Elevated CSF Lactate levels reflect CNS anaerobic metabolism due to tissue hypoxia. Lactate measurement is used as an adjunctive test in differentiating viral meningitis from bacterial , mycoplasma, fungal & tubercular meningitis in which routine parameters yield equivocal results. In adult patients with viral meningitis, lactate levels are usually < 25 mg/dL and almost always < 35 mg/dL. Bacterial meningitis shows levels > 35 mg/dL. Persistently elevated ventricular CSF lactate levels are associated with a poor prognosis in patients with severe head injury.