I wasn't clear on whether we have any guidance about how to mix/match the codes and the values. for this particular test, LOINC definitions suggest that valueCodeableConcepts like LA11883-8 ("Not Detected") should be used, but my SHC has a SNOMED code of 260415000 ("Not Detected").
https://chat.fhir.org/#narrow/stream/284830-smart.2Fhealth-cards/topic/My.20COVID.20Test.20Result