Reliability of medical tests is a major public-health challenge to establish the correct diagnosis and adjust treatments. In order to evaluate and improve comparability and reliability of medical tests, the input of metrology is necessary. As imposed by the Directive 98/79/EC on in vitro diagnostic medical devices establishing results traceability to higher-order reference methods and materials helps harmonizing results obtained in different laboratories, even if they do not use the same method. Then, using reference methods to value assign quality control materials makes it possible to assess trueness of routine methods. However, a prerequisite is that quality control materials are commutable, that is to say that they should mimic the behavior of patient samples that were not modified. In this lecture, the importance of reference methods and commutability will be explained through different examples.