<biology> A laboratory measurement of biological activity within the body that indirectly indicates the effect of treatment on disease state. ... CD4 cell counts and viral load are examples of surrogate markers in HIV infection. ... (19 Jan 1998) ... 
An entity, test, or measure used to substitute for another measure, for example, biochemical bone markers as a surrogate marker of bone loss or formation.
see endpoint.
an indirect measure of disease progression. In HIV disease, the number of CD4+ T cells per cubic millimeter of blood is often used as a surrogate marker.
an indirect measure of disease progression.