Operation
Definition
The
meets the common need for searching for the most recent or last n=number of observations
for a subject. For example, retrieving the last 5 temperatures for a patient to view trends
or fetching the most recent laboratory results or vitals signs. To ask a server to return
the last n=number of observations, the
queryuses the
defined for the Observation resource. However, rather than their normal use, they are
interpreted as inputs - i.e.. instead of requiring that the resources literally contain
the search parameters, they are passed to a server algorithm of some kind that uses them
to determine the most appropriate matches.
'GROUP BY'
limited to the number of requested responses per group specified by the optional max query
parameter
The set of returned observations should represent distinct real world observations and
not the same observation with changes in status or versions. If there are no matches,
the
query SHALL return an empty search set with no error, but may include an operation outcome
with further advice.
is an optional input parameter to the
query operation. It is used to specify the maximum number of Observations to return
from each group. For example for the query "Fetch the last 3 results for all vitals
for a patient"
= 3.
The set of most recent N Observations that match the
query search criteria.
The key differences between this query operation and simply searching Observation using
the combination of
and
parameters are:
The
query returns
the last N resource grouped by code. Using the _count query method doesn't restrict
the total mathches so you may need to page through several "A" Observations
before getting to Observation "B".
The server is responsible for grouping the observations by codes. This frees the client
from needing to determine which codes she should ask for.
<?xml version="1.0" encoding="UTF-8"?>
This operation cannot be performed on observations that the user is not authorized to
see. It is assumed that the server has identified and secured the context appropriately,
<OperationDefinition xmlns="http://hl7.org/fhir">
<id value="Observation-lastn"/>
<text>
<status value="generated"/>
<div xmlns="http://www.w3.org/1999/xhtml">
<p class="res-header-id">
<b> Generated Narrative: OperationDefinition Observation-lastn</b>
</p>
<a name="Observation-lastn"> </a>
<a name="hcObservation-lastn"> </a>
<p> URL: [base]/Observation/$lastn</p>
<h3> Parameters</h3>
<table class="grid">
<tr>
<td>
<b> Use</b>
</td>
<td>
<b> Name</b>
</td>
<td>
<b> Scope</b>
</td>
<td>
<b> Cardinality</b>
</td>
<td>
<b> Type</b>
</td>
<td>
<b> Binding</b>
</td>
<td>
<b> Documentation</b>
</td>
</tr>
<tr>
<td> IN</td>
<td> max</td>
<td/>
<td> 0..1</td>
<td>
<a href="datatypes.html#positiveInt">positiveInt</a>
</td>
<td/>
<td>
<div>
<p>
<code> max</code> is an optional input parameter to the
<em> lastn</em> query operation. It is used to specify the maximum number of Observations to return
from each group. For example for the query "Fetch the last 3 results for all
vitals for a patient"
<code> max</code> = 3. </p> </div> </td> </tr> <tr> <td> OUT</td> <td> return</td> <td/> <td> 1..1</td> <td> <a href="bundle.html">Bundle</a> </td> <td/> <td> <div> <p> The set of most recent N Observations that match the <em> lastn</em> query search criteria. </p> </div> </td> </tr> </table> <div> <p> The key differences between this query operation and simply searching Observation
using the combination of
<code> _count</code> and <code> _sort</code> parameters are: </p> <ul> <li> The <em> lastn</em> query returns <strong> only</strong> the last N resource grouped by code. Using the _count query method doesn't restrict
the total matches so you may need to page through several "A" Observations
before getting to Observation "B".
</li> <li> The server is responsible for grouping the observations by codes. This frees the
client from needing to determine which codes she should ask for.</li>
</ul> <p> This operation cannot be performed on observations that the user is not authorized
to see. It is assumed that the server has identified and secured the context appropriately,
and can either associate the authorization context with a single patient, or determine
whether the context has the rights to the nominated patient, if there is one. If there
is no nominated patient (e.g. the operation is invoked at the system level) and the context
is not associated with a single patient record, then the server should return an error.
Specifying the relationship between the context, a user and patient records is outside
the scope of this specification.
The *lastn query* meets the common need for searching for the most recent or last n=number
of observations for a subject. For example, retrieving the last 5 temperatures for a patient
to view trends or fetching the most recent laboratory results or vitals signs. To ask
a server to return the last n=number of observations, the *lastn* queryuses the [normal
search parameters](observation.html#search) defined for the Observation resource. However,
rather than their normal use, they are interpreted as inputs - i.e.. instead of requiring
that the resources literally contain the search parameters, they are passed to a server
algorithm of some kind that uses them to determine the most appropriate matches.
The response from an lastn query is a set of observations:
* filtered by additional parameters
* 'GROUP BY' `Observation.code`
* sorted from most recent to the oldest
* limited to the number of requested responses per group specified by the optional max
query parameter
* If no maximum number is given then only the most recent Observation in each group is
returned.
The set of returned observations should represent distinct real world observations and
not the same observation with changes in status or versions. If there are no matches,
the *lastn* query SHALL return an empty search set with no error, but may include an operation
outcome with further advice.
The key differences between this query operation and simply searching Observation using
the combination of `_count` and `_sort` parameters are:
* The *lastn* query returns **only** the last N resource grouped by code. Using the _count
query method doesn't restrict the total mathches so you may need to page through several
"A" Observations before getting to Observation "B".
* The server is responsible for grouping the observations by codes. This frees the client
from needing to determine which codes she should ask for.
This operation cannot be performed on observations that the user is not authorized to
see. It is assumed that the server has identified and secured the context appropriately,
whether the context has the rights to the nominated patient, if there is one. If
there is no nominated patient (e.g. the operation is invoked at the system level)
and the context is not associated with a single patient record, then the server
should return an error. Specifying the relationship between the context, a user
and patient records is outside the scope of this specification.</p>
</div> </div> </text> <extension url="http://hl7.org/fhir/StructureDefinition/structuredefinition-fmm"> <valueInteger value="3"/> </extension> <extension url="http://hl7.org/fhir/StructureDefinition/structuredefinition-standards-status"> <valueCode value="trial-use"/> </extension> <extension url="http://hl7.org/fhir/StructureDefinition/structuredefinition-wg"> <valueCode value="oo"/> </extension> <url value="http://hl7.org/fhir/OperationDefinition/Observation-lastn"/> <version value="6.0.0-ballot3"/> <name value="Lastn"/> <title value="Last N Observations Query"/> <status value="draft"/> <kind value="operation"/> <experimental value="false"/> <date value="2025-11-08T17:14:57+00:00"/> <publisher value="HL7 International / Orders and Observations"/> <contact> <telecom> <system value="url"/> <value value="http://hl7.org/fhir"/> </telecom> <telecom> <system value="email"/> <value value="fhir@lists.hl7.org"/> </telecom> </contact> <contact> <telecom> <system value="url"/> <value value="http://www.hl7.org/Special/committees/orders"/> </telecom> </contact> <description value="The *lastn query* meets the common need for searching for the most recent or last
n=number of observations for a subject. For example, retrieving the last 5 temperatures
for a patient to view trends or fetching the most recent laboratory results or
vitals signs. To ask a server to return the last n=number of observations, the
*lastn* query uses the [normal search parameters](observation.html#search) defined
for the Observation resource. However, rather than their normal use, they are interpreted
as inputs - i.e.. instead of requiring that the resources literally contain the
search parameters, they are passed to a server algorithm of some kind that uses
them to determine the most appropriate matches.
The request for a lastn query SHALL include:
* A `$lastn` operation parameter
* A subject using either the `patient` or `subject` search parameter
* A `category` parameter and/or a search parameter that contains a code element
in its FHIRpath expression. ( e.g., `code` or `code-value-concept`)
The request for a lastn query MAY include:
* Other Observation search parameters and modifiers
The response from a lastn query is a set of observations:
* Filtered by additional parameters
* If not explicitly filtered by status then will include statuses of 'entered-in-error'
* Ordered first by “equivalent” `Observation.code` value
* Codes SHALL be considered equivalent if the `coding.value` *and* `coding.system`
are the same.
* Text only codes SHALL be treated and grouped based on the text.
* For codes with translations (multiple codings), the code translations are
assumed to be equal and the grouping by code SHALL follow the transitive property
of equality. For example:
|Observation.code for observation a|Observation.code for observation b|Observation.code
for observation c|number of groups [codes/text in each group]|
|---|---|---|---|
|a|b|c | 3 [a],[b],[c]|
|a|b|a,c | 2 [a.c],[b]|
|a|b|a,b | 1 [a,b]|
|'textM'|'Text'|'t e x t'|3 ['text'],['Text'],['t e x t']|
* The ordering of the “equivalent” code groups is not specified.
* Ordered secondly (within each code group) from most recent effective time to
the oldest effective time. For further guidance, refer here on [sorting](https://www.hl7.org/
fhir/search.html#_sort).
* Limited to the number of requested responses per group specified by the optional
*max* query parameter
* In case of a tie—when the effective times for more than one Observation in
the same code group—both will be returned. Therefore, more Observations may be
returned than is specified in *max*. For example, 4 Observations instead of 3 if
the 3rd and 4th most recent observation had the same effective time.
* If no maximum number is given then only the most recent Observation in each group
is returned.
Note that the individual Observation.code values used for grouping might not be
explicit in the request, for example if a category of Observations is requested,
or if a code is specified with the `:below` modifier.
The set of returned observations should represent distinct real-world observations
and not the same observation with changes in status or versions. If there are no
matches, the *lastn* query SHALL return an empty search set with no error, but
may include an operation outcome with further advice."/>
<jurisdiction> <coding> <system value="http://unstats.un.org/unsd/methods/m49/m49.htm"/> <code value="001"/> <display value="World"/> </coding> </jurisdiction> <affectsState value="false"/> <code value="lastn"/> <comment value="The key differences between this query operation and simply searching Observation
using the combination of `_count` and `_sort` parameters are:
* The *lastn* query returns **only** the last N resource grouped by code. Using
the _count query method doesn't restrict the total matches so you may need to page
through several "A" Observations before getting to Observation "B".
* The server is responsible for grouping the observations by codes. This frees
the client from needing to determine which codes she should ask for.
This operation cannot be performed on observations that the user is not authorized
to see. It is assumed that the server has identified and secured the context appropriately,
and can either associate the authorization context with a single patient, or determine
whether the context has the rights to the nominated patient, if there is one. If there
is no nominated patient (e.g. the operation is invoked at the system level) and the context
is not associated with a single patient record, then the server should return an error.
Specifying the relationship between the context, a user and patient records is outside
the scope of this specification.
`max` is an optional input parameter to the *lastn* query operation. It is used to specify
the maximum number of Observations to return from each group. For example for the query
"Fetch the last 3 results for all vitals for a patient" `max` = 3.
whether the context has the rights to the nominated patient, if there is one. If
there is no nominated patient (e.g. the operation is invoked at the system level)
and the context is not associated with a single patient record, then the server
should return an error. Specifying the relationship between the context, a user
and patient records is outside the scope of this specification."/>
<resource value="Observation"/> <system value="false"/> <type value="true"/> <instance value="false"/> <parameter> <name value="max"/> <use value="in"/> <min value="0"/> <max value="1"/> <documentation value="`max` is an optional input parameter to the *lastn* query operation. It is used
to specify the maximum number of Observations to return from each group. For example
for the query "Fetch the last 3 results for all vitals for a patient"
`max` = 3."/>
<type value="positiveInt"/> </parameter> <parameter> <name value="return"/> <use value="out"/> <min value="1"/> <max value="1"/> <documentation value="The set of most recent N Observations that match the *lastn* query search criteria."/> <type value="Bundle"/> </parameter>
</
OperationDefinition
>
Usage
note:
every
effort
has
been
made
to
ensure
that
the
examples
are
correct
and
useful,
but
they
are
not
a
normative
part
of
the
specification.
Conformance
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