Estimation of premorbid intelligence in schizophrenia: a systematic literature review

INTRODUCTION: Cognitive impairment is a commonly observed characteristic of people living with schizophrenia (PLWS). Adequate monitoring of intellectual decline and status in this population requires reliable estimates of premorbid IQ because objective IQ data predating schizophrenia onset are often unavailable. However, premorbid IQ estimation in PLWS is frequently complicated by confounding factors. The primary objective of this systematic review is to explore and compare extant approaches for estimating premorbid IQ in PLWS and, in doing so, identify factors that might inform the selection of the most reliable method for this clinical population. METHOD: A systematic literature search was conducted across PubMed, Web of Science Core Collection, EBSCOhost, SCOPUS, and The Cochrane Library electronic bibliographic databases. After deduplication, study selection was conducted in line with predefined inclusion and exclusion criteria. The quality assessment of studies was conducted using the Effective Public Health Practice Project (EPHPP) Quality Assessment Tool. RESULTS: 43 articles involving 8,012 participants with schizophrenia spectrum disorders were included. Premorbid IQ estimation methods such as the National Adult Reading Test (NART), Wide Range Achievement Test (WRAT), Test of Premorbid Functioning (ToPF), Wechsler Test of Adult Reading (WTAR), Oklahoma Premorbid Intelligence Estimate (OPIE), demographic regression algorithms, and "hold" test approaches based on subtests from the Wechsler Adult Intelligence Scale (WAIS) were employed. Studies comparing these methods report inconsistent levels of agreement, with differences associated with factors such as schizophrenia chronicity, age of onset, and years of education. CONCLUSIONS: A range of measures are available for estimating premorbid IQ in people living with schizophrenia. However, their precision is limited by several factors identified in this review. We urge researchers and clinicians to recognize these limitations. We have created an evidence-based guide to support the selection of the most optimal estimation method for premorbid IQ in PLWS.