So why are surrogate outcomes used?

A common reason for using surrogate outcomes is that in order to demonstrate an effect on clinically meaningful outcomes, studies often need to large and long and therefore very expensive.

For example, a study to determine if an osteoporosis treatment reduces hip fractures would require a large number of patients to be followed up for several years as this outcome will not happen very often.

It is much quicker and cheaper to measure a surrogate outcome such as the effect of the drug on bone mineral density.