NHS Digital FHIR Implementation Guide (Retired - 2.1.50)

{
    "resourceType": "ObservationDefinition",
    "id": "snomed-871552002",
    "code": {
        "coding":  [
            {
                "system": "http://snomed.info/sct",
                "code": "871552002",
                "display": "Detection of SARS-CoV-2 (severe acute respiratory syndrome coronavirus 2) antibody"
            }
        ]
    },
    "validCodedValueSet": {
        "reference": "https://fhir.nhs.uk/ValueSet/covid-19-detection-of-antibody"
    }
}

<ObservationDefinition xmlns="http://hl7.org/fhir">
    <id value="snomed-871552002" />
    <code>
        <coding>
            <system value="http://snomed.info/sct" />
            <code value="871552002" />
            <display value="Detection of SARS-CoV-2 (severe acute respiratory syndrome coronavirus 2) antibody" />
        </coding>
    </code>
    <validCodedValueSet>
        <reference value="https://fhir.nhs.uk/ValueSet/covid-19-detection-of-antibody" />
    </validCodedValueSet>
</ObservationDefinition>