Biological markers in clinical psychological research - A systematic framework applied to HPA axis regulation in PTSD

Biological markers, particularly endocrine measurements, are increasingly being integrated into clinical psychological research. We introduce a systematic framework that classifies different functions of such biomarkers. The framework distinguishes between diagnostic biomarkers which add a biological perspective to conventional clinical assessments, prognostic biomarkers that inform about an individual's risk to develop or maintain a mental health disorder, and intervention-related biomarkers. Regarding interventions, including prevention and treatment, it further distinguishes between prescriptive biomarkers which predict an individual's response to an intervention, outcome biomarkers which evaluate intervention-related changes on a biological level and indicators of change mechanisms. We demonstrate how to apply the framework by exemplarily classifying and describing previously published systematic reviews and primary empirical studies on endogenous, peripheral cortisol concentrations as a biomarker for posttraumatic stress disorder (PTSD). The evidence on cortisol's diagnostic and prognostic value is heterogeneous and still sparse regarding parameters based on multiple cortisol measurements, such as the cortisol awakening response. With regard to interventions, most research focused on trauma-focused psychotherapy and cortisol reactivity to trauma reminders. This field of research appears to be growing and very promising due to its potential to optimize PTSD-related interventions. The proposed framework can help in gaining a systematic overview of existing research. It can assist in structuring, comparing, summarizing and evaluating empirical studies, and in identifying research gaps.