Comorbidity is the rule rather than the exception with psychiatric disorders and is consequently of great interest to both researchers and clinicians. However, many studies of psychiatric comorbidity have been based on lifetime prevalence with mixed-age samples, a practice that (1) biases the assessment of epidemiologic comorbidity and (2) creates the appearance of comorbidity even when disorders are randomly associated. This bias is what we refer to as pseudocomorbidity.
To clarify the source of the problem and to discuss strategies that might be adopted to deal hereafter with lifetime prevalence data.
A simulated example is presented to show that even when there is only random association between disorders, there will appear to be nonrandom comorbidity when lifetime prevalence is used with mixed-age samples. An actual example relating psychosis to phobia is presented to show the bias that can result and to illustrate one way of dealing with lifetime prevalence data.
Use of lifetime prevalence with mixed-age samples, used almost exclusively in psychiatric research, generates problematic results, especially when used for assessment of comorbidity, and should be viewed with some skepticism. Hereafter, we recommend that any future use of lifetime prevalence should require determination of the age of onset, even if only by retrospective report. Comorbidity then should be reported by age.
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Onset curves (logarithm of the odds of onset vs age [in years]) for psychosis and simple phobia, actual comorbid psychosis and simple phobia, and randomly comorbid psychosis and simple phobia. Ln indicates natural logarithm; p, probability.
Odds ratios of association between psychosis and simple phobia at varying ages compared with the odds ratio using lifetime prevalence to illustrate the bias that may occur when comorbidity is assessed by using lifetime prevalence.
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