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Term confusion alert 2: outcome versus endpoint

Our clinical trials course at UMBC is well under way, and we are getting some terrific questions from students. Here is one!

Q: Are outcomes surrogate endpoints or is there a distinction between the two?

The terms “outcome” and “endpoint” are not strictly defined and some people use them interchangeably. However:

It reminds me of the confusion between efficacy and effectivness, only it’s worse: there is no agreed-upon text that describes the distinction, so it is a really terminological free-for-all. Indeed, what I wrote above may end up not being true — caveat lector! As always, it is always best to ask people to clarify what they meant when they said this or that. Regardless, if someone tells you that “overall survival” (or, worse yet, “survival”) was the primary endpoint, it clearly can’t be the case. Endpoints need to be more specific than that.

Surrogate outcomes and surrogate endpoints are those which are stand-ins for what we actually care about. Here is a good video on surrogate endpoints in oncology.E.g. when we give chemotherapy to someone with cancer, we do it so that they would live longer and/or better. However, it is quicker and easier to measure if the tumor shrinks after chemotherapy (i.e. “responds” to treatment), and we believe that the tumor shrinking will lead to the patient living longer or better (which may not necessarily be the case!), so we use the response as a surrogate outcome for survival and quality of life (by how much did the tumor shrink? was it a complete or a partial response according to pre-specified criteria?). Study level surrogate endpoints would be the overall response rate, partial response rate, complete response rate, etc.

We have created so much confusion here that it is a small miracle we can communicate amongst ourselves at all.

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