In medicine and in psychiatry, comorbidity refers to:

The presence of one or more disorders (or diseases) in addition to a primary disease or disorder.
The effect of such additional disorders or diseases.

Comorbidity in medicine

In medicine, comorbidity describes the effect of all other diseases an individual patient might have other than the primary disease of interest. There is currently no accepted way to quantify such comorbidity.

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.

Charlson Index

The Charlson Index is a comorbidity measure created by Charlson and her colleagues and first published in 1987. The Index consisted of a list of 19 diagnoses obtained by medical chart review. The Index assigns a numerical value or "weight" from 1 to 6. The final score is simply the sum of weighted values. The Deyo-Charlson and Romano-Charlson methods theoretically improve upon the Charlson Index by assigning sets of 5-digit ICD-9-CM diagnoses. The Romano method is also called the Dartmouth-Manitoba score. As reported by Schneeweis et al. (2004), "These 2 scores differ only modestly in the ICD-9-CM codes mapping the conditions of the Charlson Index." For the D’Hoore implementation of the Charlson comorbidity index, the first 3 digits of the ICD-9 as described in D’Hoore et al. were used. The Ghali adaptation of the Charlson Index was calculated with the reduced set of diagnoses and new weights that improved prediction of in-hospital mortality in coronary artery bypass graft patients.


Patients who are more seriously ill tend to require more hospital resources than patients who are less seriously ill, even though they are admitted to the hospital for the same reason. Recognizing this, the DRGmanual splits certain DRGs based on the presence of secondary diagnoses for specific complications or comorbidities (CC).

Comorbidity in psychiatry

In psychiatry, comorbidity refers to the presence of more than one mental disorder occurring in an individual at the same time. On the DSM Axis I, Major Depressive Disorder is a very common comorbid disorder. The Axis II personality disorders are often criticized because their comorbidity rates are excessively high, approaching 60% in some cases, indicating to critics the possibility that these categories of mental illness are too imprecisely distinguished to be usefully valid for diagnostic purposes and, thus, for deciding how treatment resources should be allocated.