Handling Missing Data

MII Implementation Guide Core Dataset Base
2026.0.1 - Release Germany

This page is part of the MII Implementation Guide Core Dataset Base (v2026.0.1: Release) based on FHIR (HL7® FHIR® Standard) R4. This is the current published version. For a full list of available versions, see the Directory of published versions

Missing Data

There are situations when information on a particular data element is missing and the source system does not know the reason for the absence of data.

This section provides guidance for MII DIZ Servers on handling missing data in MII Base Module resources. The rules align with the FHIR core specification and are consistent with international best practices as defined in the International Patient Summary (IPS).

For general guidance on Must Support elements, see the Must Support page. For conformance expectations and coded element requirements, see the General Requirements page.

Elements with Minimum Cardinality = 0

Rule: If the source system does not have data for an element with a minimum cardinality = 0 (including elements labeled Must Support), the data element SHALL be omitted from the resource.^{§missing-data-1}

This applies to both non-coded and coded elements with min = 0. Simply omit the element from the resource instance.

Mandatory Elements (Minimum Cardinality > 0)

Rule: If the data element is a mandatory element (minimum cardinality > 0), it SHALL be present even if the source system does not have data or does not know the reason for the absence of data.^{§missing-data-2}

The approach for representing missing data in mandatory elements depends on whether the element is coded or non-coded:

Non-Coded Data Elements

For mandatory non-coded data elements where data is absent, systems SHALL use the DataAbsentReason-Extension with code unknown.^{§missing-data-3}

For non-coded data elements (e.g., string, HumanName, Address), use the DataAbsentReason-Extension in the data type with the code unknown.

Code: unknown - The value is expected to exist but is not known.

Example: Patient resource where the patient's family name is not available:

{
  "resourceType": "Patient",
  "id": "example-missing-name",
  "name": [
    {
      "extension": [
        {
          "url": "http://hl7.org/fhir/StructureDefinition/data-absent-reason",
          "valueCode": "unknown"
        }
      ]
    }
  ],
  "gender": "unknown",
  "birthDate": "1990-01-01"
}

Coded Data Elements

For coded data elements, the approach depends on the binding strength of the element:

Example, Preferred, or Extensible Binding Strengths

For mandatory coded elements with example, preferred, or extensible binding where the source has text but no coded value, systems SHALL use only the text element (or display for Coding datatypes).^{§missing-data-4} For mandatory coded elements with example, preferred, or extensible binding where neither text nor coded data is available, systems SHALL use an "unknown" concept from the bound ValueSet if one exists, or otherwise the unknown code from the DataAbsentReason Code System.^{§missing-data-6}

For elements with example, preferred, or extensible binding (CodeableConcept or Coding datatypes):

If the source system has text but no coded data: Only the text element is used.
- For Coding datatypes, the text-only data is represented as a display element.
If there is neither text nor coded data:
- Use the appropriate "unknown" concept code from the ValueSet if available.
- If the ValueSet does not have an appropriate "unknown" concept code, use unknown from the DataAbsentReason Code System.

Example: Condition resource where the mandatory Condition.code value is unknown:

{
  "resourceType": "Condition",
  "id": "example-unknown-code",
  "clinicalStatus": {
    "coding": [
      {
        "system": "http://terminology.hl7.org/CodeSystem/condition-clinical",
        "code": "active"
      }
    ]
  },
  "code": {
    "coding": [
      {
        "system": "http://hl7.org/fhir/CodeSystem/data-absent-reason",
        "code": "unknown",
        "display": "Unknown"
      }
    ]
  },
  "subject": {
    "reference": "Patient/example"
  }
}

Required Binding Strength

For elements with required binding (CodeableConcept or Coding or code datatypes):

Use the appropriate "unknown" concept code from the ValueSet if available.
For elements with a required binding, if the ValueSet does not have an appropriate "unknown" concept code, you SHALL use a concept from the ValueSet (otherwise the instance will not be conformant).^{§missing-data-5}