The objective of the article is to examine the extensions of a clinical measure of efficacy, the Number Needed to Treat (NNT), in different settings including screening, scanning, genetic testing and primary prevention, and the associated ethical implications. We examine several situations in which the use of the NNT or NNS (Number Needed to Screen) has been suggested, such as Prostate-Specific Antigen for prostate cancer, Magnetic Resonance Imaging scans, genetic testing and banning of smoking. For each application, we explore the ethical implications of the relevant measure. We have found that the different measures have different ethical implications. For example, the Number Needed to Prevent is the only measure that can be lower than one, indicating with a numerical example that prevention is better than cure. Conversely, we raise questions about the acceptability of genetic screening. In a realistic example, we show that primary prevention of the effects of arsenic in drinking water, targeted to the most susceptible, would require to genetically screen a large number of subjects, whereas giving rise to ethical concerns. We warn against the abuse of testing, in particular genetic testing, we show that different measures are associated with different ethical issues and that prevention tends to be better than cure.

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