Comment:

Suggest adding an attribute to an Observation representing the "focal point" of the observation. The focal point will represent what the observation is "about" when the observation is focused on something more specific than a particular body site on the patient. The focal point of an observation is something associated with the patient, such as a condition of the patient, the patient's environment, diet, or behavior, an allergy of the patient, a procedure, process, ot intervention. Examples include the size of a wound, the presence of allergens in the patient's home, the viral load associated with an HIV infection, measurement of disease activity in systemic lupus. Currently, only a BodySite can act as the focal point of an observation in the sense described here, although we note there is an inconsistent method of making observations about a device of a patient, where the Subject is replaced by the device, and then patient associated with the observation can only be determined by referring to the patient in the device resource. It would be more consistent if the Subject were always the patient, and instead of bodySite, there were a "focal point" attribute which could identify specifically what the observation was about, including a body site, device, condition, allergy, location, environment, intervention, encounter, behavior, etc. The option that the Subject could be an implanted Device would also be removed replaced by the "focal point" attribute.

Summary:

Add Observation.focalPoint