Measuring and Characterizing Psychiatric Multimorbidity Among Individuals with Mental Illness.

Psychiatric multimorbidity, defined as the co-occurrence of two or more psychiatric diagnoses, is increasingly recognized as a marker of clinical complexity. While prior research has examined psychiatric comorbidity in selected populations or via structured diagnostic interviews, less is known about the rates of psychiatric multimorbidity as documented in administrative claims. To address this gap, this study characterizes psychiatric multimorbidity within a statewide insured population. We conducted a cross-sectional analysis of the Massachusetts All-Payer Claims Database (2016-2018) including adults aged 21-64 years with at least one psychiatric diagnosis and insured by Medicaid or commercial insurance. Psychiatric disorders were identified using ICD-10-CM codes grouped into diagnostic categories, with complete code listings provided in supplementary material. Psychiatric multimorbidity was defined as ≥ 2 distinct psychiatric diagnoses documented within a calendar year. Substance use disorder diagnoses were unavailable due to APCD redaction policies and were excluded. Logistic regression models were used to examine associations of psychiatric multimorbidity with patient characteristics. The sample included 2,409,794 person-year observations representing 653,413 unique people with mental health disorders. 68.8% of the population had only one mental health disorder, 17.4% had two mental health disorders, and 13.8% had three or more mental health disorders. In multivariable analyses, females had higher odds of psychiatric multimorbidity compared to males (adjusted Odds Ratio (aOR) = 1.23, 95% CI = 1.22,1.24), and those with any medical comorbidity had higher odds of psychiatric multimorbidity compared to people without a medical comorbidity (aOR = 1.37, 95% CI = 1.36,1.38). Medicaid enrollment was associated with higher odds of psychiatric multimorbidity compared to those with commercial insurance (aOR = 1.25, 95% CI = 1.24,1.25). Those with serious mental illness had nearly nine times the odds of having psychiatric multimorbidity (aOR = 8.92, 95% CI = 8.84,8.99) compared to people without serious mental illness. This study extends prior population-based and specialty-care research by examining psychiatric multimorbidity in a statewide insured population spanning both public and private payers. In this sample, one in three individuals with a mental illness diagnosis had psychiatric multimorbidity. Findings underscore the complexity of psychiatric presentations in clinical care, highlight greater multimorbidity among Medicaid enrollees, and point to the need for integrated and patient-centered treatment approaches. Because substance use disorders were excluded, findings likely represent conservative estimates of psychiatric multimorbidity.