What labs are indicated for suspected meningitis?

Laboratory Tests for Suspected Meningitis

In patients with suspected meningitis, a lumbar puncture (LP) with cerebrospinal fluid (CSF) analysis is the cornerstone diagnostic test, which should be performed within 1 hour of hospital arrival if no contraindications exist. 1, 2

Initial Blood Tests

• Blood cultures should be obtained within the first hour of hospital arrival, prior to antibiotic administration whenever possible 1
• Complete blood count, electrolytes, urea, creatinine, liver function tests, and coagulation profile 1
• Blood glucose (essential for comparison with CSF glucose) 1
• Lactate measurement 1
• Procalcitonin (if available) 1
• Pneumococcal and meningococcal PCR (EDTA sample) 1
• Storage sample for potential serological testing if a cause is not identified 1

Cerebrospinal Fluid Analysis

Basic CSF Tests

• CSF opening pressure (unless LP is performed in sitting position) 1
• CSF cell count and differential 1
• CSF glucose with concurrent plasma glucose 1
• CSF protein 1
• Gram stain (sensitivity 60-90%, specificity 97-100%) 1, 2
• CSF culture (gold standard for bacterial meningitis diagnosis) 1

Additional CSF Tests Based on Clinical Suspicion

• CSF PCR for bacterial pathogens (especially valuable if antibiotics given before LP) 1
• CSF lactate (levels ≥31.53 mg/dL or ≥3.5 mmol/L strongly suggest bacterial meningitis) 3
• Cryptococcal antigen, fungal stains/cultures if immunocompromised or clinically indicated 1
• Viral PCR (HSV, enterovirus, etc.) based on clinical presentation 1
• Storage of additional CSF for potential further testing 1

Timing of Lumbar Puncture

• LP should be performed within 1 hour of hospital arrival if safe to do so 1, 2
• If LP is delayed due to need for neuroimaging or other reasons, blood cultures should be obtained and antibiotics administered immediately 1
• Even after antibiotics have been started, LP should still be performed within 4 hours as culture yield remains relatively high during this window (73% positive if LP within 4 hours vs. 11% if later) 4

Contraindications to Immediate LP

• Focal neurological signs 1, 2
• Presence of papilledema 1, 2
• Continuous or uncontrolled seizures 1, 2
• Glasgow Coma Scale ≤12 1, 2
• Signs of shock or severe sepsis (LP should be deferred until patient is stabilized) 1
• Rapidly evolving rash suggestive of meningococcemia 1

Interpretation of CSF Findings

Bacterial Meningitis Typical Findings

• Elevated opening pressure 1
• CSF WBC count >1000/μL (predominantly neutrophils) 1
• CSF protein >220 mg/dL 1
• CSF glucose <35 mg/dL 1
• CSF:blood glucose ratio <0.23 1

Pitfalls in CSF Interpretation

• Normal CSF WBC does not completely exclude bacterial meningitis (rare cases occur without pleocytosis) 5
• Partially treated bacterial meningitis may show a lymphocytic predominance 1
• Early viral meningitis can sometimes show neutrophil predominance 1
• Traumatic LP can falsely elevate WBC count (correction factor of 1:1000 for RBC:WBC can be applied) 1

Additional Considerations

• Nasopharyngeal swabs should be obtained in suspected meningococcal disease to attempt to grow the organism for surveillance and vaccine coverage determination 1
• If meningitis is suspected but LP is contraindicated or delayed, empiric antibiotics should be started immediately after blood cultures are obtained 1
• Blood cultures are positive in approximately 86% of pediatric bacterial meningitis cases and should not be overlooked as a diagnostic tool 6
• In patients with intracranial devices (e.g., ventriculostomy), CSF should be obtained from the reservoir and the catheter tip cultured if removed 1

LP Technique to Reduce Post-LP Headache

• Consider using atraumatic (non-traumatic) needles 1, 3
• Reinsert the stylet before removing the needle 1, 3
• Patient positioning and post-LP bed rest have not been proven to reduce headache risk 1, 3