Biomarkers in respiratory tract infections: diagnostic guides to antibiotic prescription, prognostic markers and mediators.

Used appropriately, biomarkers improve the assessment of respiratory tract infections and sepsis. Most prominently, circulating procalcitonin levels increase by a factor of several tens of thousands during sepsis. Using a sensitive assay, procalcitonin safely and markedly reduces antibiotic usage in respiratory tract infections and nonbacterial meningitis. Procalcitonin is the protopye of hormokine mediators. The term "hormokine" encompasses the cytokine-like behaviour of hormones during inflammation and infections. The concept is based on a ubiquitous expression of calcitonin peptides during sepsis. Adrenomedullin, another member of the calcitonin peptide superfamily, was shown to complement and improve the current prognostic assessment in lower respiratory tract infections. Other peptides share some features of hormokines, e.g. natriuretic peptide and copeptin. Hormokines are not only biomarkers of infection but are also pivotal inflammatory mediators. Like all mediators, their role during systemic infections is basically beneficial, possibly to combat invading microbes. However, at increased levels they can become harmful for their host. Multiple mechanisms of action were proposed. In several animal models the modulation and neutralisation of hormokines during infection was shown to improve survival, and thus might open new treatment options for severe infections, especially of the respiratory tract.