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ORIGINAL ARTICLES |
From the Department of Medicine & Center for Health Care Research, Medical University of South Carolina, Charleston, South Carolina and Ralph H. Johnson VA Medical Center, Charleston, South Carolina.
Address correspondence and reprint requests to Leonard E. Egede, MD, Medical University of South Carolina, Center for Health Care Research, 135 Cannon Street, Suite 403, P.O. Box 250837, Charleston, SC 29425. E-mail: egedel{at}musc.edu.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONRESEARCH DESIGN AND METHODSRESULTSDISCUSSIONNOTESREFERENCES |
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Methods: Data on 1794 adults with diabetes from the 1999 National Health Interview Survey were analyzed. Six chronic conditions were identified: hypertension, coronary artery disease, chronic arthritis, stroke, chronic obstructive pulmonary disease, and end-stage renal disease. Chronic conditions were counted and categorized into 4 groups: diabetes alone, diabetes + 1 condition, diabetes + 2 conditions, and diabetes + 3 or more conditions. Prevalence of depression was calculated by number of chronic conditions and for each condition. Adjusted odds of depression were calculated by number of chronic conditions and for each chronic condition while simultaneously controlling for covariates and for each chronic condition using multiple logistic regression. STATA was used for statistical analyses.
Results: Using diabetes alone as reference, adjusted odds of major depression by number of conditions were: diabetes + 1 (1.31; 95% confidence interval [CI], 0.67–2.55), diabetes + 2 (2.09; 95% CI, 1.06–4.12), and diabetes + 3 or more (4.09; 95% CI, 2.04–8.17). Adjusted odds of major depression for each coexisting chronic conditions were: hypertension (1.22; 95% CI, 0.78–1.90), coronary artery disease (2.00; 95% CI, 1.27–3.14), chronic arthritis (2.02; 95% CI, 1.35–3.02), stroke (2.15; 95% CI, 1.10–4.31), chronic obstructive pulmonary disease (0.96; 95% CI, 0.52–1.81), and end-stage renal disease (1.19; 95% CI, 0.57–2.49).
Conclusion: Odds of major depression are significantly increased among adults with diabetes in the presence of two or more coexisting chronic conditions, and coexistence of coronary artery disease, chronic arthritis, and stroke in particular, are associated with increased odds of major depression.
Key Words: diabetes • depression • chronic disease • health survey • health service research • health psychology
Abbreviations: CAD= coronary artery disease; COPD = chronic obstructive pulmonary disease; ESRD = end-stage renal disease; NHIS = National Health Interview Survey; CIDI-SF = Composite International Diagnostic Survey Short Form; DSM-IV = Diagnostic and Statistical Manual of Mental Disorders, 4th Edition.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONRESEARCH DESIGN AND METHODSRESULTSDISCUSSIONNOTESREFERENCES |
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Depression is highly prevalent in people with diabetes. Approximately 30% of people with diabetes have depression, and there is a two-fold increased odd of having depression among people with diabetes (2,3). Multiple studies have shown that coexisting depression in people with diabetes is associated with poorer glycemic control (4), increased risk of complications (5), increased healthcare utilization and cost (6), increased odds of functional disability (7), and lost productive work time (8).
Studies have shown that in addition to diabetes, depression co-occurs in a substantial proportion of patients with hypertension (9), coronary artery disease (CAD) (10), chronic arthritis (11), stroke (12), chronic obstructive pulmonary disease (COPD) (13), and end-stage renal disease (ESRD) (14). Because these conditions are associated with increased odds of having depression and are also highly prevalent in people with diabetes, it is plausible that the coexistence of these conditions in people with diabetes may explain the increased odds of having depression that has been observed in people with diabetes. Given that previous studies have not addressed this question, it is unclear how coexistence of these chronic conditions affects prevalence and odds of having depression in people with diabetes. In particular, it is unclear whether coexisting chronic conditions have a threshold effect on the odds of having depression and whether specific chronic conditions are associated with increased prevalence and odds of depression in people with diabetes.
To address these issues, data from the 1999 National Health Interview Survey (NHIS), a nationally representative survey of the civilian adult population of the United States, was analyzed to provide answers to the following two questions. First, is there a relationship between number of coexisting chronic conditions and the odds of having depression in adults with diabetes? And second, are there specific chronic conditions that are associated with increased odds of having depression among adults with diabetes?
It was hypothesized that increasing number of coexisting comorbidity would have an incremental effect on the odds of having depression among adults with diabetes. Based on current literature, it was further hypothesized that coexisting CAD, stroke, and ESRD would be associated with increased odds of having depression in people with diabetes.
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RESEARCH DESIGN AND METHODS |
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TOPABSTRACTINTRODUCTIONRESEARCH DESIGN AND METHODSRESULTSDISCUSSIONNOTESREFERENCES |
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Demographic and Socioeconomic Characteristics
Four age categories were created: 18–34, 35–49, 50–64, and 
65 years. Four racial/ethnic groups defined by NHIS were used: non-Hispanic white, non-Hispanic black, Hispanic, and other. Education was classified as <high school graduate and high school graduate. Household income was categorized as <$20,000 and $20,000. Two dichotomous groups were created for marital status (married vs. unmarried) and employment (employed vs. unemployed). Two categories of perceived health status were created: better or the same vs. worse, based on the respondent’s perception of the change in their health status compared with 1-year previously. Smokers were defined as individuals who reported that they were currently smoking.
Access to Care
The NHIS used a series of questions to identify individuals with access to care. These included questions such as "Is there a place you usually go to when you are sick or need advice about your health?," "What kind of place is it—a clinic, doctor’s office, emergency room, or some other place?," "What kind of place do you usually go to when you need routine or preventive care such as a physical examination or check-up?," and "What kind of health professional do you usually see—a doctor or nurse or some other health professional?" In addition, respondents were asked "During the past 12 months, have you seen or talked to any of the following health care providers about your own health?... a general doctor who treats a variety of illnesses (a doctor in general practice, family medicine, or internal medicine)." The NHIS created a summary access to care variable as follows: 1) adults with a single usual source of medical care, 2) adults with no known single usual source of medical care, and 3) undefined. For this study, access to care was defined as having a single usual source of medical care.
Chronic Medical Conditions
Six chronic medical conditions: 1) hypertension; 2) CAD (coronary heart disease, myocardial infarction, angina pectoris, and other heart disease); 3) chronic arthritis (rheumatoid arthritis, osteoarthritis, gouty arthritis, and other arthritis); 4) stroke; 5) COPD (emphysema and chronic bronchitis); and 6) ESRD were selected based on high prevalence and public health burden (17). These conditions were defined based on self-report. Evaluation of NHIS diagnostic reporting has shown that there is high agreement between self-report of most of these chronic medical disorders and medical records reviews (18). These 6 chronic conditions were coded as 0 (condition absent) and 1 (condition present). A summary chronic condition variable was created that counted the number of chronic conditions and categorized them into 4 groups: diabetes alone, diabetes + 1 condition, diabetes + 2 conditions, and diabetes + 3 or more conditions.
Diagnosis of Major Depression
In 1999, the NHIS introduced the Composite International Diagnostic Interview Short Form (CIDI-SF) as part of the survey questionnaire to assess mental illness at the population level. The CIDI-SF is a diagnostic interview designed for use by trained interviewers who are not clinicians. The CIDI-SF was developed from the longer and more complex CIDI (19), and it was revised to screen for disorders defined in the fourth edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV) (20). The CIDI-SF is a valid and reliable diagnostic interview and has classification accuracy of 93% for major depressive disorder (21). The classification accuracy of the CIDI-SF is derived from comparing the CIDI-SF cases and noncases with those derived from structured clinical interviews performed as part of the National Comorbidity Survey (22). Twelve-month prevalence estimates are reported.
Composite International Diagnostic Interview Short Form Scoring
A complete copy of the CIDI-SF questions and scoring instructions is available from the World Health Organization web site (http://www.who.int/msa/cidi/index.htm). The CIDI-SF uses a stem-branch logic in which a small number of initial diagnostic stem questions are used in each section to skip out people who are least likely to be considered case subjects before they are asked further symptom questions (23). There are two ways to meet the diagnostic stem requirement for major depressive disorder: either by endorsing all questions about having 2 weeks of dysphoric mood or by endorsing all questions about having 2 weeks of anhedonia. In addition, the symptoms of dysphoric mood and anhedonia should last at least most of the day almost every day. Respondents who deny either the existence of symptoms or the persistence of symptoms are defined as not having major depressive disorder. If the respondent endorses dysphoric mood, seven additional questions are asked about losing interest, feeling tired, change in weight, difficulty sleeping, trouble concentrating, feeling down, and thoughts about death, and then a summary major depressive disorder score is calculated based on positive responses to these additional seven questions (range, 0–7).
Similarly, respondents who endorse anhedonia are asked additional symptom questions, including questions about losing interest, feeling tired, and change in weight, difficulty sleeping, trouble concentrating, feeling down, and thoughts about death. A summary major depressive disorder score is also calculated based on number of positive responses (range, 0–7). Based on the recommendations for scoring (23), an individual was classified as having major depression if they endorsed the stem questions and had positive responses to three or more of the symptom questions. Individuals who endorsed the stem questions but had fewer than three positive responses to the symptoms questions were defined as not having major depression.
Statistical Analyses
Statistical analysis was performed with STATA (24), which accounts for the multistage sampling, clustering, and stratification design of the NHIS. Three sets of analyses were performed. First, characteristics of adults with diabetes were defined and weighted to reflect the U.S. population. Second, prevalence of major depression in adults with diabetes was calculated among those with and those without the six comorbid chronic conditions of interest and by number of chronic conditions. Third, unadjusted and adjusted odds of having depression were calculated in those with compared with those without the six comorbid chronic conditions of interest and by number of chronic conditions. For the unadjusted model, the dependent variable was depression (yes vs. no), and the independent variables were the six chronic conditions (coded as 0/1). For the adjusted model, age, sex, race/ethnicity, income, marital status, health status, smoking, and access to care were included as covariates to the unadjusted model. These covariates were selected for inclusion in the logistic models either because prior work had shown they were associated with increased prevalence and odds of having depression in people with diabetes (3) or they were clinically relevant.
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RESULTS |
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TOPABSTRACTINTRODUCTIONRESEARCH DESIGN AND METHODSRESULTSDISCUSSIONNOTESREFERENCES |
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Table 1 shows the characteristics of adults with diabetes. It is noteworthy that only 23% had diabetes alone. Approximately 34% had diabetes and an additional chronic condition, 25% had diabetes and two additional chronic conditions, and 19% had diabetes and three or more additional chronic conditions. Among those with additional chronic conditions, approximately 61% had hypertension, 33% had CAD, 30% had chronic arthritis, 9% had COPD, 7% had had a stroke, and 6% had ESRD.
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Table 2 shows prevalence and odds of having depression by number of coexisting chronic conditions. Prevalence of depression ranged from 6.7% in those with diabetes alone to 17.0% in those with diabetes and three or more additional coexisting chronic conditions. The adjusted odds of having depression (using those with diabetes alone as reference) ranged from 1.31 (95% CI, 0.67–2.55) for those with diabetes and an additional chronic condition to 4.09 (95% CI, 2.04–8.17) for those with diabetes and three or more additional chronic conditions. Only people with diabetes and two or more additional chronic conditions had statistically significant higher odds of depression compared with those with diabetes alone.
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Table 3 shows prevalence and odds of having depression by specific coexisting chronic conditions. Among those with coexisting chronic conditions, prevalence of depression ranged from 9.7% (95% CI, 7.9–12.0) in those with coexisting hypertension to 19.8% (95% CI, 11.7–31.5) in those with coexisting ESRD. Simultaneously controlling for the 6 chronic conditions of interest and other covariates, the coexistence of CAD (2.00; 95% CI, 1.27–3.14), chronic arthritis (2.02; 95% CI, 1.35–3.02), and stroke (2.15; 95% CI, 1.10–4.31) were associated with statistically significant increases in odds of having depression among adults with diabetes. Coexistence of hypertension, COPD, or ESRD was not associated with a significant increase in adjusted odds of having depression among adults with diabetes.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONRESEARCH DESIGN AND METHODSRESULTSDISCUSSIONNOTESREFERENCES |
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In this study, two methods were used to extricate the independent effects of chronic comorbidity on odds of having depression in people with diabetes. The first approach allowed for assessment of the effect of increasing number of comorbid conditions on the odds of having depression, whereas the second approach allowed for estimation of the effect of the coexistence of specific comorbid conditions on the odds of having depression. In combination, both approaches provide unique but different information about the effect of comorbidity on prevalence and odds of depression in people with diabetes. An additional strength of this study is the use of a valid and reliable diagnostic instrument to estimate major depression in a representative sample of the U.S. population. Likewise, the fact that the NHIS is a household survey, thereby capturing the prevalence of depression in nonclinic patients, a group that has been excluded in calculations of previous depression estimates also strengthens the study.
This study shows that coexisting comorbid conditions contribute to the increased prevalence and odds of having depression previously observed in people with diabetes. The overall prevalence of major depression in adults with diabetes was 9.3%. However, prevalence of major depression ranged from 6.7% in those without any of the six chronic conditions of interest to 9.7% in those with comorbid hypertension to 19.8% in those with comorbid ESRD. Similarly, adjusted odds of having major depression for all adults with diabetes (compared with adults without diabetes) was 1.72 but dropped to 1.27 when the six chronic conditions were included in the model. Furthermore, the odds of having major depression among people with diabetes increased from 1.31 in those with one additional chronic condition to 4.09 in those with three or more chronic conditions (compared with those with diabetes alone). These findings clearly show that a large component of the increased prevalence and odds of having major depression in people with diabetes is accounted for by coexisting chronic comorbid conditions.
This study has both clinical and research implications. From a clinical standpoint, this study further reinforces the importance of screening for depression in patients with diabetes. Although depression is a disease that responds well to medical treatment, and treatment of depression has been shown to decrease health resource utilization, decrease short-term disability, and decrease functional disability (25–27), fewer than 25% of depressed patients with diabetes (28) are recognized and treated appropriately. As such, strategies at the population level to not only increase awareness of depression among healthcare providers (29), but also to disseminate optimal treatment outcomes (30) and implement effective educational, and organizational interventions (31) to improve recognition and treatment of depression in patients with diabetes should remain a priority. The findings of this study may assist busy clinicians to identify subsets of people with diabetes who are at especially high risk for having depression, which will allow for tailored screening, treatment, and follow up.
From a research standpoint, these findings emphasize the need for better understanding of the biologic mechanism(s), pathways, and direction of the relationship between depression and diabetes and provide alternative hypotheses for further exploration. It is particularly important to understand how coexisting chronic comorbid conditions influence the likelihood of having depression in people with diabetes, because it appears that the coexistence of multiple comorbid conditions may largely account for the increased prevalence and odds of having depression seen in diabetes. It seems plausible that these chronic conditions are surrogates for diabetes duration and severity, and so represent increased psychosocial burden of illness. Alternatively, it is also plausible that these chronic conditions may have biologic effects that individually and collectively increase the risk of having depression. As future studies are designed to define the causal relationship between diabetes and depression, the moderating effects of coexisting chronic comorbid conditions need to be taken into consideration.
The findings of this study are subject to some limitations. First, as stated earlier, this study cannot speak to causality or temporality. In addition, because the etiology of depression was not assessed, it was not possible to differentiate organic from nonorganic causes of depression. This may have contributed to the high prevalence of depression seen in conjunction with certain comorbid conditions such as stroke. Second, as a result of unavailability of reliable data in the NHIS on chronic disease duration, disease severity, or comorbid psychiatric conditions, the confounding effect of these factors could not be ascertained. Third, it was not possible to differentiate type 1 from type 2 diabetes from the data. Finally, major depression was not assessed with the "gold standard" of a psychiatrist interview, which may lead to overestimation of the prevalence of major depression. However, the classification accuracy of 93% of the CIDI-SF indicates that the estimates of this study are reliable.
Despite these limitations, the findings of this study are important. The odds of major depression are significantly increased among adults with diabetes in the presence of two or more coexisting chronic conditions, and the coexistence of CAD, chronic arthritis, and stroke in particular, are associated with increased odds of major depression in these patients.
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NOTES |
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TOPABSTRACTINTRODUCTIONRESEARCH DESIGN AND METHODSRESULTSDISCUSSIONNOTESREFERENCES |
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Dr. Egede is supported by grant #5K08HS11418 from the Agency for Health Care Research and Quality, Rockville, Maryland. Dr. Egede is a Staff Physician at the Ralph H. Johnson VAMC, Charleston, SC. The contents of this publication are solely the responsibility of the author and do not necessarily represent the official views of the Agency for Health Care Research and Quality or the Centers for Disease Control and Prevention.
DOI:10.1097/01.psy.0000149260.82006.fb
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