Compelling or irrelevant? Using number needed to treat can help decide.

OBJECTIVE: The metric of number needed to treat (NNT), defined as the number of patients who need to be treated to achieve one additional favorable outcome, can help clinicians appraise claims that one intervention is meaningfully superior to the other.

METHOD: A review of the use of NNT to evaluate the differences between interventions in the treatment of depression, schizophrenia and bipolar disorder. Instead of using disparate measures such as point change on a rating scale, kilograms gained over time or relative differences, results can be converted into a common unit of measure -'patient units'- so that the clinician can anticipate how often actual differences between interventions would be expected to be observed. Calculation of NNT is demonstrated using reports published in the psychiatric literature, together with different graphical techniques to display this.

RESULTS: Clinical trial results expressed as NNT can be easily summarized and communicated effectively to patients, their families and payers. Limitations include ensuring that the NNT metric is calculated from well-designed and well-conducted research that enrolls subjects similar to patients that one treats in actual clinical practice, with doses of medications similar to what is used in the 'real world'. Direct calculation of NNT is limited to binary or dichotomous outcomes.

CONCLUSION: Using NNT can help predict treatment response in terms of both efficacy and tolerability.