When should an effective treatment be used? Derivation of the threshold number needed to treat and the minimum event rate for treatment

Clinicians and patients must decide when treatment effects are large enough to more than offset the adverse effects and costs of therapy. Calculation of the number of patients one needs to treat (NNT) in order to prevent one patient from having the target event is one tool to help with this decision . Clinicians should treat patients when the NNT is lower than a threshold N NT at which point the therapeutic risk equals the therapeutic benefit. We a imed: (1) to identify the determinants of the threshold NNT, and (2) to der ive equations for the explicit estimation of the threshold NNT and of the m inimum expected rate of target event, without treatment, above which treatm ent is justified. We conceived the threshold number needed to treat to prev ent one target event as the point at which the benefits of treating that nu mber of patients equal the negative consequences of treating that same numb er of patients. After identifying the various elements comprising the treat ment impact, we equated the benefits and negative consequences and solved t he equation for threshold NNT. We then derived the minimum expected rate of target event which would justify treatment. We derived an equation that re lates the threshold NNT to the treatment-attributable adverse event rates ( AER) and the relative values (RV) of the adverse events compared to that of the target event prevented. Specifically, the threshold NNT, denoted NNTT is given by NNTT = 1/(AER(1).RV1 +... + AER(n).RVn). We also derived a more complex equation which addresses the costs incurred by treatment and costs avoided by preventing target events. Solving the equation that includes co sts requires specifying both the value of preventing a target event and the values of adverse treatment effects in economic units. The threshold NNT a nd the relative risk reduction (RRR) for the target event determine the min imum target event rate above which treatment is justified. This minimum eve nt rate for treatment is 1/(NNTT.RRR). The values that clinicians and patie nts use determine the threshold NNT and therefore also the minimum target e vent rate, without treatment, above which treatment is justified. Quantific ation of the determinants of the threshold NNT and of the minimum event rat e to justify treatment can assist clinicians and patients in the explicit u se of underlying values when making treatment decisions. (C) 2001 Elsevier Science Inc. All rights reserved.