[Measuring the impact of medical interventions at the individual level]

Medical interventions have a variable response among individuals. It is desirable to detect patients who are getting a therapeutic benefit. Any medical intervention has to show a favorable effect in survival and/or control of symptoms in order to be considered useful. In many clinical scenarios, laboratory test results are not enough to be confident of the effectiveness of a treatment. In order to do a clinical evaluation focused on patients' interests, we suggest the use of instruments to measure quality of life. Currently there are quality of life scales that have undergone a rigorous process of validation and reliability. Sub-group analysis is frequently used to predict an individual benefit of medical interventions. In the interpretation of sub-group analysis, it is important to be aware of the risk of misinterpretation, making false positive or false negative conclusions about the effect of the treatment. The N-of-1 trial is a valid scientific alternative to define individual effectiveness of therapy. Defining the degree of therapeutic benefit timely we avoid useless therapies, unnecessary side effects, and sequelae secondary to the lack of opportune treatment.