The existence of two chronic diseases in one person at the same time, for example, a patient with the joint disease Rheumatoid Arthritis and the skin disease Psoriasis. In medicine, the term “comorbid” can be either medical condition(s) existing simultaneously but independently with another condition; or it can indicate a related medical condition or conditions.
Many tests attempt to standardize the “weight” or value of comorbid conditions, whether they are secondary or tertiary illnesses. Each test attempts to consolidate each individual comorbid condition into a single, predictive variable that measures mortality or other outcomes. Researchers have validated such tests because of their predictive value, but no one test is as yet recognized as a standard.
The term “comorbid” has three definitions:
to indicate a medical condition existing simultaneously but independently with another condition in a patient (this is the older and more “correct” definition)
to indicate a medical condition in a patient that causes, is caused by, or is otherwise related to another condition in the same patient (this is a newer, nonstandard definition and less well-accepted).
to indicate two or more medical conditions existing simultaneously regardless of their causal relationship.

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