Analyses

Sensitivity, specificity, positive predictive value (PPV) and negative predictive value (NPV) were calculated for each possible cutoff of the PHQ-2. The criteria used for comparison were the PHQ-9 diagnostic algorithm, the severity score cutoffs of 10 and 15 for moderate and severe depression, respectively, and clinician ratings of depression being of “major concern.” Optimal cutoffs were determined based on a combination of sensitivity, specificity, area under the curve (AUC), and prevalence of disorder compared to the full scale and clinicians’ ratings.

Correlations of the PHQ-2 with SF-36 subscales and summary score were calculated to examine the extent to which scores were associated with other measures of health. To assess construct validity and determine whether the full PHQ added significant information beyond the abbreviated version, the magnitude of the correlations between the PHQ-9 and measures of physical and mental health, controlling for the PHQ-2, were assessed to determine the variance in depressive symptoms accounted for by the full scale beyond that which is captured in the PHQ-2. Finally, internal consistency of the scales was assessed using Cronbach’s alpha.