Distinguishing between symptoms of schizotypal (SPD) and borderline personality disorders (BPD) is often difficult due to their partial overlap and frequent co-occurrence. We investigated correlations in self-reported symptoms of SPD and BPD in questionnaires at the levels of both total scores and individual items, examining overlapping dimensions.

Two questionnaires, the McLean Screening Instrument (MSI) for BPD and the Schizotypal Personality Questionnaire Brief (SPQ-B) for SPD, were filled in by patients with mood disorders (n = 282) from specialized psychiatric care in a study of the Helsinki University Psychiatric Consortium. Correlation coefficients between total scores and individual items of the MSI and SPQ-B were estimated. Multivariate regression analysis (MRA) was conducted to examine the relationships between SPQ-B and MSI.

The Spearman's correlation between total scores of the MSI and SPQ-B was strong (rho = 0.616, P < 0.005). Items of MSI reflecting disrupted relatedness and affective dysregulation correlated moderately (rφ varied between 0.2 and 0.4, P < 0.005) with items of SPQ. Items of MSI reflecting behavioural dysregulation correlated only weakly with items of SPQ. In MRA, depressive symptoms, sex and MSI were significant predictors of SPQ-B score, whereas symptoms of anxiety, age and SPQ-B were significant predictors of MSI score.

Items reflecting cognitive-perceptual distortions and affective symptoms of BPD appear to overlap with disorganized and cognitive-perceptual symptoms of SPD. Symptoms of depression may aggravate self-reported features of SPQ-B, and symptoms of anxiety features of MSI. Symptoms of behavioural dysregulation of BPD and interpersonal deficits of SPQ appear to be non-overlapping.

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.