A biological marker is something that can be measured which points to the presence of a disease, a physiological change, response to a treatment, or a psychological condition. For example, glucose levels are used as a biomarker in managing diabetes, and brain images can provide information about the progression of multiple sclerosis.
Biomarkers are used in many scientific fields. They are used in different ways at different stages of medicines development, including in some cases as a surrogate endpoint to indicate and measure the effect of interventions, such as medicines, in trials.
For example, haemoglobin levels have been used in Phase III trials to support development of therapies for Type 1 Gaucher disease (a rare disease that affects multiple organ systems and shortens life expectancy, but which can take years to show changes in clinical symptoms).