Note that DiagnosticReportLab does have a binding to US Core http://hl7.org/fhir/us/core/STU3.1/ValueSet-us-core-diagnosticreport-lab-codes.html . The QI-Core Observation profile represents any observation (I.e., non-lab) and many of the items referenced will have LOINC codes but many do not. Therefore, SNOMED may be more appropriate based on the use case. However, changing the binding for Observation.code to LOINC with preferred binding is appropriate and does not limit use of SNOMED or other terminology. Therefore, change the binding to "preferred" for observables from "example".