What is a biomarker?: From its discovery to clinical application

The term biomarker in medicine most often stands for a protein measured in the circulation (blood) whose concentration indicates a normal or a pathological response of the organism, as well as a pharmacological response to the applied therapy. From a wider perspective, a biomarker is any indicator that is used as an index of the intensity of a disease or other physiological state in the organism. This means that biomarkers have a very important role in medical research and practice providing insight into the mechanism and course of a disease. Since a large number of biomarkers exist today that are used for different purposes, they have been classified into: 1) antecedent biomarkers, indicating risk of disease occurrence, 2) screening biomarkers, used to determine a subclinical form of disease, 3) diagnostic biomarkers, revealing an existing disease, 4) staging bio markers, that define the stage and severity of a disease, and 5) prognostic biomarkers, that confirm the course of disease, including treatment response. Regardless of their role, their clinical significance depends on their sensitivity, specificity, predictive value, and also precision, reliability, reproducibility, and the possibility of easy and wide application. For a biomarker to become successful, it must undergo the process of validation, depending on the level of use. It is very important for every suggested biomarker, according to its purpose or its nature, to possess certain characteristics and to meet the strict requirements related to sensitivity, accuracy and precision, in order for the proper outcome to be produced in the estimation of the state for which it is intended. Finally, the development of guidelines for biomarker application is very important, based on well defined and properly conducted assessments of biomarker determination, providing the means by which research is translated into practice and allowing evidence based on facts to promote the clinical application of new biomarkers.