Working from LogicaHealth IG
Modifier extension adding certainty of absence. See https://covid-19-ig.logicahealth.org/StructureDefinition-covid19-symptoms-absent.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.
|Condition.code||COVID 19 Signs and Symptoms Value Set|
|Condition.modifierExtension:certaintyOfAbsence||CertaintyOfAbsenceVS||Does the existence of the modifier change the interpretation of the Condition instance? See https://www.hl7.org/fhir/extensibility.html#modifierExtension http://hl7.org/fhir/R4/condition.html#22.214.171.124 and http://hl7.org/fhir/R4/condition.html#126.96.36.199|