The start date isweighed, not assumed.
Follow-up has to begin somewhere, and the wrong choice bakes in bias that no later analysis can remove. Each candidate is assessed on whether eligibility is knowable at that moment, whether treatment can be assigned there, whether follow-up can begin there, and what it risks. Discharge, for instance, looks convenient and quietly excludes everyone who died first.
Candidate start date
Eligibility known
Assignment possible
Follow-up can start
Immortal time risk
Default
First treatment order
Yes
Yes
Yes
Low
Preferred
First eligible lab value
Yes
Usually
Yes
Low
Preferred
Diagnosis confirmation
Yes
Usually
Yes
Low
Preferred
Qualifying procedure
Yes
Yes
Yes
Low
Preferred
Hospital admission
Usually
Sometimes
Yes
Moderate
Acceptable
First administration
Usually
Yes
Yes
Moderate
Acceptable
ICU admission
Usually
Usually
Yes
Moderate
Caution
Symptom onset
Rarely
Rarely
Sometimes
High
Caution
Study enrolment
Yes
Usually
Yes
Low
Caution
Hospital discharge
Yes
Sometimes
Yes
High
Avoid