Union of http://hl7.org/fhir/us/core/ValueSet/us-core-laboratory-test-codes and http://hl7.org/fhir/ValueSet/observation-codes

This is the ValueSet that contains all the codes in US Core Laboratory Test Codes (http://hl7.org/fhir/us/core/ValueSet/us-core-laboratory-test-codes) and LOINC Codes (http://hl7.org/fhir/ValueSet/observation-codes). E.g. what you have to deal with if you get resources containing codes in either of them

CodeSystem