Does change over time in delusional beliefs as measured with PDI predict change over time in belief flexibility measured with MADS?

Delusional beliefs and their behavioral consequences are predominant symptoms in patients with psychosis and play an important role in the treatment. Delusional beliefs are a multidimensional concept which can be divided into three components: distress, preoccupation and conviction of delusions. These can be measured using Peters delusions inventory (PDI-21). We question, whether changes in delusional beliefs over time during treatment measured with the PDI-21 can predict changes in belief flexibility measured with the Maudsley assessment of delusions schedule (MADS). We used a group of patients from a randomized controlled trial for a cognitive intervention for psychosis or psychotic symptoms. Aside standard treatment for psychosis, half of the patients took part in a group treatment “Michael’s game”. Patients were assessed at baseline (T1), at 3 months (T2), and at 9 months (T3). We measured delusional beliefs using PDI-21 and belief flexibility with the MADS. One hundred seventy-two patients were included in the analysis. We measured a main effect of PDI-21scores on belief flexibility measured with MADS. PDI-21 Conviction scores predicted outcomes for all measured MADS items. Increasing PDI Distress and Preoccupation scores were predictors for being more likely to dismiss beliefs and change conviction. Time itself was a predictor for changing conviction and being able to plan a behavioral experiment. Overall the changes in PDI scores predicted outcomes for belief flexibility measured with MADS items. The PDI-21 could be a simple and effective way to measure progress in treatment on delusional beliefs