Flag First Observation Rule (flag_first_observation)

Identifies first measurable observation and flags that record. This is used in BLQ handling to identify BLQ values before and after the first observation.

Field	Description	Required
source_column	Column containing the concentration and BLQ values. This rule assumes that numeric values are measurable concentrations.	✅ Yes
destination_column	Column to be populated with the values before, at, or after	✅ Yes
before	Numeric value populated in destination_column for all records before the first numeric value.	✅ Yes
at	Numeric value populated in destination_column for the record with the first measuraable observation (i.e. first numeric value).	✅ Yes
after	Numeric value populated in destination_column for all records after the first numeric value.	✅ Yes

Example:

{
    "type": "flag_first_observation",
    "source_column": "AVALC",
    "destination_column": "blq_flag",
    "before": 0,
    "at": 1,
    "after": 2
}

Behavior: Flags timepoints as before, at, or after the first measurable concentration based on concentration values. This method assumes ANY numeric value (including 0) is measurable. All BLQ values should have text in the source_column for this to work properly.