Differential diagnosis between bipolar disorder (BD) and borderline personality disorder (BPD) is often challenging due to some overlap in symptoms and comorbidity of disorders. We investigated correlations in self-reported symptoms of BD and BPD in screening questionnaires at the levels of both total scores and individual items and explored overlapping dimensions.

The McLean Screening Instrument (MSI) for BPD and the Mood Disorder Questionnaire (MDQ) for BD were filled in by patients with unipolar and bipolar mood disorders (n=313) from specialized psychiatric care within a pilot study of the Helsinki University Psychiatric Consortium. Pearson's correlation coefficients between total scores and individual items of the MSI and the MDQ were estimated. Relationships between MDQ and MSI were evaluated by exploratory factor analysis (EFA).

The correlation between total scores of the MDQ and MSI was moderate (r=0.431, P<0.001). Significant correlations were found between the MSI items of “impulsivity” and “mood instability” and all MDQ items (P<0.01). In the EFA, the MSI “impulsivity” and “mood instability” items had significant cross-loadings (0.348 and 0.298, respectively) with the MDQ factor. The MDQ items of “irritability”, “flight of thoughts” and “distractibility” (0.280, 0.210 and 0.386, respectively) cross-loaded on the MSI factor.

The MDQ and MSI items of “affective instability”, “impulsivity”, “irritability”, “flight of thoughts” and “distractibility” appear to overlap in content. The other scale items are more disorder-specific, and thus, may help to distinguish BD and BPD.