Working from LogicaHealth IG
Modifier extension adding certainty of absence. See https://covid-19-ig.logicahealth.org/StructureDefinition-covid19-symptoms-present.htmlabsent.html ('Refuted' as a value is being discussed by the HL7 Patient Care Workgroup - steward of the Condition Resource). LBI: Clinical Assessment 2.3 contains some verification values.
See analysis spreadsheet by Linda Bird
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