The present study revealed that a diagnosis of diabetes mellitus type II at or after the age of forty was associated with an increased likelihood of developing a subsequent depression or depressive feelings. After statistical adjustment, practice identification code, age and a diagnosis of depression preceding the diagnosis of diabetes mellitus appeared to be significantly related to a subsequent diagnosis of depression.
This is the first longitudinal study on the association between diabetes mellitus and subsequent depression based on data from general practices. A meta-analysis by Anderson et al. [1
] involving 42 cross-sectional studies reported that patients with type II diabetes are twice more likely to experience depressive symptoms than their peers without diabetes. In contrast, the present results suggest that patients with diabetes mellitus are 1.12–1.41 (CI) times more likely to develop depression and 1.18–1.46 times (CI) more likely to develop depression and/or depressive feelings. Our findings confirm the outcome of earlier studies in showing an increased incidence of depression among patients diagnosed with diabetes mellitus. However, this association appears to be weaker than found in earlier studies. This discrepancy can be due to a number of reasons. Firstly, it has been shown that depression rates are two to three times higher in studies that use self-reports [5
]. For example, self-report measures may identify a broader spectrum of depressive disorders or symptoms that may reflect co-morbid psychiatric illness or general distress [5
], which could result in an overestimation of the prevalence of depression [13
]. Secondly, using a diagnosis of depression preceding DM as a confounder can have reduced the association between diabetes type II and depression. Although the association between diabetes and depression attenuated without this correction, it remained statistical significant. Finally, in the present study GPs were not instructed to systematically screen patients for possible depression or depressive symptoms which could have led to a lower risk estimate. If this is indeed the case, the present study reports an underestimation of depression in diabetes patients. It is important for GPs to be aware of the fact that patients with type II diabetes are more likely to experience depression than their peers without diabetes. Several studies have shown that adherence to a variety of self-care activities [6
] and metabolic control [7
] decreases in diabetes patient as a consequence of co-morbid depression. Diabetes patients with co-morbid depression also show a decline in quality of life [8
]. Moreover, for approximately 75% of the people diagnosed with diabetes mellitus in the Netherlands, the GP is the primary medical caregiver [23
], making the GP the proficient person to detect depression or depressive feelings. The present findings imply that GPs who are consulted by patients diagnosed with DM type II should be especially aware of the patient’s increased risk of developing depression in the near future. In short, general practitioners should be alert to possible early signs of depression in diabetic patients to ensure early detection and possibly even prevention of a depressive disorder. Nurse practitioners for diabetes mellitus, who assist the general practitioners and provide a broad range of health care services, could fulfill an important role in this aspect. Nurse practitioners focus on patients’ conditions as well as on the effects of the illness on the lives of the patients and their families and can therefore serve as a “point of entry” for physical as well as mental problems diabetes patients encounter. Nurse practitioners could enhance diagnostics by systematically screen diabetes patients for possible depression or depressive symptoms in order to prevent under diagnosis of depression in diabetes patients.
Another intriguing finding of the present study was that practice identification code was significantly associated with an enhanced likelihood of developing a depression. It appears that in 3 of the 21 practices involved, a relatively high percentage of patients is diagnosed with depression (ranging from 9.5 to 9.8% of the patients) while in 3 of the 21 practices this percentage is noticeably smaller (ranging from 4.0 to 4.7% of the patients). Hence, it seems plausible that GPs may differ regarding their inclination to diagnose a depression. After investigating the most important characteristics of the practices incorporated in the present study, such as geographic place (defined by postal code) of a general practice, total number of diagnosed depressive disorders in the general practice, number and gender of patients, education of the patients, and number and gender of GPs in a practice, we were not able to identify any specific characteristics that could explain this effect. This diagnostic variability may have important implications for general practices [24
Our study has several advantages over previous studies. This is the first longitudinal study that evaluated the causal relationship between diabetes mellitus type II and subsequent occurrence of depressive disorder and/or depressive feelings in a general practice based setting. Consequently, the results seem more representative for the general population than results of studies conducted in smaller and more homogeneous samples [17
]. Also, the sample size used in the present study supports the robustness of our risk estimates. Moreover, prior studies commonly relied on a variety of self-report surveys which are known to overestimate the prevalence of depression [5
]. The six criteria following diagnosis of depressive disorder in the RNH database are essentially comparable to the nine symptoms of a depressive episode described in the DSM-IV of the American Psychiatric Association [21
]. These criteria are a solid foundation for making a uniform diagnosis of depression by GPs.
Despite the previous mentioned strengths, our findings must be interpreted in light of some possible limitations. First, since the RNH database does not make a clear distinction between diabetes type I and type II, the present study only included patients diagnosed with diabetes mellitus at or after the age of 40 years. Consequently, it can not be ruled out that none of the diabetes type I patients were included. Second, the total number of conditions registered in the RNH database reflects the GPs perspective of the health status and relevant health problems of his patients. As a result, some health problems may be missing because the patient did not report them to the GP or because the GP does not judge them to be clinically significant [26
]. The number of missing health problems, however, appears to be rather small [19
]. Furthermore, GPs have a tendency to use a diagnosis primarily as a mean to reach the goal of helping the patient and not as a goal in itself [18
]. This is not the case in questionnaires and self-reports, which could have resulted in an underestimation of the prevalence of depression. Due to the very large follow-up period (01-01-1980 to 01-01-2008) it may be argued that no state of the art impression of the association between diabetes and subsequent depression is given. However, an additional analysis involving a smaller, more recent period of time (01-01-1995 to 01-01-2008) yielded similar results. Moreover, an analysis with a follow-up period of only 6 months was also conducted, which gave similar results. Finally, one of the drawbacks of studies that combine data from multiple practices is the between practice variability [19
]: it is generally assumed that some variability exists between general practitioners in making a diagnose [24
]. Possibly, this between practice variability may have resulted in the effect that code of practice has on the development of depression.
Further research is warranted to investigate a multitude of unanswered questions. More well-conducted research with adequate control for confounding factors is needed to investigate the causal relationship between diabetes and depression mellitus more in depth. In particular, the influence of practice setting on the longitudinal association between diabetes and depression should be studied in more detail. Future research should also explore the role of other confounding factors. For example, the influence of psychosocial and social-economic factors on the association between diabetes and depression needs to be elucidated.
In conclusion, the present research adds to the evidence concerning the association between diabetes mellitus type II and depression, in that this association also holds in a longitudinal setting in a large general practice population. The present results indicate, however, that patients with diabetes mellitus are less likely to develop a subsequent depression than was expected based on previous research.