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 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.