What laboratory tests and treatment are recommended for a patient suspected of having meningitis?

Laboratory Evaluation and Treatment for Suspected Meningitis

All patients with suspected meningitis require immediate blood cultures followed by empiric antibiotics within 1 hour of hospital arrival, with lumbar puncture performed immediately if no contraindications exist, or after CT imaging if high-risk features are present. 1, 2

Critical Time-Sensitive Actions

First Hour Priorities

• Obtain blood cultures before antibiotics but do not delay antibiotic administration beyond 1 hour of hospital arrival 1, 2
• Assess for CT scan indications before lumbar puncture, including: immunocompromised state (HIV, transplant, immunosuppressive therapy), history of CNS disease (mass lesion, stroke, focal infection), new onset seizure within 1 week, papilledema, abnormal level of consciousness, or focal neurologic deficits (including dilated nonreactive pupils, abnormal ocular motility, gaze palsy, arm/leg drift) 3
• Administer empiric antibiotics within 1 hour regardless of whether lumbar puncture has been performed 3, 1

Lumbar Puncture Decision Algorithm

• Proceed directly to lumbar puncture if patient is alert, oriented, and has no contraindications 2, 4
• Perform CT head first if any high-risk features present, then proceed with lumbar puncture only if no mass effect or elevated intracranial pressure 3, 2
• If antibiotics given before LP, perform lumbar puncture within 4 hours of antibiotic initiation when possible to minimize impact on CSF culture yield 2, 4

Essential CSF Laboratory Tests

Core CSF Analysis (All Patients)

• Gram stain examination - highly specific (97%) for bacterial meningitis, with sensitivity varying by pathogen: 90% for S. pneumoniae, 86% for H. influenzae, 75% for N. meningitidis, 50% for gram-negative bacilli, and 33% for L. monocytogenes 3
• Cell count with differential - bacterial meningitis typically shows >2,000 leukocytes/mm³ or >1,180 neutrophils/mm³ 3
• CSF glucose and simultaneous serum glucose - CSF glucose <34 mg/dL or CSF:blood glucose ratio <0.23 predicts bacterial meningitis with 99% certainty 3
• CSF protein - levels >220 mg/dL strongly suggest bacterial rather than viral meningitis 3, 5
• Bacterial culture - remains the gold standard despite lower sensitivity after antibiotic administration 3

Advanced Molecular Testing

• PCR for common meningeal pathogens (N. meningitidis, S. pneumoniae, H. influenzae, S. agalactiae, L. monocytogenes) has 87-100% sensitivity and 98-100% specificity, particularly valuable when antibiotics given before LP 3, 6
• Broad-range bacterial PCR (16S ribosomal RNA) demonstrates 100% sensitivity and 98.2% specificity for excluding bacterial meningitis 3
• Multiplex PCR platforms can detect multiple pathogens simultaneously with reduced time and increased sensitivity 3

Tests NOT Routinely Recommended

• Latex agglutination tests are no longer recommended as they have been surpassed by PCR, rarely modify clinical management, and can produce false-positive results 3
• Limulus lysate assay for gram-negative meningitis is not sensitive enough and does not influence treatment decisions 3

Empiric Antibiotic Regimens

Standard Adult Regimen (<60 Years, Immunocompetent)

• Ceftriaxone 2g IV every 12 hours OR Cefotaxime 2g IV every 6 hours 3, 1
• PLUS Vancomycin 15-20 mg/kg IV every 8-12 hours (or Rifampicin 600mg IV/PO every 12 hours) 3, 1

Modified Regimens for High-Risk Populations

• Adults ≥60 years or immunocompromised (including diabetics, alcohol misuse): ADD Ampicillin/Amoxicillin 2g IV every 4 hours to cover Listeria monocytogenes 3, 1
• Recent travel to areas with penicillin-resistant pneumococci (within 6 months): Ensure vancomycin or rifampicin included in regimen 3
• Penicillin/cephalosporin anaphylaxis: Chloramphenicol 25 mg/kg IV every 6 hours 3

Adjunctive Corticosteroid Therapy

• Dexamethasone 10mg IV every 6 hours should be administered immediately before or with the first antibiotic dose, continued for 4 days if pneumococcal meningitis confirmed 1, 4

Distinguishing Bacterial from Viral Meningitis

CSF Parameters Predicting Bacterial Meningitis (99% Certainty)

• CSF glucose <34 mg/dL 3
• CSF:blood glucose ratio <0.23 3
• CSF protein >220 mg/dL 3, 5
• CSF leukocyte count >2,000/mm³ 3
• CSF neutrophil count >1,180/mm³ 3

Serum Biomarkers

• Procalcitonin (PCT) combined with CSF protein provides optimal discrimination: AUC 0.998,100% sensitivity, 97.1% specificity for bacterial meningitis 5
• PCT alone has AUC 0.951, while CSF protein alone has AUC 0.996 5

Additional Diagnostic Considerations

Nasopharyngeal Swabs

• Obtain nasopharyngeal swabs for meningococcal disease when PCR-only diagnosis made (no cultured isolate), as meningococci isolated from nasopharynx in up to 50% of cases even after antibiotics started 3
• Do NOT obtain nasopharyngeal swabs for pneumococcal disease as multiple strains carried asymptomatically and relationship to meningitis strain unclear 3

Infection Control

• Respiratory isolation with droplet precautions required until meningococcal disease excluded or 24 hours of effective antibiotics completed 3
• Healthcare worker chemoprophylaxis only needed if direct exposure to respiratory secretions (e.g., intubation without mask) 3

Common Pitfalls to Avoid

• Delaying antibiotics for imaging or lumbar puncture - this increases mortality and must be avoided; give antibiotics first if any delay anticipated 1, 2, 4
• Failing to add ampicillin for Listeria coverage in patients ≥60 years or immunocompromised 3, 1
• Performing lumbar puncture with contraindications present - can cause fatal cerebral herniation; obtain CT first when high-risk features exist 3, 2
• Inadequate antibiotic dosing - must use high doses to achieve adequate CSF penetration (ceftriaxone 2g, not 1g) 3, 1
• Neglecting blood cultures - obtain before antibiotics as they may be positive when CSF culture negative 1, 2