Formal Thought Disorder and Functional Outcome in a Community Sample of Patients with Schizophrenia Spectrum Psychosis

Formal thought disorder (FTD) and impaired psychosocial functioning are hallmarks of schizophrenia, yet the relation between these two features of the illness is poorly understood. This study had two main objectives: (1) to examine a performance-based measure of FTD, the Thought Disorder Index (TDI), as a predictor of functional outcome, and (2) to examine the incremental validity of the TDI over interview-based measures of FTD. Adults with schizophrenia (n=40) completed measures of social and role functioning, clinical symptoms (negative/ positive symptoms, duration of illness), and cognition (estimated IQ, verbal fluency). FTD was measured using the TDI, the Scale for the Assessment of Positive Symptoms global FTD score, and the Brief Psychiatric Rating Scale (BPRS) version 4.0 Conceptual Disorganization item. Although the Total TDI score was a significant predictor of role functioning after accounting for clinical symptoms, it did not incrementally predict functional outcome compared with interview-based measures. Among the FTD measures, the BPRS Conceptual Disorganization item was the best predictor of psychosocial functioning, after considering the relative costs (subject burden, investigator training) of each of these measures. The effects of both negative symptoms and FTD were enhanced when used in combination to predict functional outcome. These results support the validity of FTD in improving understanding of functional deficits in schizophrenia and indicate that measures less labor intensive than the TDI are adequate for this purpose.