Psychometric Properties and Diagnostic Criteria of the Scale for Positive Symptoms Assessment - Saps in Patients With Schizophrenia

Abstract

Objectives. To assess psychometric properties of Scale for the Assessment of Positive Symptoms (SAPS) and to develop criteria for assessing the severity of positive symptoms in the dynamics of therapy, to clinically verify SAPS metric criteria. Material and methods. The study involved 157 patients with the diagnosis of schizophrenia. Psychometric analysis of SAPS was carried out on the basis of the Rasch metric system with the development of criteria for assessing the severity of positive symptoms based on the index of minimum detectable changes for 95% probability (Minimum detectable change 95% - MDC95%). Clinical verification was conducted by statistical analysis of contingency tables of clinical and metric criteria using the Clinical Global Impression Scale (CGI). Results. All items of SAPS have reasonable level of construct validity. The scale is characterized by moderate reliability (r= 0.84) and high discrimination. Factor analysis has revealed heterogeneity of SAPS metric structure: items in the first group assess hallucinatory and delusional symptoms, items in the second group characterize behavioral disorders and formal thinking disorders. The heterogeneity of SAPS reflects dimensional structure of positive syndrome, which allows to consider total score on the scale as an indicator of this psychopathological construct severity. On the basis of metric approach, criteria for assessing the severity of positive symptoms in the dynamics of therapy have been developed. Statistically significant linear relationship has been found between the levels of SAPS and CGI scales: SAPS has shown higher discriminativeness in assessing severe levels and less marked discriminativeness in assessing low levels of positive symptoms compared to CGI. Conclusions. SAPS has acceptable psychometric characteristics. The developed diagnostic criteria are suitable for the use in clinical practice and research.