We adopted a population-based approach to examine mental illness-related disparities in disease prevalence, quality of care and outcomes for a major medical condition - diabetes. The results showed that MHCs had higher diabetes prevalence, fewer recommended pathology tests for ongoing diabetes monitoring, and higher risks of hospitalisation for diabetes complications, diabetes-related mortality and all-cause mortality, compared with non-MHCs. Disparities were most marked in MHCs with alcohol/drug disorders, schizophrenia, affective disorders, other psychoses and personality disorders.
The strengths of our study compared with previous published studies are the: (i) use of population-based linked data with over 400,000 people in the study populations, (ii) inclusion of a wide spectrum of mental disorders in the analysis, (iii) use of a comparison group of people with no mental illness, (iv) rigorous identification of diabetes within the study populations, and (v) long-term follow-up (up to 16.5 years).
The limitations of our study included, first, the lack of data for private psychiatrists and GPs treating mental disorders. This limits the extrapolation of our findings for people with mental illness in Australia. Patients in the MHR account for about 40% of people with mental illness (8% of the estimated 20% of Australian adults who have clinically diagnosable mental illness), generally with moderate to severe mental illness. Their physical disease burden and physical health care disparities were probably greater than the remainder of people with mental illness. Nevertheless, using the MHR to identify mental health group as did our previous study [6
] ensures the continuity and integrity of our investigations and findings.
Second, while the domain restriction to the WA electoral roll enhanced the internal validity, it reduced the external validity. It is likely that MHCs who were not on the electoral roll have different socio-demographic profiles to MHCs who are on the electoral roll. They may represent a group who are younger than 18 years, have severe mental illness, are homeless, Indigenous, overseas visitors or new migrants [6
]. The disparities in this group may be greater. Using Indigenous people as an example, they are known to have high rates of both diabetes and mental illness. They are more likely over-represented in MHCs who were not on the electoral roll and may experience greater barriers to receiving good quality of care. Also the MHR captured only about 40% of patients with mental illness, thus our non-MHC group almost certainly included some people with mental illness. This may result in underestimation of the true difference between MHCs and non-MHCs.
Third, MBS and PBS data do not contain diagnostic information and thus diabetes cases identified from these data are implied rather than definite. However, importantly, we used the same definition to identify diabetes in both groups as the focus of the study was on differences rather than absolute measures. Nevertheless, our diabetes prevalence measures for schizophrenia and affective disorders were consistent with other studies (10 to 15%) [16
]. Also, regular examinations of eyes and feet, and weight and blood pressure monitoring are all part of a comprehensive ongoing monitoring recommended by the Australian clinical practice guidelines for people with diabetes to prevent the development and progression of macrovascular and microvascular complications [18
]. However, MBS does not have enough good data to be able to evaluate these. Fourth, we had no information on lifestyle risk factors or detailed clinical information so the effects of these factors could not be adjusted in the analyses. We also had no information on the clinical decision-making processes leading to pathology testing or hospitalisation, so we can only speculate on the relative roles of patient and doctor in driving or inhibiting the recommended tests or hospitalisation. Last, the ascertainment of diabetes-related death from death certificates may have improved over time, but this would apply equally to both MHCs and non-MHCs leading to negligible change in the relative differences reported in this study.
Previous research on the association between mental illness and diabetes prevalence has exclusively focused on a single mental illness (schizophrenia or affective disorders) and almost all investigations have used the general population as their comparison group [16
] rather than a non-mental health comparison group we used. Our study showed that all evaluated mental disorders were associated with a higher prevalence of diabetes not just people with severe mental illness. Diabetic MHCs received fewer recommended pathology tests than non-MHCs, after adjustment for socio-demographics and case mix, which is consistent with other studies [19
], except that we also assessed the long-term disparities (up to 16.5 years). This indicates that the quality of diabetes care may be poorer in MHCs as the differences in the intensity of testing for diabetes monitoring cannot be explained by the differences in socio-demographics, case mix or access to primary care. Our finding that diabetic MHCs had a higher rate of diabetes-related hospitalisations than diabetic non-MHCs suggests that diabetic MHCs may have more diabetic complications than diabetic non-MHCs, possibly due to their lower use of monitoring tests [20
]. Another possible reason may be because diabetic MHCs have a lower threshold for diabetes-related admissions than diabetic non-MHCs. Among MHCs, schizophrenia had the lowest relative risk of hospitalisation for diabetes complications, possibly because of their higher diabetes-related mortality rate. In fact, schizophrenia had the highest rate of fatal first complication (adjusted rate ratio 2.81, 1.84 to 4.29).
Possible reasons for MHC-related disparities include factors related to the: (i) patient's cognitive impairment or poor communication skills [21
], (ii) provider's bias against 'difficult' patients [22
], (iii) time constraints of competing conditions [24
], and (iv) fragmented health system [25
]. The interpersonal aspects of the patient-provider relationship may contribute to more pronounced disparities in quality of care in patients with alcohol/drug disorders and personality disorders [19
], particularly those with alcohol/drug disorders who are unlikely to receive any preventive care [27
Our study contributes to emerging evidence which shows that mental illness-related disparities in physical disease burden and physical health care are real and substantial, and present fundamental public health and ethical challenges. While access to care is a prerequisite for good quality care, increasing access may not overcome barriers for good quality care in MHCs. While it is important to promote early detection, diagnosis and treatment of mental illness in primary care settings, attention to physical health conditions in people with existing mental illness is also critical. Given the established overall high rate of GP visits in MHCs [6
], there is an opportunity for quality improvement and savings in life-years and costs. Potential interventions to improve physical health care should focus on approaches that highlight both mental health issues and physical care requirements in the consultation. Incentives are required that promote preventive care in the routine management of diabetic patients with comorbid mental illness, particularly those with severe mental illness and behavioural disorders and those with multiple risk factors. Mental and physical health care services need to integrate their efforts to provide a holistic, patient-centred approach to improving health outcomes and quality of life in patients with mental illness and comorbid physical diseases.