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Laboratory tests
If meningitis is suspected based on clinical examination, the patient should be given antibiotics. The next step is to perform laboratory tests on the blood and cerebrospinal fluid (CSF).
CSF is obtained by means of a lumbar puncture (LP). However, if the patient is at risk for elevated intracranial pressure, lumbar puncture may be contraindicated because of the possibility of fatal brain herniation. In such cases a CT or MRI scan should be performed prior to the lumbar puncture in order to make sure there are no large masses compressing the brain. Patients at risk for elevated intracranial pressure include those with recent head trauma, the immunocompromised, those with a known CNS neoplasm, or focal neurologic deficits such as papilledema or altered consciousness. Otherwise, the CT or MRI should be performed after the LP, with MRI preferred over CT due to its superiority in demonstrating areas of cerebral edema, ischemia, and meningeal inflammation.
The opening pressure is noted during the LP and the CSF sent for examination of white blood cell, red blood cell, glucose, protein, Gram stain, culture, and possibly latex agglutination test, limulus lysates, or PCR for bacterial or viral DNA. If the patient is immunocompromised, the doctor may also consider testing the CSF for toxoplasmosis, Epstein-Barr virus, cytomegalovirus, JC virus and fungal infection.
CSF analysis in bacterial meningitis
* Opening pressure: > 180 mm H2O
* White Blood Cells: 10-10,000/uL with neutrophil predominance
* Glucose: < 40 mg/dL
* CSF glucose to serum glucose ratio: < 0.4
* Protein: > 4.5 mg/dL
* Gram stain: positive in >60%
* Culture: positive in >80%
* Latex agglutination: may be positive in meningitis due to Streptococcus pneumoniae, Neisseria meningitidis, Haemophilus influenzae, Escherichia coli, Group B Streptococci
* Limulus lysates: positive in Gram-negative meningitis
Cultures are often negative if CSF is taken after the administration of antibiotics. In these patients, PCR can be helpful in arriving at a diagnosis. It has been suggested that CSF cortisol measurement may be helpful.
CSF analysis in viral meningitis
* Appearance is clear
* Will contain lymphocytes (lymphocytosis)
* Normal level of protein
* Normal or increased level of glucose
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Important notice:
The content is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other
qualified health provider with any questions you may have regarding a medical condition.
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