Comorbidity

In medicine and in psychiatry, comorbidity is defined as a preexisting concomitant but unrelated disease or diseases, in addition to a primary disease, disorder, initial diagnosis, or index condition. The term comorbidity is also used to describe the effect of all other disorders or diseases an individual patient might have other than the primary diagnosis or disease of interest. Results from the first National Comorbidity Survey, released in 1994, revealed that 79% of all seriously ill people (inclusive of all diseases) were comorbid. Comorbidity has serious implications for the diagnosis, treatment, rehabilitation, and outcome of affected individuals. Comorbidity may also affect the ability of affected individuals to function and may be used as a prognostic indicator for length of hospital stay, cost, and mortality.

According to the Diagnostic and Statistical Manual (DSM) published by the American Psychiatric Association, anxiety and major depressive disorders commonly occur together or are common comorbid disorders. Such comorbidity is found among about half of all the individuals with these disorders. Comorbidity is also common among substance users, both physiologically and psychologically (e.g., substance use/misuse and bipolar disorder). The presence of mental disorders associated with substance use and dependence—the dually diagnosed—among those attending substance use treatment services has been reported to be between 30% and 90%. A survey in 1994 found that 65% of those attending mental health services reported alcohol use disorders. Moreover, alcohol use–related disorders are also common among persons diagnosed with schizophrenia.

There are currently no standardized means of quantifying or classifying prognostic comorbidity. Many tests attempt to standardize the ‘weight’ or predictive value of specific complications or comorbid conditions, based on the presence of secondary or tertiary diagnoses. The Charlson Co-Morbidity Index attempts to consolidate each individual comorbid condition into a single, adjusted variable that measures or predicts the 1-year mortality or other outcomes for a patient who presents with a range of comorbid conditions. The Charlson Co-Morbidity Index has demonstrated excellent predictive validation and contains 19 categories of comorbid conditions, primarily defined using the International Statistical Classification of Diseases and Related Health Problems, Version 9, Clinical Modification (ICD-9-CM) diagnoses codes. The comorbidity score reflects the cumulative increase in likelihood of 1-year mortality due to the severity of the effect of comorbidities; the higher the score, the more severe the burden of comorbidity.

Social scientists, health care scholars, and policymakers advised caution in the use and reliance on the Charlson Index because the ICD-9-CM codes used in the indexes or composite variables often lead to difficulties in distinguishing between complications and comorbidities. A complication is usually defined as a medical condition that is acquired during a hospital stay and could have been prevented. If these disease categories cannot be reliably differentiated, it is possible that the burden of comorbid conditions might be overestimated. Those urging caution in using the Charlson Index coding schema have also noted that the comorbidity and complication codes often fall in the same disease category resulting in a lack of distinction between principal diagnoses present at admission versus those that developed or occurred during the hospital stay.

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