CSF lactic acid levels are also useful in assessing the significance of RBCs in the CSF in
patients with a decreased CSF glucose. If the diagnosis is between HSV-1 andL. monocytogenes
meningitis, inL. monocytogenesmeningoencephalitis, CSF lactic acid levels will be highly
elevated, i.e.,6 mmol/dL, whereas the CSF lactic acid levels will be normal/near normal in
HSV-1. A normal CSF lactic acid level in the absence of RBCs from a trauma or traumatic tap is
the best way to differentiate aseptic from septic meningitis. If the Gram stain is negative and CSF
lactic acid levels are normal, then the clinician can confidently wait for CSF cultures to be
reported as negative during the next one to three days. No empiric antimicrobial therapy is
needed if the CSF lactic acid level is normal and the CSF Gram stain is normal. CSF lactic acid
levels may be obtained serially to determine if antimicrobial therapy of the meningitis is
effective, and also may be used at the end of therapy as a test of cure (1,30–34) (Fig. 1).
Other CSF Tests (CRP, PCT, LDH)
Another test that has been used to differentiate aseptic/viral meningitis from ABM is
C-reactive protein (CRP). CSF CRP is elevated in bacterial meningitis but is not as highly
elevated in viral/aseptic meningitis. CSF procalcitonin (PCT) levels are highly elevated in
ABM but not in aseptic/viral meningitis (35).
Other CSF parameters have been used, i.e., lactate dehydrogenase (LDH) to differentiate
the various types of meningeal pathogens, but lack sensitivity and specificity. The CSF antigen
tests, i.e., counter immunoelectrophoresis (CIE) techniques of the CSF are generally unhelpful.
The problems with the CSF CIE assays are lack of sensitivity and specificity. When a CNS
pathogen is demonstrated by Gram stain and culture and there is no doubt about the
diagnosis, the CIE is not infrequently negative (1,3–5,8).
Other tests are useful in selected CNS disorders. The CSF C 4 level is decreased and
diagnostic of SLE meningitis/ cerebritis although clonal bands in the CSF may be present in
SLE as well as multiple sclerosis. Cytology of the CSF may indicate meningeal carcinomatosis,
which may mimic ABM (1,19–21,28,29).
Other tests are useful in the CSF for selected pathogens. PCR technique is useful to make
the diagnosis of enteroviral meningitis, HSV-1/2 and HHV-6 aseptic meningitis. PCR is also
useful to diagnose acute TB meningitis (1,5,25,27,36).
Serum Tests (CRP, PCT, and Ferritin Levels)
The serum CRP has also been useful to differentiate ABM from aseptic/viral meningitis. In
ABM, serum CRP levels are higher than in viral/aseptic meningitis. Similarly, serum PCT
levels are more highly elevated than viral/aseptic meningitis versus ABM (35,37,38).
Highly elevated serum ferritin levels appear to be a marker for West Nile encephalitis
(WNE). In WNE, serum ferritin levels are highly elevated but are unelevated/minimally
elevated in aseptic/viral and bacterial meningitis (39,40).
Figure 1 Temporal relationships of CSF lactic acid levels in bacterial meningitis.Source: adapted from Ref. 31.
Meningitis and Its Mimics in Critical Care 147