Terminologies and Coded Values

FHIR® resources often use coded values to clearly represent various healthcare concepts. These coded values are supported by FHIR®'s standardized data format and a range of operations to ensure their effective use. A collection of codes relevant to a specific domain, along with their meanings, is known as a code system. For instance, LOINC is a code system for lab tests, measurements, survey answers, and other observable healthcare data. While many code systems are defined by large organizations, FHIR® also allows for the use of locally-defined code systems.

Representing Coded Values

A base code consists of just the coded value string, such as 8867-4 from LOINC. Without context, it can be challenging to understand what this code represents. Different code systems might use the same code for different meanings. In FHIR®, plain code values are used only when the code system is clear from the context. Typically, FHIR® uses Coding values to represent coded values unambiguously, for example:

{
  "system": "http://loinc.org",
  "code": "8867-4",
  "display": "Heart rate"
}

When using coded values, it might be necessary to combine multiple codes for the same concept. This is often done to record equivalent codes from different systems or multiple related codes that describe a more complete concept. The CodeableConcept data type in FHIR® represents this grouping and is the most common way to represent coded values in FHIR® resources, for example:

{
  "coding": [
    {
      "system": "http://loinc.org",
      "code": "8867-4",
      "display": "Heart rate"
    },
    {
      "system": "http://snomed.info/sct",
      "code": "364075005",
      "display": "Heart rate (observable entity)"
    }
  ],
  "text": "Heart rate"
}

Defining and Using Code Systems

The FHIR® CodeSystem resource type defines how a code system should be used by the FHIR® server. It is the authoritative source of information about the codes from the system. A CodeSystem is primarily identified in FHIR® by its URL, for example:

{
  "resourceType": "CodeSystem",
  "url": "http://snomed.info/sct",
  "name": "SNOMEDCT_US",
  "title": "SNOMED CT, US Edition",
  "status": "active",
  "content": "example",
  "concept": [
    {
      "code": "364075005",
      "display": "Heart rate (observable entity)"
    }
  ]
}

Large code systems often have too many codes to fit into a single FHIR® resource. To use them effectively, a small subset of codes related to a specific use case is defined. A ValueSet resource specifies a group of codes from one or more code systems that relate to a common use case. These codes can be specified explicitly or by criteria specific to the code system. Like CodeSystem resources, a ValueSet is identified primarily by its URL, for example:

{
  "resourceType": "ValueSet",
  "url": "http://example.com/ValueSet/vitals",
  "name": "vitals",
  "title": "Vital Signs",
  "status": "active",
  "compose": {
    "include": [
      {
        "system": "http://loinc.org",
        "concept": [
          {
            "code": "8310-5",
            "display": "Body temperature"
          },
          {
            "code": "8462-4",
            "display": "Diastolic blood pressure"
          },
          {
            "code": "8480-6",
            "display": "Systolic blood pressure"
          },
          {
            "code": "8867-4",
            "display": "Heart rate"
          },
          {
            "code": "9279-1",
            "display": "Respiratory rate"
          }
        ]
      },
      {
        "system": "http://snomed.info/sct",
        "filter": [
          {
            "property": "concept",
            "op": "descendent-of",
            "value": "118227000"
          }
        ]
      }
    ]
  }
}

For more detailed information, feel free to contact us .