In addition to widely-used code systems like [LOINC](/content/docs/careplans/loinc/index.html), FHIR makes it possible to define and use your own codes and terminology.

## Creating a CodeSystem [​](/content/docs/terminology/local-codes#creating-a-codesystem "Direct link to Creating a CodeSystem"/index.html)

The full set of available codes is defined using a `CodeSystem` resource, which should contain a canonical `url` by which to identify the code system. If the total number of codes is relatively small (on the order of hundreds, or fewer), they can be included directly within the resource. Setting `CodeSystem.content` to `complete` :

```js
{

"resourceType": "CodeSystem",

"url": "http://example.com/CodeSystem/local-medications",

"status": "active",

"content": "complete",

"concept": [

{ "code": "AB", "display": "Albuterol" },

{ "code": "AC", "display": "Acetominophen" },

{ "code": "AL", "display": "Alanine" },

{ "code": "AS", "display": "Aspirin" },

{ "code": "AX", "display": "Amoxicillin" },

]

}
```

### Hierarchical Terminologies [​](/content/docs/terminology/local-codes#hierarchical-terminologies "Direct link to Hierarchical Terminologies"/index.html)

If codes have a parent-child relationships, where one is a more specific instance of the other, this can be represented by nesting the codes in the `CodeSystem` definition. The type of hierarchy should be defined, with `is-a` being the most typical:

```js
{

"resourceType": "CodeSystem",

"url": "http://example.com/CodeSystem/local-medications",

"status": "active",

"hierarchyMeaning": "is-a",

"content": "complete",

"concept": [

{ "code": "AB", "display": "Antibiotics", "concept": [

{ "code": "AX", "display": "Amoxicillin" },

] },

{ "code": "AN", "display": "Analgesics", "concept": [

{ "code": "AC", "display": "Acetominophen" },

{ "code": "AS", "display": "Aspirin" },

]},

{ "code": "AM", "display": "Amino acids", "concept": [

{ "code": "AL", "display": "Alanine" },

]}

]

}
```

## Defining ValueSet(s) [​](/content/docs/terminology/local-codes#defining-valuesets "Direct link to Defining ValueSet(s/index.html)")

In order to make use of the codes, they should be combined into one or more `ValueSet` resources, which define useful groupings of the codes. In the simplest case, where all codes in the `CodeSystem` should be available, simply include the entire system:

```js
{

"resourceType": "ValueSet",

"url": "http://example.com/ValueSet/medications",

"status": "active",

"compose": {

"include": [

{ "system": "http://example.com/CodeSystem/local-medications" }

]

}

}
```

If the code system includes a hierarchy or other properties, you can use those to define subsets for specific use cases:

```js
{

"resourceType": "ValueSet",

"url": "http://example.com/ValueSet/painkillers",

"status": "active",

"compose": {

"include": [

{

"system": "http://example.com/CodeSystem/local-medications",

"filter": [

// is-a hierarchy uses the special property "concept"

{

"property": "concept",

"op": "is-a",

"value": "AN"

}

]

}

]

}

}
```

## Importing Larger Systems [​](/content/docs/terminology/local-codes#importing-larger-systems "Direct link to Importing Larger Systems"/index.html)

If the number of codes in the code system is too large to fit in a single resource, Medplum server supports a proprietary [`$import` operation](/content/docs/api/fhir/operations/codesystem-import/index.html) that allows Super Admin users to load codes and their metadata into the database. Although codes imported this way will not appear in the `CodeSystem` resource, they are available for use as normal. To use this method, create the `CodeSystem` as normal, but set `CodeSystem.content` to `not-present`:

```js
{

"resourceType": "CodeSystem",

"url": "http://example.com/CodeSystem/everything",

"status": "active",

"content": "not-present",

}
```

Subsequently, codes and any metadata properties associated with them can be sent in batches using the operation endpoint.

- [Creating a CodeSystem](/content/docs/terminology/local-codes#creating-a-codesystem/index.html)
  - [Hierarchical Terminologies](/content/docs/terminology/local-codes#hierarchical-terminologies/index.html)
- [Defining ValueSet(s)](/content/docs/terminology/local-codes#defining-valuesets/index.html)
- [Importing Larger Systems](/content/docs/terminology/local-codes#importing-larger-systems/index.html)