Answer No 1: There is no such a thing as ICD-10. Well, there is, except there are many. Every country has their own. The American version, ICD-10-CM, is the largest with over 100k codes. The original WHO ICD-10 has less than 20k. So, obviously it’s not the same. But even the codes that overlap are not always the same between the versions.
Answer No 2: ICD-10 is suboptimal for research, because it has a lousy one-parent only hierarchy. SNOMED has a way stronger hierarchical organizations, plus other links connecting the concepts.
Answer No 3: There are conditions in other terminologies. For example, HCPCS M1062 “Patient immunocompromised” is a Condition not stored in ICD-10. Internationally you have plenty of those.
Happens often with people who have used their definitions for a long time and don’t want to change their ways. I have empathy.
Just map them using the mapping tables provided by OHDSI. We have all ICD-10s from most versions mapped over.
The power and beauty of OMOP is in the standardized data model and the terminology harmonization. The one and only one standard concept_id to represent an idea, the hierarchies, the classification concepts, domains to identify the table where the data lives, and the ability to represent your source data ‘as is’ with a 2 billionaire source concept_id make the OMOP CDM amazing!