The word 'surrogate' is equivalent to 'substitute', and strictly a surrogate end-point should be able to substitute completely for the end-point of interest; there is, however, much misuse of the term. This short communication discusses a number of end-points that appear to have a useful role to play in the development of new agents, but even they could not be strictly called surrogates and may be better described as intermediate end-points.
The reason to seek these intermediates is that they generally provide earlier information and are easier and cheaper to use. They are also frequently more ethical to obtain, achieve or monitor than the clinical end-point, and in addition there are circumstances in which the surrogate end-point may be superior to the clinical end-point, in that it assesses the underlying disease process in a more direct manner . It is argued below that change in proliferation may be one such end-point in relation to estimating clinical benefit from new agents.
Certain dangers must be considered in the application of supposed surrogates, in that these may not always reflect the disease process despite all evidence to date indicating that they do. In addition, the end-point will very often not (and maybe cannot) capture the overall risk/benefit of a treatment, but this will normally be readily apparent.