USMLE Prep - Medical Reference Library

Bacterial Meningitis — Adult Empiric Therapy

System: Infectious Diseases • Reviewed: Aug 31, 2025 • Step 1Step 2Step 3

Synopsis:

Suspect with fever, neck stiffness, and altered mental status; give empiric IV antibiotics and dexamethasone without delay, obtain blood cultures, and perform lumbar puncture when safe.

Key Points

  • Stabilize ABCs; begin targeted evaluation without delaying life-saving therapy.
  • Use system-specific risk tools to guide testing and disposition.
  • Order high-yield tests first; escalate imaging when indicated.
  • Start evidence-based initial therapy and reassess frequently.

Algorithm

  1. Primary survey and vitals; IV access and monitors.
  2. Focused history/physical; identify red flags and likely etiologies.
  3. Order system-appropriate labs and imaging (see Investigations).
  4. Initiate guideline-based empiric therapy (see Pharmacology).
  5. Reassess response; arrange consultation and definitive management.

Clinical Synopsis & Reasoning

Suspect bacterial meningitis in acute febrile encephalopathy with meningeal signs or unexplained sepsis; do not delay therapy for diagnostics in unstable patients. Check glucose and electrolytes, obtain blood cultures, and perform head CT prior to LP only with focal deficits, seizures, immunocompromise, or altered consciousness suggesting mass effect. CSF analysis (cell count, glucose, protein, Gram stain, PCR) directs pathogen‑specific management.


Treatment Strategy & Disposition

Give empiric, age‑adjusted IV antibiotics and dexamethasone when pneumococcal disease is suspected, ideally before or with the first antibiotic dose. Tailor coverage as culture/PCR results return, manage complications (seizures, raised ICP), and implement droplet precautions for meningococcal disease. Admit to ICU when neurologic monitoring, airway protection, or vasopressors are required; arrange prophylaxis for close contacts when indicated.


Epidemiology / Risk Factors

  • Immunosuppression, devices; recent hospitalization

Investigations

TestRole / RationaleTypical FindingsNotes
CBCInflammation/infectionLeukocytosis/leukopenia
LactateHypoperfusionElevatedTrend
Blood culturesPathogen IDPositive/negativeBefore antibiotics if feasible

Empiric Regimens (Adult)

GroupExample
Age 18 to 50Ceftriaxone plus vancomycin
Age >50 or immunocompromisedAdd ampicillin for Listeria

Pharmacology

MedicationMechanismOnsetRole in TherapyLimitations
Ceftriaxone + vancomycinβ-lactam + glycopeptideHoursEmpiric adult therapyAllergy; nephrotoxicity (vanc)
Ampicillin (add >50 or immunocompromised)β-lactamHoursListeria coverageAllergy
DexamethasoneGlucocorticoidHoursAdjunct for S. pneumoniaeHyperglycemia

Prognosis / Complications

  • Depends on host and source control; sepsis/organ failure risk

Patient Education / Counseling

  • Explain red flags and when to seek emergent care.
  • Reinforce medication adherence and follow-up plan.

Notes

Prophylaxis for close contacts in meningococcal disease. Manage complications such as seizures and raised intracranial pressure.


References

  1. IDSA Bacterial Meningitis Guideline — Link
  2. WHO Meningitis Resources — Link