Slicing

Slicing Errors

Slicing errors occur when elements in a repeated list do not conform to the slicing rules declared in the StructureDefinition. FHIR slicing allows profiles to define sub-groups (slices) within a repeating element, each with its own constraints. The validator checks that elements match the correct slices, that slice cardinality is respected, and that closed slicing rules are not violated.

Error Codes

ID	Severity	Message
`SLICE_UNMATCHED_CLOSED`	error	Element at ‘{path}’ does not match any slice (closed slicing)
`SLICE_MIN_NOT_MET`	error	Slice ‘{slice}’ requires minimum {min} occurrence(s), found {count}
`SLICE_MAX_EXCEEDED`	error	Slice ‘{slice}’ allows maximum {max} occurrence(s), found {count}

SLICE_UNMATCHED_CLOSED

An element in a sliced list does not match any of the defined slices, and the slicing is declared as closed (i.e., slicing.rules is "closed" or "openAtEnd"). In closed slicing, every element must match exactly one slice.

Example – invalid resource:

A profile defines slicing on Observation.component with closed rules, defining two slices: systolic and diastolic. An element that does not match either slice triggers this error:

{
  "resourceType": "Observation",
  "status": "final",
  "code": {
    "coding": [{ "system": "http://loinc.org", "code": "85354-9" }]
  },
  "component": [
    {
      "code": {
        "coding": [{ "system": "http://loinc.org", "code": "8480-6" }]
      },
      "valueQuantity": { "value": 120, "unit": "mmHg" }
    },
    {
      "code": {
        "coding": [{ "system": "http://loinc.org", "code": "8462-4" }]
      },
      "valueQuantity": { "value": 80, "unit": "mmHg" }
    },
    {
      "code": {
        "coding": [{ "system": "http://loinc.org", "code": "8867-4" }]
      },
      "valueQuantity": { "value": 72, "unit": "/min" }
    }
  ]
}

If the third component (heart rate, LOINC 8867-4) does not match either the systolic or diastolic slice and the slicing is closed, the validator produces this error.

Validation output:

ERROR: Element at 'Observation.component[2]' does not match any slice (closed slicing)
  Path: Observation.component[2]
  MessageID: SLICE_UNMATCHED_CLOSED

Fix: Remove the unmatched element or use a profile that allows it (open slicing).

Slicing Rules

The slicing.rules field in the ElementDefinition determines how unmatched elements are handled:

Rules	Behavior
`open`	Unmatched elements are allowed (no error)
`closed`	All elements must match a slice (error if unmatched)
`openAtEnd`	Unmatched elements are allowed only at the end of the list

SLICE_MIN_NOT_MET

A slice has a minimum cardinality that is not satisfied. Each slice can have its own min value that requires a minimum number of elements matching that slice.

Example – invalid resource:

A blood pressure profile requires at least one systolic component (min: 1) and at least one diastolic component (min: 1). A resource with only the systolic component:

{
  "resourceType": "Observation",
  "status": "final",
  "code": {
    "coding": [{ "system": "http://loinc.org", "code": "85354-9" }]
  },
  "component": [
    {
      "code": {
        "coding": [{ "system": "http://loinc.org", "code": "8480-6" }]
      },
      "valueQuantity": { "value": 120, "unit": "mmHg" }
    }
  ]
}

Validation output:

ERROR: Slice 'Observation.component:diastolic' requires minimum 1 occurrence(s), found 0
  Path: Observation.component
  MessageID: SLICE_MIN_NOT_MET

Fix: Add the required diastolic component:

{
  "resourceType": "Observation",
  "status": "final",
  "code": {
    "coding": [{ "system": "http://loinc.org", "code": "85354-9" }]
  },
  "component": [
    {
      "code": {
        "coding": [{ "system": "http://loinc.org", "code": "8480-6" }]
      },
      "valueQuantity": { "value": 120, "unit": "mmHg" }
    },
    {
      "code": {
        "coding": [{ "system": "http://loinc.org", "code": "8462-4" }]
      },
      "valueQuantity": { "value": 80, "unit": "mmHg" }
    }
  ]
}

SLICE_MAX_EXCEEDED

More elements match a slice than the slice’s maximum cardinality allows. Each slice can have its own max value that limits how many elements can match it.

Example:

A profile defines a slice Observation.component:systolic with max cardinality of "1". If two components match the systolic discriminator:

{
  "resourceType": "Observation",
  "status": "final",
  "code": {
    "coding": [{ "system": "http://loinc.org", "code": "85354-9" }]
  },
  "component": [
    {
      "code": {
        "coding": [{ "system": "http://loinc.org", "code": "8480-6" }]
      },
      "valueQuantity": { "value": 120, "unit": "mmHg" }
    },
    {
      "code": {
        "coding": [{ "system": "http://loinc.org", "code": "8480-6" }]
      },
      "valueQuantity": { "value": 118, "unit": "mmHg" }
    }
  ]
}

Validation output:

ERROR: Slice 'Observation.component:systolic' allows maximum 1 occurrence(s), found 2
  Path: Observation.component
  MessageID: SLICE_MAX_EXCEEDED

Fix: Ensure only the allowed number of elements match each slice.

How Slicing Works

Slicing is defined in the StructureDefinition by a slicing element on the base path, followed by individual slice definitions. The slicing.discriminator specifies how elements are matched to slices:

{
  "id": "Observation.component",
  "path": "Observation.component",
  "slicing": {
    "discriminator": [
      {
        "type": "pattern",
        "path": "code"
      }
    ],
    "rules": "open"
  }
}

Each slice then uses a pattern or value on the discriminator path to define which elements belong to it:

{
  "id": "Observation.component:systolic",
  "path": "Observation.component",
  "sliceName": "systolic",
  "min": 1,
  "max": "1",
  "patternCodeableConcept": {
    "coding": [{ "system": "http://loinc.org", "code": "8480-6" }]
  }
}

Slicing rules, discriminators, and slice cardinalities are all read from the StructureDefinition. The validator does not hardcode any assumptions about which elements can be sliced or how slices are defined.

Last updated on March 1, 2026

Fixed/Pattern Profiles