The Index Date in a cohort study is typically defined as the date of cohort entry based on which event?

• A. Cohort entry: first prescription; first diagnosis; first treatment visit
• B. Date of death
• C. Date of last contact
• D. Enrollment date

Answer: (A)

Index date is the moment follow-up starts for each person, defined by the event that marks cohort entry and establishes exposure or eligibility. In practice, this is often tied to the date of the event that sets the subject’s exposure status: the first prescription (when exposure begins), the first diagnosis (when a disease state is established), or the first treatment visit (when management of the condition starts). Starting follow-up from this point ensures that the risk period is aligned with when the exposure or condition of interest is actually present, making comparisons across individuals meaningful and minimizing time-related biases.

Dates like the date of death are outcomes that happen during follow-up, not the entry point for starting risk. The date of last contact serves as censoring, not the initiation of risk. The enrollment date can be before exposure is defined and may not correspond to when the individual actually becomes at risk under the study’s exposure criteria.