Our results are consistent with those from prior studies: persons with schizophrenia have significant increased medical comorbidity for conditions related to modifiable behaviors (e.g., chronic obstructive pulmonary disease), as well as conditions that may influence the course of mental symptoms (e.g., hypothyroidism). We add to prior studies by identifying increased risk for conditions not previously reported, such as deficiency anemias, hypothyroidism, neurological disorders, and fluid and electrolyte conditions. It is possible that such conditions were diagnosed in this insured population because subjects had greater financial access to physician and diagnostic services than the uninsured or underinsured who have previously been studied. It is also noteworthy that the ORs for ischemic heart disease and hypertension were not elevated, given the increased odds for nicotine abuse. We speculate that this is likely because of underdiagnosis of these conditions in this population.
Finally, compared with controls without mental illness, persons with schizophrenia were more likely to have a greater number of conditions spanning several disease categories including cardiovascular, pulmonary, neurological, and endocrine diseases. One-third of this young population (average age 40 years) had 3 or more chronic comorbidities, and only 29% of persons with schizophrenia compared with 54% of controls had no claims for comorbidities.
The impact of medical comorbidity in schizophrenia is significant because medical comorbidity affects quality of life and delivery of psychiatric and medical services. Dixon et al. reported that not only did persons with clinically diagnosed schizophrenia sampled from a variety of community and treatment settings have at least 1 medical problem, these persons also had worse perceived physical health status, more psychosis, more depression, and a greater likelihood of suicide attempt. Medical comorbidity can either cause or exacerbate the psychotic illness.17
Because medical conditions may go unrecognized in this population, it is possible that unrecognized medical conditions contribute to prolonged hospitalizations and treatment failure.17–20
Primary care and behavioral health providers dealing with apparent exacerbations of mental illness in their patients with schizophrenia may need to consider whether symptoms are being driven by undiagnosed medical conditions. Unfortunately, failure to treat medical conditions in persons with schizophrenia is a common problem,17–19,21
and these patients are more likely to report substantial barriers to care including economic barriers and delays in seeking care.10
Even among insured persons with mental disorders, risks for delaying care or not receiving needed care are substantial.22
Reasons include failure of psychiatric providers to ask about medical issues and patient inability to identify primary care provider by name.23
Recognition of treatment barriers has led to calls for integration of physical and mental health treatment services,12,22,24–28
and integrated services have been successfully demonstrated in inpatient settings, outpatient clinics, detoxification units, and smoking cessation programs.28–31
Furthermore, primary care providers may play an essential role in providing care to persons with schizophrenia, and may be first in line to assess medical conditions, especially in homeless shelters, walk-in clinics, or emergency treatment venues. Our results may guide the evaluation of persons with schizophrenia. For instance, the high rates of alcohol and polysubstance abuse increase the likelihood that presenting signs and symptoms of worsening psychosis may be related to substance intoxication, withdrawal, or medical conditions (e.g., hypothyroidism, congestive heart failure, diabetes). These findings also indicate that systems of care for primary and secondary prevention are important, especially for conditions related to smoking and infection.
Our study has several important strengths. Unlike studies conducted in a single hospital or clinic setting, our study analyzed a large population-based sample of adults. The data represent practice patterns of a diverse group of physicians in a wide geographical area. Because these subjects were commercially insured, the findings represent a population rarely studied, the commercially insured chronically mentally ill. We examined 6 years of claims data, with a follow-up period of approximately 40 months for subjects with schizophrenia. The use of rigorous case-finding methodology further ensures specificity of the schizophrenia-spectrum diagnoses and the generalizability of these finding to other men and women having schizophrenia. The use of the Klabunde comorbidity measure also ensures that the medical conditions are likely to be valid.
The limitations of this work should also be considered. First, this study included insured adults from Iowa, a racially homogeneous state. These results are generalizable to similarly insured populations but may not apply to racial and ethnic minorities and the uninsured. Second, limitations inherent to the analysis of claims data may have affected our results. Subjects who did not visit health care providers during the study period are not represented and could have only been captured if enrollment data were available. Thus, the “true” rates of comorbidity may be different from those reported. Physician failure to bill for services or failure to code medical diagnoses may have resulted in lower than expected rates of comorbidity. We have no reason to suspect that this differentially affected either the cases or controls. However, physician failure to provide needed medical assessments of persons with schizophrenia may have resulted in lower rates of claims for specific medical diagnoses. Subjects with multiple insurers may also have resulted in lower than expected rates of service receipt represented in these data. Differences because of disparate length of follow-up were controlled for in the adjusted analyses. Yet, it is possible that persons with schizophrenia had longer follow-up times for fear of losing health benefits.22
Finally, we had limited access to data regarding tobacco use, and risk for some conditions may be changed if tobacco use were entered into statistical models. However, logistic models controlling for smoking did not change the elevated risk for respiratory conditions in the Sokal et al.11
In summary, this research contributes to the growing literature on medical and psychiatric comorbidity by describing a commercially insured population of men and women with schizophrenia compared with contemporaries without administrative claims for mental illness. Using rigorous methodology, we confirmed the findings of prior studies (e.g., increased comorbid diabetes) and extended these studies by identifying other comorbid medical conditions (e.g., hypothyroidism, anemia) in persons with schizophrenia. Our findings support the development and dissemination of coordinated medical and psychiatric systems of care, especially those directed at detection and the primary and secondary prevention of these medical conditions.