[Bayesian tragedy, from clinical iatrogenesis to biotechnology].

Diagnostic algorithms, as well as the biotechnological design, require the calculation of conditional probability, given the presence of certain positive data, in the context of prevalence, sensitivity and specificity; It is necessary to estimate the probability that the patient has a certain disease. Sometimes, with a test of scrutiny, it goes from a probability of 1/1000 to 1/20, constituting a great diagnostic advance, reducing the uncertainty spectacularly; However, the tragedy is that most doctors believe that the probability changed from 0.1% (1/1000) to more than 90%, which is outrageously wrong. Iatrogeny arises from the error in answering the question: "given that the test is positive, what is the probability that the patient has the disease?" In other cases, tragedy is to apply a test to an individual belonging to a subpopulation for which it was not designed. In addition, it is evident that the fascination for the sensitivity avoids the application of less sensitive methods in populations that are abandoned; It is not a matter of making better tests than those that the State does to the patients it attends, but of making less accurate tests for the patients that the State does not attend.