Hi @KSimon,
There are cases when obsolete concepts that cannot be consistently mapped to Standard equivalents still widely persist in the clinical data tables. Since users may need to query them in their studies, there’s a workaround approach applied in OMOP vocabulary system: the concepts turned into Standard and valid, but with the validity end date defined in the past.
Please read the forum discussion and find more information here.
In short, if you cannot find a standard, valid concept with a 12/31/2099 valid_end_date, you may use such concepts in your study.