Seven percent of adults in the USA have been diagnosed with Major Depressive Disorder (MDD) and in adults with chronic diseases, such as diabetes, this increases to over eleven percent . Although the causal connection between the two remains unclear, the consequences are far-reaching. Having both diabetes and depression is associated with poor glycaemic control, resulting in more severe complications and a lower quality of life [2, 3]. With the increasing severity of diabetes, the prevalence of depression also increases, and especially in vulnerable patients such as those with diabetes-related complications. Depression has severe consequences; underlining the importance of focus on the prevention of depression.
Depression often remains unrecognized, and although several screening questionnaires are available, unfortunately, most questionnaires have been validated for use in primary care in patients with less complex medical illnesses. It is expected that patients with severe diabetes and depression are frequently present in specialized outpatient clinics or hospitals, but specialists often do not have the necessary time or skills to recognize depression. The recognition of depression is very important and the Patient Health Questionnaire (PHQ-9) was developed for this purpose [4, 5]. This instrument has already been validated for primary care patients, cardiac patients in general hospitals , and diabetes patients in primary care , but not for diabetes patients in specialized outpatient clinics. Besides that, the present study is the first one assessing operating characteristics for the PHQ-9 in diabetes patients. In patients with chronic medical diseases, co-morbid MDD can be difficult to identify, because the symptoms of the two may overlap. The effect of symptom overlap on the performance of screening instruments for depression, such as the PHQ-9 , would be that higher cut-off points are necessary to correctly identify MDD in the chronically ill than in a population with less severe illnesses. The overall effect would be that both sensitivity and specificity would decline.
In this study we assessed the criterion validity, in terms of sensitivity, specificity, positive and negative predictive value. What is new is that Receiver Operator Curves (ROC) were assessed of the PHQ-9 for MDD in diabetes patients in specialized outpatient clinics. These specialized clinics differ from general diabetic care clinics in that in these specialized clinics foremost patients with severe diabetes with complications are present and specialized clinical diabetes care is provided by a team of a diabetologist, a specialized diabetes nurse and a dietician.