Table 4.

Biomarkers studied in respiratory tract infections.

Biomarker	Description
C-reactive protein	Acute phase protein synthesized by hepatocytes
Procalcitonin	Prohormone of calcitonin which is induced by the activation and adherence of monocytes to the endothelial layer of blood vessels
Proadrenomedullin	Precursor for adrenomedullin which is involved in immuno-modulation
sTREM-1	Soluble triggering receptor expressed on myeloid cells -1 (sTREM-1) levels rise following an increase of TREM-1 expression on neutrophils, granulocytes, monocytes and alveolar macrophages. TREM-1 expression is increased by microbial products
Pentraxin-3	Acute inflammatory marker and a component of innate immunity
Neopterin	Produced in monocytes and macrophages. A marker of cell-mediated immunity. Levels rise in viral infections
Copeptin	Stable byproduct of vasopressin biosynthesis
Lipocalin-2	Protein involved in innate immunity. It limits bacterial growth by sequestering iron-containing siderophores
Syndecan-4	Receptor in intracellular signaling. Present in alveolar macrophages
Midregional proatrial natriuretic peptide (MR-proANP)	A byproduct of atrial natriuretic peptide (ANP) biosynthesis. ANP regulates macrophage activity in innate and acquired immunity