primary endpoint

The outcome a trial names in its protocol, before it begins, as the most important one for judging whether the treatment worked. Most trials have one; some have more than one.

ClinicalTrials.gov, the trial registry run by the United States National Library of Medicine, defines an outcome measure as a planned measurement described in the protocol that is used to determine the effect of an intervention or treatment on participants. The primary outcome measure is the one that is most important for evaluating that effect. A secondary outcome measure, in the registry's words, is not as important as the primary one but is still of interest.

The word planned is doing the work. The measure is written into the protocol before anyone is treated, rather than picked out of the results afterwards, and a trial collects many measurements while resting on a short list of them. Journals usually print end point where the registry prints outcome measure, and they name the same thing.

The registry also records that most clinical studies have one primary outcome measure, but some have more than one. A trial with two is asking two questions rather than one, and the two answers need not agree, so a trial can meet one primary endpoint and miss the other. That is a result about which question was asked, not a failure to complete the trial.

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