The functional relevance of a short assessment of formal thought disorder in psychosis

Background Formal thought disorder (FTD) is a highly disabling transdiagnostic feature that impedes communication and social ties. Progress in understanding and treating FTD has been hampered by the uncertainties in its assessment. Aims We examined if a short 3–5min assessment of transcribed speech can capture the latent dimensions and network structure of FTD and predict functional outcomes. Method In a transdiagnostic sample ( N = 666) with a single longitudinal follow-up over 3–12 months ( n = 244), we administered the short form of the Thought and Language Index to measure eight individual features of FTD. We determined the baseline factor structure of FTD, its temporal invariance at follow-up, and the predictive validity of FTD dimensions on the global single-item Social and Occupational Functioning Assessment Scale scores at baseline and follow-up. We identified the most influential and putative primary phenomena within the FTD syndrome, using network analysis. Results Factor analyses revealed a stable three-factor model of FTD: impoverishment (poverty of speech, weakening of goal), loosening (looseness, illogicality) and peculiarities (peculiar words, peculiar sentences), with excellent fit (Comparative Fit Index: 0.997, root mean square error of approximation: 0.040) and metric invariance over time. Impoverishment and peculiarities predicted functioning at baseline and 3–12 months later (cross-sectional: β = –0.196, p < 0.001 and β = –0.298, p = 0.001, respectively; longitudinal: β = –0.201, p = 0.037 and β = –0.336, p = 0.042, respectively). Looseness and poverty of speech were putative primary features influencing other FTD phenomena. Weakening of goal and peculiar sentences were the most connected phenomena. Conclusions By integrating latent variable and network approaches, we provide a unified, empirically grounded framework to interpret FTD assessed using a brief speech task. We report a replicable three-dimensional structure, identify central symptoms that may maintain the FTD syndrome, and the specific dimensions that influence functional disability. These findings clarify the prognostically valuable features of FTD for future mechanistic and interventional research.