Aseptic meningitis laboratory tests


 * Associate Editor-In-Chief:

Laboratory Tests
Usually the history and examination will arouse suspicion. Confirmation is mainly through CSF findings:
 * Less than 500 mononuclear cells/mm³ (pleocytosis) should develop with 8-48 hours
 * Normal glucose
 * Elevated pressure
 * Elevated protein
 * No findings which suggest another diagnosis - e.g. negative bacteria antigen tests, no lactate
 * PCR may identify a causative organism

Viruses may be cultured from swabs of other areas, such as the throat.

Blood tests are rarely helpful in establishing the diagnosis (but may be of use to establish baseline chemistry). Imaging is useful in excluding other diagnoses, or identifying other features of infection by an organsim - for example, a chest X-ray may be useful if tuberculosis is suspected.

Our main task is to identify bacterial or other treatable causes of meningitis as well as to decide if empiric antibiotic / antiviral therapy is necessary. Additionally, we should be able to identify benign etiologies in order to prevent unnecessary testing / treatment. The history should focus on exact details of disease onset and associated symptoms as well as past medical history (including immunosuppression), travel, exposure to animals, insects and their excrement, and HIV risk factors. One should also consider likely etiologies depending upon geography and time of year. Spanos et.al. developed a predictive model based on age, month, CSF/serum glucose ratio and the CSF poly count.
 * As rough guidelines go, AM tends to have a CSF WBC count < 500 (though > several thousand can be seen) with a mononuclear cell predominance. The protein is generally normal (can be high) and glucose is usually normal.
 * PMNs can predominate in the 1st 48h in up to 2/3 of patients.
 * Other lab tests, such as the C-reactive protein, the Limulus lysate assay, LDH etc. have been looked at, but are generally not recommended due to their poor sensitivity and specificity.
 * Unfortunately, there are no formal recommendations for which treatment for bacterial processes can be withheld and therefore Rx needs to be individualized.