Adjustment disorder (AD) is characterized by abnormal distress or functional impairment in reaction to a stressor of unspecified intensity or duration. Though symptoms must be clinically significant to warrant an AD diagnosis, it is a subthreshold, temporally circumscribed diagnosis with a good prognosis if the stressor is not persistent (American Psychiatric Association, 2013; O’Donnell et al., 2019). However, the diagnostic criteria for AD, as characterized in the Diagnostic and Statistical Manual of Mental Disorders (5th ed.; DSM-5; American Psychiatric Association, 2013), are vague (O’Donnell et al., 2019; Strain, 2019), and the disease course is not always predictable. If the stressor or its consequences are chronic and the AD symptoms last for more than six months, then the disorder is defined as chronic (DSM-IV-TR; American Psychiatric Association, 2000) or persistent (DSM-5; American Psychiatric Association, 2013). Its status as a subthreshold disorder—it should not be diagnosed if the stress-related disturbance meets criteria for another disorder—may contribute to poor diagnostic stability. This may occur, for example, if time criteria are subsequently met for another disorder, and that diagnosis replaces the AD diagnosis (Casey, 2018). In fact, AD has been described as a gateway to potentially more severe disorders, such as posttraumatic stress disorder (PTSD), anxiety, and depression (O'Donnell et al., 2016; O'Donnell et al., 2016; O'Donnell et al., 2016; O’Donnell et al., 2019). The diagnostic course of AD and its association with occupational and other functional outcomes is not well understood. A better understanding of factors that predict a worsening course of AD and its diagnostic and functional outcomes is important for treatment planning as well as informing policy decisions and evaluating disease burden.

Only a few studies have looked at the diagnostic course of AD to better understand long-term mental health outcomes. Andreasen and Hoenk (1982) found that 21 % of adults with AD at baseline had a mental health disorder at five year follow-up, and Lorenz et al. (2018) found that a substantial proportion of patients with AD at baseline continued to have medium or high symptom severity up to 15 months following the precipitating stressor. A study of Danish military conscripts found that nearly a quarter of those diagnosed with AD at baseline had AD or another mental health disorder at 10 year follow-up (Hageman et al., 2008). A more recent longitudinal study also suggests that AD may lead to worse outcomes than previously believed. O'Donnell et al. (2016) found that 56 % of study participants who had an AD diagnosis three months following a stressful event continued to have a mental health diagnosis nine months later.

Very little is known regarding AD prognostic factors. For hospital in-patients diagnosed with AD, baseline sociodemographic variables (age, sex, marital status, and race) did not predict differing readmission rates (Jones et al., 2002). In a five-year follow-up study of adults with AD at baseline, male sex and greater behavioral symptoms at baseline predicted mental health issues at follow-up, while a number of other variables (e.g., age, social class, symptom duration) did not (Andreasen and Hoenk, 1982). Studies examining predictors of AD disease course or associated functional impairment are needed.

As with many mental health disorders, AD may be characterized by difficulties with occupational, social, or other areas of functioning (American Psychiatric Association, 2013), and little is known about the long-term functional impact of AD. A study of civilians admitted to the hospital following an accident found that the severity of functional impairment associated with AD lay between that of other mental health disorders and no mental health disorder (O'Donnell et al., 2016). In the military, AD is the most commonly diagnosed mental health disorder (Stahlman and Oetting, 2018; Taal et al., 2014; Armed Forces Health Surveillance Branch, 2020). In the US Armed Forces, AD accounted for 30.8 % of incident mental health diagnoses, more than for posttraumatic stress disorder (7.7 %), anxiety disorders (16.7 %), and depressive disorders (16.5 %), in the period of 2016 to 2020 (Armed Forces Health Surveillance Division, 2021). Within this population, separation from service has been used as a measure of functional impairment (Hoge et al., 2002, Hoge et al., 2005). Several studies have shown that mental health disorders increase risk of military separation (Creamer et al., 2006; Hoge et al., 2005, Hoge et al., 2006; Garvey Wilson et al., 2009). Studies addressing AD show a mixed impact on functional impairment. High rates of medical evacuees from the battlefield were diagnosed with AD, but the majority of mental health evacuees were returned to duty (Rundell, 2006; Williams et al., 2017). Similarly, AD was prevalent among Army aviation personnel with mental health disorders but was more likely to be associated with return to duty than were other mental health diagnoses (Britt et al., 2018). And while AD was a prevalent mental health diagnosis in military trainees recommended for military separation, it was also strongly associated with return to duty (Cigrang et al., 1998; Englert et al., 2003).

A recent analysis of gaps in published and ongoing AD research (Morgan et al., 2021) revealed a dearth of research examining the longitudinal course of AD. The purpose of the current study was to provide an overview of the course of AD in active duty service members (SM) without a prior mental health history. Specifically, our primary aim was to examine the diagnostic transition from an incident AD diagnosis (IADx) to one of three diagnostic states at 15 months after IADx: 1) another mental health disorder, 2) persistent AD, or 3) no mental health diagnosis recorded. Secondary aims were to examine potential predictors of these follow-up diagnostic states, and to assess associated functional impairment as measured by separation from service. In keeping with AD as a subthreshold, temporally circumscribed diagnosis with a good prognosis (American Psychiatric Association, 2013), we anticipated that the majority of SM would not have a mental health diagnosis at follow-up. Because previous studies have shown that patients with ADs have a level of severity (e.g., quality of life, health care utilization, disability, anxiety and depression symptoms) in between that of other mental health disorders and no mental health disorder (Fernández et al., 2012; Jones et al., 2002; O'Donnell et al., 2016), we anticipated a similar pattern regarding functional impairment, with chronic AD lying between those who subsequently received another mental health diagnosis and those whose mental health diagnostic status resolved.