Diagnostic Approach to Bacterial Meningitis

CSF Parameters and Diagnostic Tests

The typical CSF picture in bacterial meningitis includes pleocytosis with predominantly polymorphonuclear leukocytes, low glucose concentration, low CSF-to-blood glucose ratio, and elevated protein levels 1
CSF lactate is often elevated in bacterial meningitis and has better diagnostic accuracy than CSF white blood cell count in differentiating bacterial from other types of meningitis 1
No single CSF test is fully reliable in distinguishing bacterial meningitis from other forms of meningitis, and results should be interpreted in the clinical context 1

Multiple diagnostic algorithms have been developed to help differentiate between bacterial and viral meningitis, but none are 100% sensitive 2
CSF culture remains the gold standard for diagnosis of bacterial meningitis, with positive results in 70-85% of patients who have not received prior antimicrobial therapy 3
CSF Gram stain permits rapid identification of the causative bacterium in 60-90% of patients with community-acquired bacterial meningitis 3

Prior antibiotic treatment may modify CSF findings, though CSF WBC count may not be greatly affected by antibiotics given before lumbar puncture 1
If bacterial meningitis is suspected and lumbar puncture is delayed for any reason, empirical antibiotic therapy should be started immediately after blood cultures are obtained 1
In immunocompromised patients, normal CSF parameters do not rule out meningitis, and a high index of suspicion should be maintained until cultures are final 1