(1) A measure of the biological activity inside the body that indicates the effect of treatment on the state of disease.
(2) A laboratory biomarker that measures the effect of a treatment to indicate, reflect or substitute a clinical endpoint.
In clinical or laboratory experiments, surrogate markers are used when a clinical endpoint is undesired or unethical to wait for (e.g. death) or when there are limited number of cases.
An example is CD4 cell counts as a surrogate marker for death from HIV infection.
Also called: surrogate endpoint
See also: clinical endpoint