Observation

Overview

The Observation API can be used to interact with Observation records for a patient. When querying for data it must always be in the context of a single patient.

  • Retrieving or searching for Observation records is performed using a HTTP GET described below.

GET API - Observation Retrieval

id and _id

The Observation resource can be retrieved directly if the id of the resource is known. By appending the id to the url will cause the Observation resource to be retrieved. Unlike all other retrieval queries, the response will be a single resource as opposed to a Bundle.

GET /Observation/[id]

An alternate method for retrieval is to make use of the _id search parameter. In this case, the result will be a Bundle. If the id is present the Bundle will contain a single Observation resource, otherwise, the Bundle will be empty.

GET /Observation?_id=[id]

Query Parameters

Unless retrieving an Observation resource by the use of id the subject must always be present. The Observation can only be targeted against a single patient at any one time.

Subject

GET /Observation?patient=Patient/[id]

When searching for NHS Numbers use the system https://fhir.nhs.uk/Id/nhs-number. For example:

GET /Observation?patient.identifier=https://fhir.nhs.uk/Id/nhs-number|1234567890

All the search examples shown below will accept the patient parameter in either form.

Code

When searching for Observations with a specific code the query can be constructed using different combinations of the code system and the code value.

GET /Observation?patient=[value]&code=[code]
GET /Observation?patient=[value]&code=[system]|[code]

For example to search for a BMI observation using SNOMED then a query would look like the following:

GET /Observation?patient=[value]&code=http://snomed.info/sct|60621009

It is possible to construct a search using text.

GET /Observation?patient=[value]&code:text=[value]

It is possible to construct a search using a predefined ValueSet that defines a collection of codes.

GET /Observation?patient=[value]&code:in=[ValueSet url]

The ValueSets that are available for this function will evolve over time. Speak to Graphnet if you intend to use this capability.

Date

To search on the date an Observation was made the following search constructs can be used.

GET /Observation?patient=[value]&date=[value]

To improve searching the API supports the use of search modifiers. The modifiers supported for date are as below:

Modifier Description
lt Returns dates less than the search term
le Returns dates less than or equal to the search term
gt Returns dates greater than the search term
ge Returns dates greater than or equal to the search term
eq Returns dates equal to the search term 
GET /Observation?patient=[value]&date=lt[value]
GET /Observation?patient=[value]&date=le[value]
GET /Observation?patient=[value]&date=gt[value]
GET /Observation?patient=[value]&date=ge[value]
GET /Observation?patient=[value]&date=eq[value]

So to search for an Observation on 23rd January 2021 the query would be:

GET /Observation?patient=[value]&date=2021-01-23

To search for all Observations before the 23rd December 2020 the query would be:

GET /Observation?patient=[value]&date=lt2020-12-23

Identifier

To search for Observations using an identifier on the Observation record, the following search constructs can be used.

GET /Observation?patient=[value]&identifier=[system]|[code]

For example:

GET /Observation?patient=[value]&identifier=http://acme.org/obsdata|21323-123213

Category

To search for Observations classified by type, the following search constructs can be used.

GET /Observation?patient=[value]&category=[code]
GET /Observation?patient=[value]&category=[system]|[code]

It is possible to construct a search using text.

GET /Observation?patient=[value]&category:text=[value]

Status

The search for Observations by status the following construct can be used.

GET /Observation?patient=[id]&status=[value]

_lastUpdated

To retrieve Observations based on the last updated date of the record.

GET /Observation?patient=[id]&_lastUpdated=[value]

To improve searching the API supports the use of search modifiers. The modifiers supported for date are as below:

Modifier Description
lt Returns dates less than the search term
le Returns dates less than or equal to the search term
gt Returns dates greater than the search term
ge Returns dates greater than or equal to the search term
eq Returns dates equal to the search term 
GET /Observation?patient=[value]&_lastUpdated=lt[value]
GET /Observation?patient=[value]&_lastUpdated=le[value]
GET /Observation?patient=[value]&_lastUpdated=gt[value]
GET /Observation?patient=[value]&_lastUpdated=ge[value]
GET /Observation?patient=[value]&_lastUpdated=eq[value]

So to search for an Observation last updated on 23rd January 2021 the query would be:

GET /Observation?patient=[value]&_lastUpdated=2021-01-23

To search for all Observations last updated before the 23rd December 2020 the query would be:

GET /Observation?patient=[value]&_lastUpdated=lt2020-12-23

_summary

Adding the _summary=count query parameter will change the behaviour of the query to return just the count of records, rather than the records themselves.

GET /Observation?patient=[id]&_summary=count

This will return a response like:

{
    "resourceType": "Bundle",
    "type": "searchset",
    "total": 15,
    "id": "df23fd8b-d30c-47c6-a2bd-64c8a24b1166"
}

This can be added to any query construct.

_format

The API supports both XML and JSON formats. The default format is JSON. The format can be requested either using HTTP headers or via the use of the _format query parameter as per the following table.

Format Value API Response Format
xml XML
text/xml XML
application/xml XML
application/fhir+xml XML
json JSON
application/json JSON
application/fhir+json JSON 
GET /Observation?patient=[id]&_format=[format value]

For example the query below would return a response in XML format.

GET /Observation?patient=[id]&_format=xml

This can be added to any query construct.

Sort Parameters

The default sort order for Observation records is in descending date order (i.e. most current first). The following sort parameters are also available.

Date

GET /Observation?_sort=date
GET /Observation?_sort=-date

Paging

_count

The parameter _count is defined as a hint to the server regarding how many resources should be returned in a single page.

GET /Observation?patient=[id]&status=[code]&_count=[n]

For example the query below would return a Bundle with 5 records per page.

GET /Observation?patient=[id]&status=[code]&_count=5