Comorbid Diagnoses are for diseases the patient has had for some time. Comorbidities can be a factor in increasing the patient risk of dying; see Charlson Comorbidity Index.
Many ICD10 diagnosis code can be used as a comorbid/pre-existing diagnosis -- the general criterion is that it was present PRIOR to admission AND is still present and clinically relevant.
If the diagnosis under consideration qualifies as an Admit Diagnosis, then in general it is not appropriate to code it as a comorbidity.
If the diagnosis is acute then in general it is not appropriate to code it as a comorbidity.
Code these even if the diagnosis of the condition was only made during the current hospital admission but it is quite clear that it must have existed before admission (even if that wasn't known).
Example 1: If a patient is admitted with pneumonia and on further workup is found to have CA of the lung, then this is coded in comorbid as it is obvious that the cancer must have been there for a while prior to admission.
Example 2: Patient comes in with abdominal pain. Diagnosed as gastroenteritis but incidentally pt is found to be HIV +ve. You would code HIV +ve as a comorbid. Again, this is obvious that the pt had this problem for a while prior to admission to the hospital.
Past medical history
-There is, in ICD10, a small list of codes that represent previous procedures or medical situations that can't be captured in another way -- category:Past medical history
Priorities
You will need to enter priorities for comorbids to group them for Combined ICD10 codes. Don't worry about actually prioritizing them, for comorbids the priorities will only be used for grouping.