Abstract
BACKGROUND: Although widely employed in clinical research, no studies have evaluated the psychometric properties of the English version of the Borderline Symptom List–23 (BSL-23), including the equivalence of its mean score to that of the German version. Using Item Response Theory, we: a) examined the psychometric properties of the English BSL-23 when administered to treatment-seeking adults diagnosed with Borderline Personality Disorder (BPD); and b) tested if language-related bias needs to be considered when comparing or combining mean scores from the German and English versions.
METHOD: We evaluated the psychometric properties of the English BSL-23 by fitting a graded response model (GRM) to data from 321 treatment-seeking adults diagnosed with BPD. We used log-likelihood ratio tests to test for differential item functioning (DIF) between English and German items, fitting a multi-group GRM to data from the English sample and a sample of 327 German treatment-seeking adults diagnosed with BPD. We also tested for the cumulative effect of DIF on the mean score to assess potential bias in the mean score between the two versions.
RESULTS: The English BSL-23 mean score demonstrates high internal consistency(Cronbach’s α ≥ 0.90) across a wide range of BPD severity among treatment-seeking adults with BPD. Sensitivity of the English mean score to individual differences was greatest when BPD severity was mild to moderate. We found DIF at the item level but these effects cancelled out when items were summed, resulting in a positive bias in the mean score among English participants of no more than 0.10 (2.55%), possibly reflecting language differences.
CONCLUSION: The English BSL-23 mean score is highly reliable across a broad range of underlying BPD severity among treatment-seeking adults meeting 5 or more DSM-IV/DSM-5 criteria for BPD. Sensitivity is greatest when BPD severity is mild to moderate. Mean scores from the current versions of the English and German BSL-23, administered to treatment-seeking adults diagnosed with BPD, can be combined or compared without needing to adjust for language differences.
SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s12888-026-07863-y.