Describe how to query ward encounters

input/pagecontent/StructureDefinition-fi-base-encounter-intro.md

@@ -0,0 +1,43 @@
+### Scope and Usage
+
+
+### How to find a patient in a ward?
+
+Many systems have a need to answer the question: "How to find patients/encounters that are at ward
+X at moment T?". 
+
+There are key points to consider when forming the search criteria:
+* We are most likely interested only in `IMP` (inpatient encounter) encounters, so we should filter by `class`.
+* We are interested only in `in-progress` encounters, so we need to filter by `status`.
+* We are most likely interested in querying by organizational hierarchy, the organizational unit (ward) responsible for the patient (not physical location).
+
+Organizational ward should be stored in `serviceProvider`. 
+
+FHIR request parameters:
+`GET /Encounter`
+* class = `IMP`
+* status = `in-progress`
+* period `gt` and `le` -filters as per target moment
+* serviceProvider = id of the ward organization resource
+
+Historically some systems keep the ward encounter active when the patient is sent to ICU. Leaving 
+the encounter in an `in-progress` state would cause a gotcha: encounter says that it's active but
+patient is not really there. Using encounter´s `location` elements could be used to express that 
+the patient has no active location in the ward, but this might be difficult to process for client
+systems and depend heavily on the organizational and location structures of a facility. Because of
+these problems, the encounter should not be kept `in-progress` when the patient is in an ICU period
+(if there is a need for "recycling" the ward period encounter, it should be set to `finished` and 
+then back to `in-progress` upon return from the ICU).  
+
+#### Why not use Location.managingOrganization?
+`Encounter`s `location` -field is a reference to `Location` that has a field 
+`managingOrganization`. Use of this to find the organization responsible for the patient is 
+problematic. `Location.managingOrganization` might change for different reasons (a room is 
+assigned to another organization) and this would have an unexpected effect on the 
+responsibility-concern. `Location.managingOrganization` is "too far" from the encounter to be 
+useful in this context.
+
+#### Querying by physical location
+Querying by location is also an option. Here the focus is on the physical location and not the 
+organizational responsibility towards the patient. [This blog post](https://fhirblog.com/2013/10/24/adventures-in-searching-getting-a-list-of-patients-in-a-ward/)
+will describe the process.