Table 4.

FHIR/US Core IG validation errors that could not be resolved.

Validation issue	Explanation and examples
Error: Element cardinality	“MedicationRequest.requester” is a required element in US-Core. In rare cases, the OmOp PROVIDER_ID field was blank. By design, a “fake” entry was not created to meet the cardinality requirement.
Error: Code not from code system	A small number of valid SNOMED-CT codes were flagged as not valid by the terminology server. On manual verification, these were found to be US-only SNOMED-CT codes. Examples include condition and smoking related codes. Similarly, a small number of valid RxNorm codes were not recognized as RxNorm codes by the terminology service. Manual verification showed them to be influenza or remapped codes.
Error: Violations of FHIR invariant rules)	On very rare occasions, OMOP data contained datetime values where the start date was later than the end value in the Medication.validityPeriod field. These resources were kept “as is” because this is an accurate representation of the original data and has no impact on the current use case.
Warning: Code not from value set	US-Core binds the “Medication.code” element to a “Medication Clinical Drug” value set derived from RxNorm. Validation indicated RxNorm codes that did not belong to this value set. Manual examination revealed valid RxNorm codes for the package form, but the value set did not include package-related codes.
Warning: No coding from the value set	Validation indicated certain records did not contain any RxNorm codes when one was required. These Drug_Exposure records contained HCPCS (J1200) and OMOP RxNorm extension codes (OMOP1088103), CVX, CPT but no RxNorm code.
Warning: Label not matching from terminology server	The validator verifies code label text and warns if there is not an exact match with the label string in the terminology service. A few of the OMOP concept labels had variations from the original labels, such as additional spaces around hyphens. Also, OMOP truncates all labels at 256 characters.
Info: Unknown extensions	A FHIR Bundle extension was added to hold the OMOP-specific metadata found in the OMOP CDM_SOURCE table. This extension was correctly flagged as an unknown extension.
Info: Unknown code systems	Terminologies not known to the terminology server but that are still assigned URLs by HL7 are present in OMOP source values and source codes. The data elements holding these codes could not be verified because the code system is not available at the terminology server. Examples are HCPCs, OMOP RxNorm extension, and CPT.