Observation is the most commonly used and the most commonly misused resource type in FHIR.
At its heart Observation captures key / value pairs. The “key” is a CodeableConcept identifying what type of Observation it is. The “value” stores a piece of data as one of 11 or more allowed data types.
Examples are:
- Heart rate
- Body Weight
- Height
- Laboratory reading
Some Observations capture more complicated values that are comprised of multiple parts. To store values of this type, the Observation’s component list is used instead of the simpler “value” element.
The component list is itself a list of key / value pairs. It sounds complicated but it’s not — not when you use it correctly.
A good example of an Observation that uses the component list is blood pressure.
This contains:
- An Observation code for Blood pressure
- One component key / value pair for a diastolic reading
- One component key / value pair for a systolic reading
The component list of Observation is often used incorrectly.
How do you know if you’re using it incorrectly? Ask yourself this question:
Can any of the values in my component list be interpreted in isolation from the rest of the Observation?
If they can, if they have value on their own, independent of the other values in the component list, then you are misusing the Observation.component list and you need to go back to the drawing board.
Here’s an example of an Observation using the component list incorrectly. It has:
- An Observation code for Vital Signs
- One component key / value pair for a patient’s body weight
- One component key / value pair for a patient’s height
This is incorrect, as both values have meaning in their own right and have no reliance on or dependency on the other.
An Observation should capture one piece of data. That data may be simple, such as a patient’s weight or it may be complicated, such as a patient’s blood pressure. If it’s capturing two or more pieces of independent data, you need two or more Observations.
More about the Observation component list: https://hl7.org/fhir/R4/observation.html#gr-comp
---