Agree with these comments from Jay Lyle:

These profiles use Condition to represent presence or absence of symptoms, contrary to guidance provided in the specification: "This resource is not typically used to record information about subjective and objective information that might lead to the recording of a Condition resource. Such signs and symptoms are typically captured using the Observation resource." This guidance was further confirmed and emphasized in meetings with the Patient Care and MnM workgroups.

We recommend, again, that these phenomena be recorded as Observations,

1. in compliance with existing FHIR documentation

Viz.,

"This resource is not typically used to record information about subjective and objective information that might lead to the recording of a Condition resource. Such signs and symptoms are typically captured using the Observation resource" (as above)

"It is appropriate to capture a "refuted" Condition record if the patient or anyone else had reason to believe that a patient did have a condition for a period of time and subsequent evidence has demonstrated that belief was mistaken. In this case, a concrete statement acknowledging the belief as well as the refutation of it is useful.

It is common as part of checklists prior to admission, surgery, enrollment in trials, etc. to ask questions such as "are you pregnant", "do you have a history of hypertension", etc. This information should NOT be captured using the Condition resource but should instead be captured using QuestionnaireResponse or Observation. In this case, the combination of the question and answer would convey that a particular condition was not present."

1. to avoid unnecessary and risky introduction of modifying extensions
2. due to undefined scope of negation - the intent seems to be a categorical assertion of absence, but Condition represents instances of conditions, not general patient state, so the actual scope of absence asserted by the extension is insufficient. It wouldn't mean "the patient has no skin ulcers"; it would mean "the patient doesn't have this skin ulcer."

We understand that the guidance provided for how to do this with Observation leaves room for interpretation. But with Condition it is discouraged.