> ## Documentation Index
> Fetch the complete documentation index at: https://docs.corti.ai/llms.txt
> Use this file to discover all available pages before exploring further.

# Mention extension

> Look up the definition of the Corti mention extension, which links a FHIR Extraction field to the words in the note that support it

The mention extension links one field of a FHIR Extraction resource to the words in the clinical note that support its value. It is a complex extension: it has no value of its own, only sub-extensions. [Every value points back to the note](https://docs.corti.ai/fhir/how-it-works.md#every-value-points-back-to-the-note) shows how to use it.

## Definition

| Property | Value |
| - | - |
| Canonical URL | `https://docs.corti.ai/fhir/StructureDefinition/mention` |
| FHIR version | R4 (4.0.1) |
| Context | Condition, AllergyIntolerance, FamilyMemberHistory, Procedure, Observation, Patient, Encounter |
| Cardinality on a resource | `0..*`: one mention extension per field that has evidence |

Systems that do not know the extension can ignore it. It never changes the meaning of the resource.

## Sub-extensions

| Sub-extension `url` | Cardinality | Value | Definition |
| - | - | - | - |
| `field` | `1..1` | `valueString` | The field the mention supports, for example `code` or `valueQuantity` |
| `evidence` | `1..*` | nested `start`, `end`, `text`, `sectionId`, `sectionStart`, `sectionEnd` | One span of the note that supports the field |
| `evidence.start` | `1..1` | `valueInteger` | Character offset where the span starts, counted from the start of the full note |
| `evidence.end` | `1..1` | `valueInteger` | Character offset where the span ends. The character at `end` is not part of the span. |
| `evidence.text` | `0..1` | `valueString` | The words of the note between `start` and `end` |
| `evidence.sectionId` | `0..1` | `valueString` | The `id` of the `context` section the span comes from |
| `evidence.sectionStart` | `0..1` | `valueInteger` | Character offset where the span starts, counted from the start of that section's title and body joined together |
| `evidence.sectionEnd` | `0..1` | `valueInteger` | Character offset where the span ends in that section's title and body. The character at `sectionEnd` is not part of the span. |

`start` and `end` count characters of the full note. A `context` string is the note as it is. With `context` sections, the note is every section written as `Title:` on its own line, then the body, then a blank line. Only spans from a request with sections carry `sectionId`, `sectionStart`, and `sectionEnd`, which count characters of the section's `title` followed directly by its `body`, so you can look up a span without rebuilding the note. A span that crosses from one section into the next has no section offsets.

## Field values

`field` holds the R4 element name the mention supports. A choice element such as `onset[x]` uses its base name (`onset`), whichever type the resource carries. Nested elements use a dotted path.

| Resource | `field` values |
| - | - |
| Condition | `abatement`, `bodySite`, `category`, `clinicalStatus`, `code`, `onset`, `severity`, `stage`, `verificationStatus` |
| AllergyIntolerance | `category`, `clinicalStatus`, `code`, `criticality`, `lastOccurrence`, `onset`, `reaction`, `type`, `verificationStatus` |
| FamilyMemberHistory | `age`, `condition.code`, `condition.onset`, `relationship`, `sex`, `status` |
| Procedure | `bodySite`, `category`, `code`, `performed`, `performer`, `status` |
| Observation | `bodySite`, `code`, `effective`, `interpretation`, `status`, `valueCodeableConcept`, `valueInteger`, `valueQuantity`, `valueQuantity.unit`, `valueRatio`, `valueRatio.numerator.unit` |
| Patient | `birthDate`, `gender` |
| Encounter | `class`, `period`, `priority`, `reason`, `status` |

A mention can exist for a field the resource leaves out. For example, a time stated as "last month" grounds `lastOccurrence` even though that field only accepts dates and is omitted.

## Example

```json theme={null}
{
  "url": "https://docs.corti.ai/fhir/StructureDefinition/mention",
  "extension": [
    { "url": "field", "valueString": "code" },
    {
      "url": "evidence",
      "extension": [
        { "url": "start", "valueInteger": 5 },
        { "url": "end", "valueInteger": 11 },
        { "url": "text", "valueString": "asthma" }
      ]
    }
  ]
}
```

## Related pages

* [Alternative extension](https://docs.corti.ai/fhir/StructureDefinition/alternative.md): other codes that could also fit a field
* [FHIR datatypes](https://docs.corti.ai/fhir/datatypes.md#extension): how extensions sit on a resource


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