δEPCD: the electrophysiologic coefficient of depressiveness

• Published in: Biomarkers 2021-11, Vol.26 (8), p.752-759
• Main Authors: Bou Khalil, Rami, El Khoury, Rhéa
• Format: Article
• Language: English
• Subjects: biological markers; Biomarkers - blood; Depression; Depressive Disorder, Major - blood; Depressive Disorder, Major - diagnosis; Depressive Disorder, Major - physiopathology; Heart Rate - physiology; heart rate variability; Humans; Kynurenine - blood; Kynurenine pathway; Quinolinic Acid - blood; Reproducibility of Results; Sensitivity and Specificity; Stress, Physiological - physiology; Tryptophan - blood; tryptophan metabolism
• ISSN: 1354-750X; 1366-5804
• DOI: 10.1080/1354750X.2021.1995497

Despite research advances, recently identified biological markers for depression are either non-specific or impractical in daily clinical practice. Hence, we aim to identify a novel biomarker: δ EPCD , the electrophysiologic coefficient of depressiveness. δ EPCD must be sensitive and specific to the vulnerability towards depression. It should also detect the presence of a depressive clinical state and be able to quantify its severity. Moreover, it should be easily accessible and cost-effective. Accordingly, combining high-frequency heart rate variability (HF-HRV), which reflects a reduction in vagal tone, and tryptophan metabolism, which influences serotonin synthesis pathway, may have a good diagnostic and prognostic accuracy in depression. δ EPCD is the multiplication of the intrinsic difference between state 0 (rest) and state 1 (exposure to stress) of HF-HRV and the plasma concentration ratio between quinolinic acid and kynurenine. δ EPCD theoretically fluctuates between −1000 and 0 where being closer to 0 signifies no vulnerability to depression. Individuals with a score between −16.7 and −167 have a high vulnerability to depression. Finally, individuals with a δ EPCD closer to −1000 have the most severe forms of depression. δ EPCD is theoretically conceived to be easy to assess and monitor which makes it a candidate for further evaluation of reliability and validity. CLINICAL SIGNIFICANCE Depression is currently diagnosed based on emotional and behavioural symptoms; however there is currently a rising interest in the field of neurobiological markers that could improve diagnostic accuracy. Many current biological approaches are primarily based on single neurobiological markers that are either non-specific or impractical in daily clinical practice. Among other neurological effects, depression may modify the parasympathetic nervous system tone and disturb the tryptophan metabolism. The electrophysiological coefficient of depressiveness δ EPCD combines heart rate variability (HRV) and tryptophan metabolism to reflect the intrinsic individual vulnerability towards depression and the inherent severity of an index depressive disorder. δ EPCD is the intrinsic difference between state 0 (without stress) and state 1 (exposed to a stressful task) of the high-frequency heart rate variability multiplied by the intrinsic difference between both states, e.g. state 0 and 1, of the plasma concentration ratio of quinolinic acid over kynurenine.