first subject in 

EVIDENCE-BASED MEDICINE

A self-explanatory term used in the context of a clinical trial, for the first subject meeting the trial’s inclusion or exclusion criteria.

The first subject in is dated and typically, randomised. 

Synonym First patient in

Reference http://www.appliedclinicaltrialsonline.com/appliedclinicaltrials/article/articleDetail.jsp?id=700052&sk=&date=1&pageID=17