Weaknesses of the NNT Metric
The idea of NNT provides clinicians with a method of explaining the relative benefit or harm of a given therapy for a patient. Because of its simplicity, NNT has the following weaknesses:
- It is usually described as a point estimate instead of a confidence interval of the observed therapeutic effect. This has led some authors to report NNT with a corresponding confidence interval (eg, NNT 5 (95% CI 3 to 9))
- As with other descriptions of benefit, NNT does not account for a patient’s baseline risk. If a patient’s individual risk is higher or lower than that studied in a trial, his or her NNT will be lower or higher, respectively.
- When describing NNT, the comparator is an essential component. The NNT of a given treatment will be very different when describing the value versus placebo instead of another active therapy.
- The time frame of a given study is important and the benefit of a treatment is usually not linear over time. For example, if a treatment was conducted over a mean of 4 years, its NNT should be expressed with the same time component (eg, 12 patients need to be treated over about 4 years…).
