In medicine
Medicine is the science and art of healing. It encompasses a variety of health care practices evolved to maintain and restore health by the prevention and treatment of illness....

, comorbidity is either the presence of one or more disorders (or diseases) in addition to a primary disease or disorder, or the effect of such additional disorders or diseases.

In medicine

In medicine, comorbidity describes the effect of all other diseases an individual patient might have other than the primary disease of interest.

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 two definitions:
  1. to indicate a medical condition existing simultaneously but independently with another condition in a patient (this is the older and more "correct" definition)
  2. 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).

Charlson index

The Charlson co-morbidity index predicts the ten-year mortality for a patient who may have a range of co-morbid conditions such as heart disease
Heart disease
Heart disease, cardiac disease or cardiopathy is an umbrella term for a variety of diseases affecting the heart. , it is the leading cause of death in the United States, England, Canada and Wales, accounting for 25.4% of the total deaths in the United States.-Types:-Coronary heart disease:Coronary...

Acquired immune deficiency syndrome or acquired immunodeficiency syndrome is a disease of the human immune system caused by the human immunodeficiency virus...

, or cancer
Cancer , known medically as a malignant neoplasm, is a large group of different diseases, all involving unregulated cell growth. In cancer, cells divide and grow uncontrollably, forming malignant tumors, and invade nearby parts of the body. The cancer may also spread to more distant parts of the...

 (a total of 22 conditions). Each condition is assigned with a score of 1,2,3 or 6 depending on the risk of dying associated with this condition. Then the scores are summed up and given a total score which predicts mortality. There are many variations of the Charlson comorbidity index including the Charlson/Deyo, Charlson/Romano, Charlson/Manitoba, and Charlson/ D'Hoores adaptations of the Charlson comorbidity index.

The clinical conditions and scores are as follow:
1 each: Myocardial infarct, congestive heart failure, peripheral vascular disease, dementia, cerebrovascular disease, chronic lung disease, connective tissue disease, ulcer, chronic liver disease.
2 each: Hemiplegia, moderate or severe kidney disease, diabetes, diabetes with complication, tumor, leukemia, lymphoma.
3 each: Moderate or severe liver disease.
6 each: Malignant tumor, metastasis, AIDS.

For a physician, it is helpful in knowing how aggressively to treat a condition. For example, a patient may have cancer, but also heart disease and diabetes so severe that the costs and risks of the treatment outweigh the short term benefit from treatment of the cancer.

Since patients often do not 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, MacKenzie CR (1987). A new method of classifying prognostic comorbidity in longitudinal studies: development and validation. J Chron Dis, 40(5): 373-383.

Elixhauser Index

The Elixhauser comorbidity measure was developed using administrative data from a statewide California inpatient database from all non-federal inpatient community hospital stays in California (n=1,779,167). The Elixhauser comorbidity measure developed a list of 30 comorbidities relying on the ICD-9-CM coding manual. The comorbidities were not simplified as an index because each comorbidity affected outcomes (length of hospital stay, hospital changes, and mortality) differently among different patients groups. The comorbidities identified by the Elixhauser comorbidity measure are significantly associated with in-hospital mortality and include both acute and chronic conditions. Walraven et al. has derived and validated an Elixhauser comorbidity index that summarizes disease burden and can discriminate for in-hospital mortality.

Diagnosis-related group

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 diagnosis-related group
Diagnosis-related group
Diagnosis-related group is a system to classify hospital cases into one of originally 467 groups. The 467th was "Ungroupable." The system of classification was developed as a collaborative project by Robert B Fetter, PhD of the Yale School of Management, and John D Thompson, MPH of the Yale...

 (DRG) manual splits certain DRGs based on the presence of secondary diagnoses for specific complications or comorbidities (CC). The same applies to Healthcare Resource Groups (HRGs) in the UK.

Mental health

In psychiatry
Psychiatry is the medical specialty devoted to the study and treatment of mental disorders. These mental disorders include various affective, behavioural, cognitive and perceptual abnormalities...

, psychology and mental health counseling comorbidity refers to the presence of more than one diagnosis occurring in an individual at the same time. However, in psychiatric classification, comorbidity does not necessarily imply the presence of multiple diseases, but instead can reflect our current inability to supply a single diagnosis that accounts for all symptoms. On the DSM
Diagnostic and Statistical Manual of Mental Disorders
The Diagnostic and Statistical Manual of Mental Disorders is published by the American Psychiatric Association and provides a common language and standard criteria for the classification of mental disorders...

 Axis I, Major Depressive Disorder is a very common comorbid disorder. The Axis II personality disorder
Personality disorder
Personality disorders, formerly referred to as character disorders, are a class of personality types and behaviors. Personality disorders are noted on Axis II of the Diagnostic and Statistical Manual of Mental Disorders or DSM-IV-TR of the American Psychiatric Association.Personality disorders are...

s 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.

The term 'comorbidity' was introduced in medicine by Feinstein (1970) to denote those cases in which a 'distinct additional clinical entity' occurred during the clinical course of a patient having an index disease. Although the term has recently become very fashionable in psychiatry, its use to indicate the concomitance of two or more psychiatric diagnoses is said to be incorrect because in most cases it is unclear whether the concomitant diagnoses actually reflect the presence of distinct clinical entities or refer to multiple manifestations of a single clinical entity. It has been argued that because "'the use of imprecise language may lead to correspondingly imprecise thinking', this usage of the term 'comorbidity' should probably be avoided".
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