Diagnostic- and age-based measurement invariance of irritability in youths.

Irritability is one of the most common transdiagnostic symptoms. It spans across mood, anxiety, and other disorder classes and is listed as a top problem in outpatient settings. Irritable mood is a symptom for at least 10 disorders in the DSM-5, and it is also developmentally normal across the lifespan. Given this background, we investigated how irritability maps across disorders and ages of children and adolescents. Participants (N = 186) ages 8-16 were enrolled in two ongoing IRB-approved studies investigating the nature of irritability in a transdiagnostic sample of children and adolescents. Participants completed both a child and parent version of the Affective Reactivity Index (ARI-C and ARI-P, respectively) and the Kiddie Schedule for Affective Disorders and Schizophrenia (KSADS) was administered by a trained clinician. Multiple indicators multiple causes (MIMIC) modeling was used to assess for differential item functioning among items in the ARI-P/C compared to diagnoses of major depressive disorder (MDD), generalized anxiety disorder (GAD), oppositional defiant disorder (ODD), and attention-deficit hyperactive disorder (ADHD). ARI-P showed differential item functioning (DIF) with MDD, GAD, and ADHD and participant age. ARI-C showed DIF with ADHD. However, the DIF effect sizes were small. These results indicate child and parent reports of child irritability manifest similarly regardless of specific diagnosis or age. Further understanding of how irritability presents itself across disorders and the lifespan can allow for improved assessment and treatment.