Qualification of the Concepts of Unqualified Success and Unmitigated Failure
To the Editor:
By mathematically compensating for adverse effects, Mancini and Schulzer1 refine the concept of the numbers needed to be treated (NNT) for a therapeutic success and introduce the use of the terms: (1) the numbers needed to be treated for an unqualified success (NNTUS) or (2) an unmitigated failure (NNHUF). Nevertheless, there appear to be important limitations to these concepts. First, the authors define an unqualified success as “…a successful outcome unaccompanied by treatment-related adverse events…”. However, in the context of practice, as one moves from the NNT to the NNTUS, more adverse events will occur (eg, in the first study in the Table shown in the article, the NNT of 58.82 would be associated with 1.18 adverse events, whereas the NNTUS of 60.02 would be associated with a proportionate increase in adverse events to 1.20). The only time a success would truly be unqualified would be if there were no adverse events associated with a given therapy, but then the calculation of the NNTUS would become unnecessary. Second, neither the NNTUS nor the NNHUF compensates for the adverse events that occur in the proportion of the population who cannot benefit from treatment. This is an essential concept since (1) these patients will be harmed never having had the possibility of benefit (they are, in essence, bystanders) and (2) the majority of adverse events will emanate from this portion of the population whenever p1 is <50%. Again using the authors’ first example in the Table, 94.8% of adverse events would emanate from the group of patients who could not have benefited from treatment. Therefore, the number needed to be treated to have an adverse event in this “bystander” group is 52.74, fewer patients than are needed for a therapeutic success (NNT=58.82) and far fewer patients than the NNHUF of 1428.57. Finally, the interpretation of NNTUS and NNHUF is confounded whenever the primary end point(s) of therapy and the treatment-related adverse event(s) are not comparable. For example, the NNTUS and NNHUF mathematically equate the desired effect of the maintenance of sinus rhythm at 12 months with the undesired effect of death (Table 1, Reference 9) and equate the prevention of mortality with cough, taste disturbance, and dizziness (Table 1, Reference 15). Therefore, the interpretation of the absolute NNTUS and NNHUF, as well as the size of the change between the NNT and NNTUS or the NNH and NNHUF, depends on the nature and comparability of desired and undesired end points of therapy.
- Copyright © 2000 by American Heart Association