Bacterial or viral meningitis? Measuring lactate in CSF can help you know quickly. Measurement of cerebrospinal fluid (CSF) lactate is a useful test when properly applied to the appropriate clinical situation and can offer information other CSF parameters cannot provide. Except for Gram's stain of CSF, lactate level is the most important CSF parameter in the early differentiation of viral (aseptic) and bacterial purulent meningitis. 