Comorbidity

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 co-morbidity index predicts the 1 year mortality for a patient who may have a range of co-morbid conditions such as heart disease, AIDS, or cancer. For a physician, it's helpful in knowing how aggressively to treat a condition. e.g. A patient may have cancer, but also heart disease and diabetes so severe that it doesn't make sense to treat the cancer with life-changing measures such as chemotherapy.

Since patients often don't know how severe their conditions are, originally to calculate the index nurses were supposed to go through the patient's chart and determine whether the patient had a particular condition. Subsequent studies have adapted it to a questionnaire for patients.

The original citation follows: Charlson ME, Pompei P, Ales KL, McKenzie CR (1987). A new method of classifying prognostic comorbidity in longitudinal studies: development and validation. J Chron Dis, 40(5): 373-383.

DRG
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. Comorbidity is also found to be high in drug addicts, both physiologically and psychologically.

Komorbidität Comorbiditeit