In a cohort study, the Index Date is defined as what?

• A. Date of birth
• B. Enrollment date
• C. Cohort entry: first prescription; first diagnosis; first treatment visit
• D. Date of last follow-up

Answer: (C)

The index date is the moment a person enters the cohort and starts follow-up. In many cohort studies, this entry point is tied to an exposure-related event, such as the first prescription, the first diagnosis, or the first treatment visit. This date defines the baseline for that individual and starts the clock for observing outcomes.

Why this is the best choice: it directly lines up the start of risk time with a defined exposure or care event, which is how cohorts are typically constructed in pharmacoepidemiology and other follow-up studies. It ensures the exposure status is established and that follow-up begins consistently.

Why the other ideas don’t fit: birth date is only a demographic attribute, not the point at which follow-up starts. Enrollment date can mark when someone enters the system or study, but the index date in exposure-based cohorts is specifically the exposure-related entry event. The date of last follow-up marks the end of observation, not the start.