A patient may have multiple conditions, some medical and some psychiatric. There is some variation in multimorbidity is defined and interpreted in such a patient.
Halstead et al defines:
(1) >= 2 chronic physical conditions as physical multimorbidity
(2) > =3 psychiatric conditions as psychiatric multimorbidity
Other authors combine both groups into a single number of conditions while others use 2 psychiatric conditions as indicating multimorbidity.
Other issues:
(1) how to handle conditions of different significance or severity. This may be handled by weighting while others favor a simple tally of conditions.
(2) how to handle mood responses to chronic illness such as depression or anxiety
(3) how conditions are diagnosed, as some medical conditions may be diagnosed as psychiatric and vice versa.
(4) how substance use disorders are classified (although more seem to favor including them as a psychiatric disorder
Significance of multimorbidities:
(1) Clinical trials tend to exclude people with multiple conditions, limiting the evidence for a particular patient.
(2) The conditions interact and may exacerbate each other.
(3) The presence of multimorbidity makes the person prone to worse outcomes.
(4) The presence of multimorbidity may impact management. It may alter drug kinetics or compliance with therapy.
(5) They contribute to polypharmacy, drug interactions, and adverse drug effects.
(6) They are more expensive to manage.
(7) The presence of multimorbidity may negatively impact socioeconomic status.
