Tricuspid regurgitation: standardized stepwise work-up from referral to expert heart valve centre.

Major depressive disorder (MDD) is a common disease with an estimated global prevalence of 4.4% []. Although candidate markers related to pathological hypotheses, such as excitation/inhibition disturbances, monoaminergic systems, inflammation, neuroimaging, hypothalamic–pituitary–adrenal axis, blood metabolome, and gut microbiota, have been investigated [–], establishing practical biomarkers remains challenging owing to the elusive mechanisms of MDD. Thus, there is a need to identify biomarkers of MDD that can be used in daily clinical settings. Considering the high prevalence of MDD, there is an urgent need to establish functional biomarkers of MDD for diagnosis, state evaluation, treatment response, and subtyping to facilitate a novel biology-oriented classification for psychiatric disorders [] or endophenotype-oriented translational research []. Our study group was the first to report the significant reduction of cerebrospinal fluid (CSF) ethanolamine (EA) levels in patients with MDD compared with healthy controls []. Remarkably, CSF EA levels were associated with depression severity (i.e., depressed, but not remitted, patients with MDD had lower CSF EA levels than healthy controls), suggesting CSF EA as a potential state-dependent marker for MDD or its subtype). EA (CHNO; also known as monoethanolamine, 2-aminoethanol, or 2-aminoethyl alcohol) is a metabolite that exists in all living species, from bacteria to humans []. It belongs to the class of ethanolamines (including di- and triethanolamines) and is composed of ethane with an amino and a hydroxy substituent []. EA is essential for life as a crucial structural component of cells, a substrate for enzymatic reactions, and a precursor for the production of bioactive molecules [–]. It is distributed in every cell in the human body as the head group of phosphatidylethanolamine and other lipids [], including bioactive lipid mediators []. EA plasmalogens are essential components of mammalian cellular membranes [], and EA is also present as free EA at varying concentrations in bodily fluids [].