In medicine, the term specificity refers to the ability of a test to recognize a "negative" value in a healthy subject. Sensitivity, on the other hand, is the ability of a test to detect a "positive" value in a sick person. The terms negative and positive are to be understood, in the case of a prenatal screening based on DNA, respectively as the absence of the mutation or the presence of the mutation. Each test, depending on its intended use, will have a higher sensitivity or specificity. The aim is to detect all probable positives in a very large population, which must then be confirmed through specific tests.