would reflect risk while on therapy rather than the preventive impact. It’s also not a threshold for deciding treatment nor a measure of cost; those are separate concepts (decision thresholds and cost-effectiveness, respectively).