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Psychosomatic Medicine 67:233-240 (2005)
© 2005 American Psychosomatic Society

ORIGINAL ARTICLES

Potentially Modifiable Factors Associated With Disability Among People With Diabetes

Michael Von Korff, ScD, Wayne Katon, MD, Elizabeth H. B. Lin, MD, MPH, Gregory Simon, MD, MPH, Evette Ludman, PhD, Malia Oliver, Paul Ciechanowski, MD, MPH, Carolyn Rutter, PhD and Terry Bush, PhD

From the Center for Health Studies (M.v.K., E.H.B.L., G.S., M.O., C.R., T.B.), Group Health Cooperative, Seattle, WA; and the Department of Psychiatry & Behavioral Sciences (W.K., E.L., P.C.), University of Washington School of Medicine, Seattle, WA.

Address correspondence and reprint requests to Michael Von Korff, ScD, Center for Health Studies, Group Health Cooperative, 1730 Minor Ave. Suite 1600, Seattle, WA 98101. E-mail: vonkorff.m{at}ghc.org


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 METHODS
 RESULTS
 DISCUSSION
 NOTES
 REFERENCES
 
Objective: This article seeks to identify potentially modifiable factors associated with disability among people with diabetes.

Study Design and Setting: Among people with diabetes (N = 4357) in a large health maintenance organization, disease severity, psychologic and behavioral risk factors for disability were assessed. Disability was evaluated by the WHO Disability Assessment Scale (WHO-DAS-II), the SF-36 Social Functioning scale, and days of reduced household work.

Results: Depression was associated with a tenfold increase in elevated WHO-DAS-II and low SF-36 Social Functioning scores, and a fourfold increase in 20+ days of reduced household work. Minor depression and the presence of three or more diabetic complications were associated with approximately a twofold increase in disability risk. Diabetic symptoms, chronic disease comorbidity, and reduced exercise were also associated with disability.

Conclusion: Among people with diabetes, depression, diabetic complications, and exercise are potentially modifiable factors associated with disability. This suggests that integrated, biopsychosocial approaches may be needed to understand and to ameliorate disability among people with diabetes.

Key Words: disability • diabetes • depression • chronic disease • risk factor • survey

Abbreviations: BMI = body mass index; GHC = Group Health Cooperative; DSM-IV = Diagnostic and Statistical Manual, 4th Edition; PHQ-9 = Patient Health Questionnaire; ICD-9 = International Classification of Diseases, 9th Revision; HbA1c = glycosylated hemoglobin; WHO-DAS-II = World Health Organization Disability Assessment Schedule II.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
METHODS
 RESULTS
 DISCUSSION
 NOTES
 REFERENCES
 
The worldwide prevalence of diabetes is currently 150 million and is expected to increase to 300 million by the year 2025 (1). The epidemic of obesity in the United States (2) is contributing to a rapid increase in diabetes prevalence. People with diabetes are more likely to experience limitations in mobility, social role function, and activities of daily living (3–5). Given the growing disability burden of diabetes, a better understanding of how to help people with diabetes sustain active, productive, and fulfilling lives is needed. Identification of modifiable factors associated with disability among people with diabetes may suggest avenues for reducing the disability burden of this common chronic disease (3).

Previous studies of disability among people with diabetes have found that complications, comorbid chronic disease, diabetes symptoms, depression, obesity, low levels of exercise, increasing age, and lower educational levels are associated with increased disability, whereas current glycemic control is not correlated with disability (5–12). Among large surveys of disability risk factors among people with diabetes, the Women’s Health and Aging Study found that diabetic complications and depressive symptoms were important predictors of disability (5). The Diabetes Patient Outcomes Research Team project found that comorbid chronic diseases, depressive symptoms, obesity, lack of regular exercise, taking insulin, and lower levels of education were associated with increased disability. Smaller studies have also found that depression is associated with increased disability among diabetic patients (11,12). However, no prior study has simultaneously assessed the independent effects of the full range of physiological, psychologic, and behavioral risk factors for disability, nor have they focused on potentially modifiable risk factors for disability. This study permitted more extensive ascertainment of diabetic complications than was possible in prior large surveys assessing risk factors for disability among people with diabetes. In addition, prior large studies of disability risk factors have used symptom scales rather than a diagnostic assessment that distinguishes major depression from depressive symptoms not meeting diagnostic criteria.

Depression is associated with disability among people with a wide range of chronic conditions (13). Recent randomized, controlled trials have found that treating major depression improves disability outcomes among patients with comorbid chronic disease (14,15). Given the increased prevalence of major depression among people with diabetes (16), understanding whether major depression is strongly associated with disability after controlling for disease severity and comorbidity is important. Diabetes also provides an informative disease model for examining the joint effects of a chronic physical disease and depression on disability. The common complications of diabetes are well described, typically progressive, and influence functional status.

The goal of this article is to identify potentially modifiable risk factors for disability among people with diabetes. Our primary focus is evaluating the strength of the association of major depression with disability. We also assess the relationship of disability with other potentially modifiable risk factors for disability, including diabetic complications and exercise. The strength of the association of these variables with disability is assessed after controlling for measures of the severity of diabetes, chronic disease comorbidity, and demographic factors.


   METHODS
TOP
 ABSTRACT
 INTRODUCTION
 METHODS
RESULTS
 DISCUSSION
 NOTES
 REFERENCES
 
The data reported in this article were developed through the Pathways Study, a population survey of people with diabetes. Survey respondents were subsequently eligible for a randomized, controlled trial of depression treatment among diabetic patients with major depression. This article concerns only the initial population survey of people with diabetes. This project was carried out by a multidisciplinary team from the Center for Health Studies of Group Health Cooperative (GHC) and the Department of Psychiatry at the University of Washington. GHC is a nonprofit health maintenance organization with 25 primary care clinics in western Washington State. The study protocol was reviewed and approved by Institutional Review Boards at GHC and the University of Washington.

Study Setting
Nine GHC primary care clinics in western Washington were selected for the study. We selected clinics based on three criteria: 1) large number of diabetic patients, 2) within a 40-mile geographic radius of Seattle, and 3) greatest possible racial and ethnic diversity.

Sample Recruitment
Case identification used GHC’s diabetes registry that supports patient care (17). Patients are added to the diabetes registry based on: 1) current use of any diabetic agent, or 2) a fasting glucose ≥126 mg/dL confirmed by a second out-of-range test within 1 year, or 3) a random plasma glucose ≥200 mg/dL also confirmed by a second test within 1 year, or 4) a hospital discharge diagnosis of diabetes at any time during GHC enrollment or two outpatient diagnoses of diabetes (17).

Patients were screened by mail in sequential waves with approximately 700 questionnaires sent per month. A $3 gift certificate for a local store was included with the mailing to encourage response. Patients received two mailings and then were contacted by telephone.

Disability Risk Factors
The questionnaire obtained information about age, gender, years of education, race/ethnicity, height, weight, age of onset of diabetes, and initial treatment for diabetes. Patients were classified as having type 1 diabetes if their diabetes age of onset was less than age 30 and insulin was the first treatment prescribed.

The Patient Health Questionnaire (PHQ-9) was used to assess depressive illness. This questionnaire yields major and minor depression diagnoses according to Diagnostic and Statistical Manual, 4th Edition (DSM-IV) criteria and a continuous severity score (18,19). The PHQ-9 diagnosis has high agreement with a major depression diagnosis based on structured interview (18). The criteria for major depression required the patient to have, for at least 2 weeks, five or more depressive symptoms present for more than half of the days with at least one of these symptoms being either depressed mood or anhedonia. To meet the criteria for minor depression, patients had to have, for at least 2 weeks, two to four symptoms present for more than half the days with one of the symptoms being either depressed mood or anhedonia.

Automated diagnostic, pharmacy, and laboratory data were used to assess diabetes complications and glycemic control. International Classification of Diseases, 9th Revision (ICD-9) codes for seven types of diabetic complications (retinopathy, nephropathy, neuropathy, cerebrovascular, cardiovascular, peripheral vascular, and ketoacidosis) were used to identify the presence of specific complications. This diabetes complication measure is similar to one previously developed and validated in a tertiary care diabetes center (20). Group Health automated data on hemoglobin A1c (HbA1c) levels for the 18 months before screening were obtained. The HbA1c level closest in time before completion date of the questionnaire was used. HbA1c is accepted as the best measure of recent glycemic control (within the last 120 days) and is used to guide clinical management (21). HbA1c values were grouped as follows: <7.0%, 7–<8%, 8–<10.0%, and >10%. Computerized pharmacy records were used to measure medical comorbidity (Rx Risk) based on prescription drug use over the previous 12 months (22). Rx Risk has been found to be comparable to ambulatory care groups (23) in predicting total future health costs (22), and has also been shown to predict risks of hospitalization and mortality. Rx Risk values, which predict the intensity (total costs) of healthcare service use in the coming year, were divided into quartiles. Participants were asked to report their recent level of exercise, as well as height and weight (24). The Self-Completion Patient Outcome Instrument (25) was used to measure diabetes symptoms, including cold hands and feet, numb hands and feet, polyuria, excessive hunger, abnormal thirst, shakiness, blurred vision, feeling faint, and fatigue. We added an item assessing pain in hands and feet. These items were rated on a Likert scale from "never" to "every day." A symptom was considered present if it was experienced at least "several days" in the past month. The symptom scale has satisfactory internal consistency, test–retest reliability, and responsiveness to change after treatment of diabetes (25).

Disability Measures
Disability was assessed using validated measures appropriate for a population that spans working age and retired adults. Global disability was assessed using the 12-item version of the World Health Organization Disability Assessment Schedule II (WHO-DAS-II) (26,27). The WHO-DAS-II assesses disability in domains defined by the WHO International Classification of Functioning, Disability and Health (ICF) (28), including self-care, mobility, understanding and communication, interpersonal relations, work and domestic responsibilities, and participation in community activities. The 12-item WHO-DAS-II provides a reliable and valid measure of global disability (26,27). WHO-DAS-II items ask about difficulty in doing specific functions as a result of health or mental health problems. Difficulty is rated as none, mild, moderate, severe, or extreme/cannot do. The WHO-DAS-II score ranges from 0 to 100 with higher scores reflecting greater disability, in which the score indicates the percent of the highest possible score obtained. We used the scoring algorithm provided by WHO with modification to exclude missing items from scoring in both the numerator and denominator of the score. The Social Functioning subscale of the SF-36 was used to assess social function (29–31). This two-item scale asks about interference with social activities with family, friends, neighbors, or groups, and with social activities resulting from physical health or emotional problems. The SF-36 Social Functioning score ranges from 0 to 100, with a score of 100 indicating no interference with social activities. In the U.S. adult population, the Social Functioning scale has a mean of 84 and a standard deviation of 22.9 (32). Based on available normative data, we classified a Social Functioning score of less than 50 and a WHO-DAS-II score of 45 or greater as indicating substantial disability.

Participants were also asked how many days in the last 30 days they reduced or completely missed household work because of their health condition. Prior research has shown reporting of activity limitation days over a 30-day reporting period to be valid (33,34). We classified 20 or more activity limitation days in the prior month as indicating substantial disability.

Assessing Nonresponse Bias
After obtaining Institutional Review Board approval, we examined differences in deidentified data between survey respondents and nonrespondents using automated healthcare data. We excluded the 372 survey participants who did not give permission to use automated medical records data. We estimated response propensity scores (the probability of being a respondent) as a function of the following variables: age, sex, most recent HbA1c value, treatment with insulin in the prior year, use of oral hypoglycemic medicines in the prior year, received specialty mental health care in the prior year, received a depression diagnosis in primary care or specialty care in the prior year, filled any prescriptions for an antidepressant medication in the prior year, hospitalization in the prior year, Rx Risk score for the prior 12 months omitting medications for diabetes and mental disorders, number of primary care visits in the prior year, number of specialty care visits in the prior year, whether the patient was on the GHC heart disease registry, and patient primary care clinic location. We predicted response/nonresponse status as a function of these variables using PROC LOGISTIC of SAS (35). Using these predictors, we estimated a response probability for each survey respondent (the response propensity score). We used a weighted analysis, with weights inversely proportional to the estimated probability of response, rescaled to sum to the observed sample size (ie, the number of survey respondents). In weighted analyses, people with a low probability of responding would be given a higher weight in the analysis to represent the larger number of nonrespondents with similar characteristics. We then compared weighted and unweighted analyses to see if postsurvey adjustment for factors related to nonresponse resulted in meaningful differences in survey estimates. Differences in estimates based on weighted and unweighted data were negligible, so we report analyses based on observed data in this article.

Disability Risk Factor Analyses
The disability risk factors of interest were: age, sex, education level, race/ethnicity, level of depressive illness (normal, minor depression, major depression), most recent HbA1c value, number of complications of diabetes, type 1 or type 2 diabetes, body mass index (≤30 kg/m2 or>30 kg/m2), number of symptoms of diabetes reported present on most days, and number of times per week engaged in physical exercise for 30 minutes or more (23). To assess gradients of effects, all variables were entered in the model as class variables. Using logistic regression (35,36), we assessed the strength of the association of each of these risk factors with the presence of substantial disability (as defined previously) after adjusting for the remaining covariates. We carried out separate analyses with the three different disability measures (WHO-DAS-II, SF-36 Social Functioning, Days Reduced Household Work). We assessed whether each variable showed a significant relationship with disability using likelihood ratio tests. We report adjusted odds ratios for the presence of substantial disability and 95% confidence intervals.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 METHODS
 RESULTS
DISCUSSION
 NOTES
 REFERENCES
 
Patients (N = 9064) on the diabetes registry were mailed the study questionnaire. A total of 1222 were not eligible for the study, including 444 who had already disenrolled from Group Health or were moving and could not be followed, 259 with a spurious diagnosis of diabetes, 202 who were too ill to participate, 99 with language problems or hearing impairment, 128 who were dead, eight with gestational diabetes, and 2 for miscellaneous other reasons. Among the 7841 eligible patients, a total of 3002 questionnaires were not returned. Of the 4839 subjects who returned questionnaires (61.7% of eligible patients), 369 did not give their permission for access to automated medical records, 79 did not have at least one HbA1c test in the past 18 months, and seven patients did not complete the PHQ-9 depression questions. There were 4391 patients included with automated data on HbA1c values and number of diabetic complications (56% of the potentially eligible sample).

The mean age was 63 years, approximately half of the participants were female, and over 20% were minority (see Table 1). Reflecting the GHC enrollment and the population of Puget Sound, most people had attended at least some college. The large majority of participants had type 2 diabetes, approximately half had a body mass index greater than 30 kg/m2, and approximately one third had low HbA1c scores (<7.0%). The percent with high levels of disability was approximately 9% on the brief WHO-DAS-II disability scale (a score of 45 or greater) and 12% on the SF-36 Social Functioning scale (a score less than 50), whereas 9% reported 20 or more days of reduced household work as a result of illness in the prior 30 days.


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  		TABLE 1. Sample Characteristics (n = 4391)

 

Odds ratios for disability risk factors (and their 95% confidence intervals) are shown for each of the three measures of disability (Table 2). The p values in Table 2 are based on the Wald test for the overall effect of each variable after adjusting for other predictors included in the logistic regression model. Predictors with a significant effect (p ≤.05) are shown in bold in Table 2. Confidence intervals of odds ratio estimates that do not include 1.00 are also shown in bold.


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  		TABLE 2. Adjusted Odds Ratios and Confidence Intervals for Disability Risk Factors Estimated by Logistic Regression for WHO-DAS-II Global Disability, SF-36 Social Functioning, and Activity Limitations Days for Household Work in the Prior Month

 

Major depression was associated with an almost tenfold increase in risk of elevated WHO-DAS-II score and low SF-36 Social Functioning score, and a fourfold increase in the risk of 20 or more days of reduced household work (effect of depression was significant at p <.0001 for all three analyses). Minor depression was also associated with increased risks of disability, with odds ratios ranging from 1.6 to 2.2. Although the increment in disability for minor depression was not nearly as great as that observed among people with major depression, the odds ratios for the minor depression group were in the range of those observed for people with three or more diabetic complications.

Level of diabetic symptoms, chronic disease comorbidity, and diabetic complications were also consistently associated with increased disability risks, but the odds ratios were not as large as for depression. Exercising three or more times a week was associated with a reduced likelihood of disability with odds ratios consistently less than one. Female gender was associated with increased risk of disability for all three disability measures. Glycemic control, type 1 versus type 2 status, and body mass index were not associated with any of the disability measures. It was surprising that poorer glycemic control showed a nonsignificant trend toward association with lower levels of disability on the WHO-DAS-II and days of reduced household work measures. Given the large sample size, the inconsistent results across the three disability measures, and the fact that these differences did not reach conventional levels of statistical significance, we believe this reflects random variation rather than a trend toward reduced disability among people with poorer glycemic control. No prior study has reported that poorer glycemic control is associated with higher levels of functional disability. Relative to whites, blacks were found to be at increased risk of disability for the WHO-DAS-II and the SF-36 measure but not for the Activity Limitation Days measure.

We screened separately for interaction effects of depression with: 1) diabetic complications, 2) chronic disease comorbidity, and 3) glycemic control. We did not observe significant interaction effects between depression and these disease severity measures in their effect on disability status. Only estimates of main effects are shown in Table 2. To permit visual inspection of the joint effects of disease severity and depression, the joint effect of level of depressive illness and number of diabetic complications on the mean number of days of reduced household work is shown in Figure 1. At each level of depressive illness, increasing diabetic complications showed a moderate effect on days of reduced household work. In contrast, for a given number of diabetic complications, the level of depressive illness showed a strong association with days of reduced household work. The mean number of days of reduced household work was approximately doubled for people with minor depression relative to people who were not depressed. Days of reduced household work doubled again for people with major depression relative to those with minor depression. It is readily apparent in Figure 1 that the increment in disability associated with depressive illness is roughly additive across the range of diabetes complications, indicating that depression and diabetic complications do not have a synergistic effect on disability.



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  		Figure 1. Days reduced household work in prior month by number of diabetic complications and depression status.

 

A similar pattern was seen for the percent with high levels of disability as assessed by the brief WHO-DAS-II and the SF-36 Social Functioning scale (see Table 3). In the top half of Table 3, the joint effect of depressive illness and of diabetic complications and chronic disease comorbidity (Rx Risk score (22)) on elevated WHO-DAS-II score is shown. Depressive illness showed a strong relationship with disability at every level of diabetic complications and at every level of chronic disease comorbidity. Both diabetic complications and chronic disease comorbidity showed a moderate effect on disability at a given level of depressive illness. A similar pattern was observed for analyses of the percent with an SF-36 Social Function score less than 50 (bottom half of Table 3).


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  		Table 3. Percent With High Levels of Disability (Brief WHO-DAS-II Score of 40 or Greater, SF-36 Social Functioning Score of Less Than 50) by Depression, Diabetes and Co-Morbidity Status

 


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 METHODS
 RESULTS
 DISCUSSION
NOTES
 REFERENCES
 
This research identified potentially modifiable risk factors for disability among people with diabetes. These included depressive illness, number of diabetic complications, and frequency of exercise. Risk factors less susceptible to modification that also showed clinically significant relationships with disability included the severity of diabetes symptoms and the extent of chronic disease comorbidity. Major depression was associated with up to a tenfold increase in risk of clinically significant disability. Minor depression was associated with a two-fold increase in disability risks, an increase in risk comparable to that observed for people with three or more diabetic complications. These results underscore the potential importance of depression as a determinant of quality of life among people managing a major chronic disease like diabetes.

Disability may cause increases in depressive symptoms and reduce exercise frequency, thereby inflating the association of these variables with disability. A bidirectional relationship between disability and many of the variables studied here (eg, depression, chronic disease comorbidity, exercise frequency) is likely. Although disability, depression, and disease severity may have reciprocal effects, the results of this study point to the need for research that assesses what kinds of interventions are effective in reducing disability among people with diabetes. Despite considerable evidence that diabetes is associated with markedly increased risks of disability, little is known about how to reduce the disability burden among people with diabetes or with other common chronic diseases.

Research concerning effects of improved disease management has generally focused on effects on disease control, not on functional outcomes. The fact that psychologic and behavioral variables are strongly associated with disability among people with diabetes suggests that a broader, biopsychosocial perspective toward management of diabetes may be needed if progress is to be made in improving quality of life in this patient population. Because depression is associated with disability in diverse chronic disease populations, knowledge gained about the relationship of depression and disability in this patient population may be relevant to the depression–disability relationship for other chronic diseases.

An important limitation of this study is its cross-sectional design. Although current glycemic control and body mass index were unrelated to disability in this study, poor long-term glycemic control and increased body mass index have been shown to increase risks of diabetic complications, which may subsequently increase disability risks. The effects of these risk factors may be realized over time. If so, a long-term prospective study of disability onsets among people with diabetes would be needed to discern their impact on disability. This study was also limited by reliance on self-report measures of exercise frequency, height, and weight. Major depression status was assessed by a well-validated self-report scale, the PHQ, rather than by a structured clinician interview. This method does not permit evaluation of psychiatric diagnoses that might exclude a diagnosis of major depression or assessment of whether depressive symptoms are explained by an underlying medical condition.

The response rate obtained in this study was low (56%), so nonresponse bias may have influenced results. However, we were able to obtain extensive medical records information for well over 90% of the people eligible for this study. Propensity score adjustment has been advocated as a means of evaluating and controlling survey nonresponse bias (37). In this study, the information on demographic, medical, and psychiatric variables that was available for the large majority of people eligible permitted more extensive propensity score adjustment than is usually possible in a population survey. Propensity score analyses adjusting for differential response by these variables did not suggest that nonresponse bias was an important factor influencing study results.

Our results suggest that depressive illness may play an important role in functional disability among diabetic patients. Prior randomized trials have shown that treating depression can improve disability outcomes among people with significant chronic physical diseases (14,15). We currently have underway a large randomized trial of depression treatment among survey respondents diagnosed with major depression or dysthymia. The results of this trial will shed light on whether enhanced treatment of major depression can improve disability outcomes among people with diabetes.

The strength of the relationship of depressive illness with disability, after controlling for disease severity and comorbidity, indicates the potential importance of the relationship of depression and disability among people with a major chronic physical disease. Depression may cause disability because it impairs high-order human adaptive capacities such as motivation, energy, and self-confidence (38). A major chronic physical disease, like diabetes, challenges these adaptive capacities, placing chronically ill individuals under stress. Managing diabetes requires difficult changes in health behaviors such as exercise and diet as well as glucose monitoring, adhering to complex medical regimens, and managing bothersome symptoms.

This cross-sectional study cannot determine whether depression is a cause or a consequence of impaired functioning. It is plausible that depression is both a cause of impaired functioning and a consequence of the psychologic and physiological stresses associated with having a major chronic disease. Prior longitudinal studies have indicated that depression and disability have reciprocal effects (39). This cross-sectional study cannot shed further light on the plausibility of reciprocal causation. However, our results do suggest that the determinants of disability among people with diabetes are likely to be multifactorial, including physiological, psychologic, and behavioral factors. Given the multifactorial correlates of disability among people with diabetes, it is plausible that if interventions are to be effective in reducing disability in this population, they will need to address the most important physiological (complications, chronic disease comorbidity), psychologic (depressive and diabetic symptoms), and behavioral (exercise) risk factors for disability.

In conclusion, our results suggest that depressive illness, diabetic complications, and exercise frequency are important factors associated with increased risks of disability among people with diabetes. The severity of diabetic symptoms and chronic disease comorbidity also showed robust relationships with disability. Current glycemic control and body mass index were not related to disability in this study. Because disability may influence depression, diabetic complications, and exercise, experimental studies are needed to assess whether treating depression, preventing diabetic complications, and increasing exercise levels can improve functional outcomes among people with diabetes.


   NOTES
TOP
 ABSTRACT
 INTRODUCTION
 METHODS
 RESULTS
 DISCUSSION
 NOTES
REFERENCES
 
Supported by grant #MH 4–1739 from the National Institute of Mental Health Services Division, Bethesda, MD.

Received for publication March 24, 2004; revision received October 4, 2004.

DOI:10.1097/01.psy.0000155662.82621.50


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 METHODS
 RESULTS
 DISCUSSION
 NOTES
 REFERENCES
 

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