“Bayes' theorem (also known as Bayes' rule or Bayes' law) is a result in probability theory, which relates the conditional and marginal probability distributions of random variables. In some interpretations of probability, Bayes' theorem tells how to update or revise beliefs in light of new evidence a posteriori” (from Wikipedia, the free encyclopaedia, where you can find more information).

The **predictive values** of a test depend not only on its **sensitivity** and **specificity**, but also **on the prevalence of the infection** (disease) in a population group, the so-called pre-test probability.