[The Basel Interview for Psychosis (BIP): structure, reliability and validity]

Background: Although several instruments have been developed to identify patients with an at-risk mental state (ARMS) for psychosis and first episode of psychosis (FEP), up to now there were no instruments for a detailed assessment of risk factors and indicators of emerging psychosis and the temporal development of psychiatric symptoms over the whole life span in these patients. We therefore developed the Basle Interview for Psychosis (BIP). The aim of this study is to describe the development of the BIP and to report about its psychometric properties.

Methods: The BIP is a comprehensive semi-structured interview that was developed for the Basel early detection of psychoses (FePsy) study. Its items were derived from the most important risk factors and indicators of psychosis described in the literature and from several existing instruments. It contains the following six sections: 1) social and physical development and family, 2) signs and symptoms, 3) vulnerability, 4) help-seeking behavior, 5) illness insight, 6) evaluation of the interview. To estimate the inter-rater reliabilities of the items of sections 2 and 3, 20 interviews were conducted and rated by 8 well-trained raters. The factorial structure of the BIP section "signs and symptoms" was explored in a sample of 120 ARMS and 77 FEP patients. On the basis of the discovered factorial structure, we created new subscales and assessed their reliabilities and validities.

Results: Of the 153 studied items of sections 2 and 3, 150 (98 %) were rated with sufficiently high agreement (inter-rater reliability > 0.4). The items of section "signs and symptoms" could be grouped into 5 subscales with predominantly good to very good internal consistencies, homogeneities, and discriminant and convergent validities. Predictive validities could be demonstrated for the subscales "Positive Psychotic Symptoms", "Disturbance of Thinking" and the total score.

Discussion: The BIP is the first interview for comprehensively assessing risk factors and indicators of emerging psychosis and the temporal development of psychiatric symptoms over the whole life span, which has been validated in ARMS and FEP patients. We could show that the BIP has excellent psychometric properties.