To flag Baseline records

As an example, the Baseline value for one assessment here is defined as the last non-missing measurement taken on or before the day of the first dosing of study drug.

Different programmers use different approaches to flag Baseline records. I used two steps (using LB domain in SDTM as an example):
1) To sort the assessment results in dataset LB througth PROC SORT by subject assessment date_time...;

PROC SORT DATA=lb;

  BY usubjid lbcat lbscat lbtestcd DESCENDING lbdtc;

RUN;

2) To flag the Baseline value with ABLFL='Y'.

 DATA lb(drop=flag); ** drop the temporary variable **;

  SET lb;

  BY usubjid lbcat lbscat lbtestcd DESCENDING lbdtc;

   ** variable flag is used to identify if one test has ABLFL marked **;

  RETAIN flag;

   ** flag=0 indicating there is no ABLFL marked yet. **; 

  IF first.lbtestcd THEN flag=0;

  ** you can add more conditions in IF logic expressions. **;

  ** TRTSDT below has the first dosing dates. **;

  IF flag=0 and LBDT <= TRTSDT and LBSTRESN ne .THEN DO;

    ABLFL='Y'; ** ABLFL='Y' indicates the baseline record. **;

    flag=1; ** flag=1 indicating the ABLFL is marked. **;

  END;

RUN;