1.5 Diagnosis codes
The hospital migrated from ICD-9 (ICD, 9th revision) to ICD-10 for recording diagnoses during the three-year period for which data were available in this study. Under the OMOP-CDM, both ICD versions could be mapped to the same standard concept ID in the “Condition Occurrence” table. For example, the ICD-9 and ICD-10 codes for “Type 2 Diabetes Mellitus without complications” are “250.02” and “E11.9”, respectively. The corresponding source concept IDs are “44836915” and “45561952”, respectively. Both of these map to the same OMOP standard concept ID “201826”.
Some data cleaning steps were required during the ETL process for diagnosis codes. For example, in the source database, the diagnosis code “I.255 Ischaemic cardiomyopathy” had the decimal place shifted by two places and should have been the ICD-10 code “I25.5”. Since both the code and text were in the source database, the proper mapping was rectified by moving the decimal two places to the right and confirming that the text with the corrected code matched the OMOP descriptor. Codes that had more granularity than present in the OMOP vocabulary were mapped to the parent code, such as “E14.69 Unspecified diabetes mellitus” which was mapped to the concept ID corresponding to the parent code of “E14.6. Unspecified diabetes mellitus” in ICD-10.