From: 2 Definitions, Terms, and Classification Systems for Co-Occurring Disorders

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Quadrant I: This quadrant includes individuals with low severity substance abuse and low severity mental disorders. These low severity individuals can be accommodated in intermediate outpatient settings of either mental health or chemical dependency programs, with consultation or collaboration between settings if needed. Alternatively, some individuals will be identified and managed in primary care settings with consultation from mental health and/or substance abuse treatment providers.
Quadrant II: This quadrant includes individuals with high severity mental disorders who are usually identified as priority clients within the mental health system and who also have low severity substance use disorders (e.g., substance dependence in remission or partial remission). These individuals ordinarily receive continuing care in the mental health system and are likely to be well served in a variety of intermediate level mental health programs using integrated case management.
Quadrant III: This quadrant includes individuals who have severe substance use disorders and low or moderate severity mental disorders. They are generally well accommodated in intermediate level substance abuse treatment programs. In some cases there is a need for coordination and collaboration with affiliated mental health programs to provide ongoing treatment of the mental disorders.
Quadrant IV: Quadrant IV is divided into two subgroups. One subgroup includes individuals with serious and persistent mental illness (SPMI) who also have severe and unstable substance use disorders. The other subgroup includes individuals with severe and unstable substance use disorders and severe and unstable behavioral health problems (e.g., violence, suicidality) who do not (yet) meet criteria for SPMI. These individuals require intensive, comprehensive, and integrated services for both their substance use and mental disorders. The locus of treatment can be specialized residential substance abuse treatment programs such as modified therapeutic communities in State hospitals, jails, or even in settings that provide acute care such as emergency rooms (see chapter 7 for an example in an emergency room setting).
From: 2 Definitions, Terms, and Classification Systems for Co-Occurring Disorders
NCBI Bookshelf. A service of the National Library of Medicine, National Institutes of Health.