Estimating Cognitive Impairment in Bipolar Disorder: Should We Account for Premorbid <scp>IQ</scp> ?

ABSTRACT Introduction Cognitive impairment estimations traditionally rely on the deviation of current cognitive performance from population norms (normative approach). Since bipolar disorder (BD) has been associated with above‐average premorbid cognitive functioning in some studies, this approach might underestimate the extent of cognitive impairment in a proportion of patients. We examined whether intraindividual deviation of cognitive performance from premorbid estimates (idiographic approach) would more accurately estimate impairment in BD and assessed the functional relevance of this approach. Methods This study pooled euthymic patients with BD ( N = 257) taking part in two cognitive remediation trials with comparable eligibility criteria and measures. All participants underwent a baseline assessment including measures of current cognition, premorbid IQ, psychosocial functioning and mood symptom severity. We estimated and compared the prevalence of normative versus idiographic impairment and examined the association of the idiographic approach with functioning. Results According to normative standards, 13%–31% of our euthymic sample was classified as cognitively impaired depending on the adopted cut‐off. After accounting for premorbid IQ, idiographic impairment rates increased to 37%–64%. The difference between the two approaches was significant for each cut‐off considered (0.5 and 1 SD). Idiographic cognitive impairment was significantly associated with psychosocial functioning, even after controlling for residual mood symptoms and demographic/clinical variables and functional impairment classification. Conclusion Common practices for characterising cognitive impairment using a normative approach may underestimate the extent of impairment for a substantial proportion of euthymic patients with BD. Considering idiographic impairment provides an alternative, functionally relevant approach for cognition trials and clinical practice.