The Index of Number of Events and Severity (INES): A Novel Instrument for Quantifying Lifetime Illness Burden in Bipolar Disorder

Bipolar disorder (BD) is a chronic psychiatric condition affecting approximately 1-2% of the population, characterized by depressive and manic episodes. BD comprises two main subtypes, defined by the presence of mania (BD-I) or hypomania (BD-II). Commonly used clinical scales, including the Global Assessment of Functioning (GAF), Clinical Global Impressions (CGI), and World Health Organization Disability Assessment Schedule (WHODAS), assess functional impairment at the time of evaluation. However, they may not adequately capture cumulative lifetime illness burden or provide a retrospective measure of clinical severity. Here, we introduce the Index of Number of Events and Severity (INES), a novel instrument designed to quantify longitudinal illness-course severity in BD by integrating cumulative clinical events with illness duration. INES incorporates psychosis and rapid cycling as dichotomous variables and quantifies hospitalizations, suicide attempts, and affective episodes as discrete categories. INES was evaluated in 307 individuals from the MadManic cohort. It correlated moderately with GAF and CGI, while its strongest association was observed with WHODAS (r=0.347). Factor analysis over the four scales supported a two-factor structure, where INES loaded alongside WHODAS, capturing the variability of structured instruments. Linear modelling indicated that traditional scales explained only 14.4% of the variance of INES, suggesting that this scale captures clinical information largely unaccounted by the other instruments. INES was the only to differentiate between BD subtypes, with higher severity observed in individuals with BD-I. These findings support INES as a reproducible tool for capturing cumulative lifetime severity in BD, with potential utility in clinical and genetic studies.