Surrogate markers are used when it is unethical to look for the end point (e.g., death) in the experiment, or when the number of end point events is very small, thus making it impractical to conduct an experiment to look for the end point. The measurement of surrogate markers provides a way to test the effectiveness of a treatment for a fatal disease without having to wait for a statistically significant number of deaths to occur. the FDA will often accept evidence from clinical trials that show a benefit to surrogate markers instead of to end points.
A commonly used example is cholesterol. A clinical trial may show that a particular drug is effective in reducing cholesterol. A high cholesterol is associated with death from heart disease, so it is believed that a treatment that is effective in reducing cholesterol must also be effective in reducing death from heart disease. "Death from heart disease" is the endpoint of interest, but "cholesterol" is the surrogate marker.
Examples of other surrogate markers include:
There have been a number of instances when studies using surrogate markers have been used to show benefit from a particular treatment, but later, a repeat study looking at endpoints has not shown a benefit, or even a harm.