Sühs KW, et al. - Researchers investigated the regulation of tryptophan-kynurenine-NAD+ pathway in CNS infections considering its significance for improving the understanding of pathophysiology and end-organ damage. Concentrations of kynurenine (Kyn) and tryptophan (Trp) were determined in 220 cerebrospinal fluid samples from patients with bacterial and viral (herpes simplex, varicella zoster, enteroviruses) meningitis/encephalitis, neuroborreliosis, autoimmune neuroinflammation (anti-NMDA-R encephalitis, multiple sclerosis), and noninflamed controls (Bell’s palsy, normal pressure hydrocephalus, Tourette syndrome). Kyn concentrations were noted to be strongly correlated with CSF markers of neuroinflammation (leukocyte count, lactate, and blood-CSF-barrier dysfunction). These were highly raised in bacterial and viral CNS infections, but were low or undetectable in anti-NMDA-R encephalitis, multiple sclerosis, and controls. In viral CNS infections and neuroborreliosis, Trp was decreased mostly. Combinations of Kyn, Trp and Kyn/Trp ratio with leukocyte count or lactate was identified as accurate classifiers for the clinically important differentiation between neuroborreliosis, viral CNS infections, and autoimmune neuroinflammation.