The validity of self-rating depression scales in patients with chronic widespread pain: a Rasch analysis of the Major Depression Inventory

Background: Assessment of depression in chronic pain patients by self-rating questionnaires developed and validated for use in normal and/or psychiatric populations is common. The aim of this study was to evaluate the psychometric properties of the Major Depression Inventory (MDI) in a sample of females with chronic widespread pain (CWP).Method: A total of 263 females diagnosed with CWP and referred for rehabilitation completed the MDI as part of the baseline evaluation. Rasch analysis was applied to this dataset. Rasch measurement models allow detailed analyses of an instrument's rating scale and further aspects of validity, including fit of individual scale items to a unidimensional model indicating assessment of a single construct (depression), as a prerequisite for measurement.Results: The Rasch analysis revealed substantial problems with the rating scale properties of the MDI and lack of unidimensionality. In contrast to somatic items, MDI items related to depressed mood and negative view of oneself were distributed at the higher end of the item difficulty measurement scale, indicating low endorsement of these items.Discussion: From the perspective of the Rasch measurement model, the MDI demonstrated insufficient psychometric properties when used to identify and quantify severity of depression in a clinical sample of females with CWP. The observed item endorsement pattern indicated that, in this study population, the relatively high depression severity scores primarily pertained to a common core of pain-related somatic symptoms. Careful consideration when interpreting questionnaire-derived scores of depression implemented in research and routine clinical care of patients with chronic pain is warranted.