index hospitalization恰当翻译

     各位老师好，请问您们应该如何恰当的翻译“index hospitalization”, 摘自文献“The first hospitalization with relevant diagnosis or procedure codes for a CV event during this period was designated as that patient’s index event. The pre-index period was defined as 1 year (12 months) prior to index hospitalization admission date.”，有见过译成“指数住院”的，但是感觉不太合适，望各位老师们指点帮助~谢谢您们~~