<p><i>Background:</i> Individual Burden of Illness for Depression (IBI-D) represents a method for assessing the degree to which depressive symptomatology reduces functioning. Although the IBI-D has been shown to provide more information than measures of depressive symptomatology alone, previous studies have relied upon participant self-reports, and have not included an objective measure of physiological functioning. Such an index might add to the validity and informational value of IBI-D data.</p>

<p><i>Methods:</i> Responses to scales measuring depressive symptoms, functional impairment due to mental and physical health, and saliva samples were collected from 111 community volunteers from rural Australia. Saliva was assayed for cortisol and C-Reactive Protein concentrations, and the ratio of these was used as an indicator of the balance between the Hypothalamic-Pituitary-Adrenal axis and the inflammatory response from the immune system.</p>

<p><i>Results:</i> Principal Components Analysis (PCA) produced two- and three-factor solutions from the psychological and biological data, and were used to form weighted models of two IBI-D equations. Stepwise regression analysis indicated that the addition of the biological index to the second IBI-D made a significant extra contribution to variance in depression score.</p>

<p><i>Limitations:</i> No longitudinal data were collected, participants were from a single geo-cultural region, and were self-selecting. Clinician interviews might augment participant self-reports.</p>

<p><i>Conclusions:</i> Valid assessment of the self-reported symptoms of depression provides one aspect of diagnostic information, but the addition of biological information can further inform clinicians and researchers about the effect that these symptoms have upon individual patient functioning.</p>