> ## 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.

# Alternative extension

> Look up the definition of the Corti alternative extension, which lists other codes that could also fit a FHIR Extraction field

The alternative extension lists other codes that could also fit a coded field of a FHIR Extraction resource. The endpoint picks one code for the field and puts up to 10 runners-up here, best first. 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/alternative` |
| FHIR version | R4 (4.0.1) |
| Context | Condition, AllergyIntolerance, FamilyMemberHistory, Procedure, Observation |
| Cardinality on a resource | `0..*`: one alternative extension per field that has alternatives |

Systems that do not know the extension can ignore it. It never changes the meaning of the resource: the code on the field itself is the one the endpoint chose.

## Sub-extensions

| Sub-extension `url` | Cardinality | Value | Definition |
| - | - | - | - |
| `field` | `1..1` | `valueString` | The field the alternatives are for |
| `alternative` | `1..10` | `valueCodeableConcept` | One alternative code, with the same coding system as the field |

## Field values

| Resource | `field` values |
| - | - |
| Condition | `code`, `bodySite` |
| AllergyIntolerance | `code` |
| FamilyMemberHistory | `condition.code` |
| Procedure | `code`, `bodySite` |
| Observation | `code`, `bodySite` |

## Example

```json theme={null}
{
  "url": "https://docs.corti.ai/fhir/StructureDefinition/alternative",
  "extension": [
    { "url": "field", "valueString": "code" },
    {
      "url": "alternative",
      "valueCodeableConcept": {
        "coding": [
          { "system": "http://snomed.info/sct", "code": "443694000", "display": "Uncontrolled type 2 diabetes mellitus" }
        ],
        "text": "Uncontrolled type 2 diabetes mellitus"
      }
    }
  ]
}
```

## Related pages

* [Mention extension](https://docs.corti.ai/fhir/StructureDefinition/mention.md): the words in the note that support a field
* [FHIR datatypes](https://docs.corti.ai/fhir/datatypes.md#extension): how extensions sit on a resource


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