Not surprisingly, a strong correlation was observed in this study between the number of comorbidities and the use of GP care, ambulatory specialist care and hospital admissions. However, we found no systematic differences between patients with either vascular or non-vascular comorbidity. Patients with both vascular and non-vascular comorbidity showed the highest health care utilization pattern. This is mainly caused by the higher number of comorbidities in this patient group.
Our results demonstrated that patients without comorbidity use little care. The large impact on health care utilization of patients with diabetes occurs when diabetes is included in a constellation of vascular and non-vascular comorbidities. Therefore, health care providers should routinely monitor patients with diabetes with respect to comorbidity. Case-finding protocols as mentioned by Gijsen et al. [8
], should be developed and implemented in integrated diabetes care programs.
Moreover, our results demonstrated that non-vascular comorbidities are as important utilization drivers as vascular comorbidities. In addition to the beneficial effects of it on the quality of life of not having any comorbidity [8
], prevention of comorbidity can possibly also curb the growing demands for health care. Until now, prevention in integrated diabetes care programs focuses mainly on micro- and macrovascular comorbidity. Our study shows that this focus is too limited, since additional non-vascular comorbidities in patients with diabetes increase the health care utilization as much as vascular comorbidities.
Different comorbid conditions have different effects on health care utilization. Diabetic foot results in a large increase in the use of GP care, but not in the use of medical specialist care and hospital care. Coronary heart diseases, stroke, depression, musculoskeletal diseases and cancer result in a substantially increase in both GP care, medical specialist care and hospital care. Our finding that the average length of stay in the hospital increases in most comorbidities, is in accordance with findings from other studies [24
Limitations of the study need to be considered in interpreting the results. Firstly, we obtained data about health care utilization by medical record linkage of different registrations, which introduced some selection bias. Linkage probabilities between older and younger patients differ because of different rates of changes of address [26
]. Older people move less often than younger people and therefore have higher linkage probabilities since area zip code is one of the linkage variables. Therefore the hospital utilization of the younger patients, i.e. the patient group with diabetes only, could be underestimated.
Secondly, the number of consultations by the medical specialist in the linkage is an underestimation of the actual number of consultations, since only 45% of the hospitals in the National Ambulant Register register all consultations. Since this inaccuracy in the registration of consultations to the medical specialist applies for all patient groups, it is not likely that this has biased our results.
Thirdly, for a large number of patients information on educational level was missing and they were therefore excluded from the analyses. Patients with missing data on educational level might differ from the included patients, which may have influenced our results. However, additional analyses showed that the health care utilization of patients with unknown educational level did not differ from the patients with known educational level.
Fourth, the prevalence's of retinopathy and nephropathy are substantially lower than observed in another study [27
]. A reason for this underregistration could be that retinopathy and nephropathy are not registered adequately in the GP records, since these vascular comorbidities are often treated and registered in integrated care programs and resulting in suboptimal registration in the GP records. This underestimation of the number of patients with retinopathy and nephropathy might lead to an underestimation of the effects of these vascualar comorbidities on GP care, medical specialist care and hospital care.
In our analyses, we focused on medical care only. Future research should also include long-term care, e.g. home care, nursing home care and rehabilitation care, in order to fully understand the effects of comorbidity.