What is the recommended initial evaluation and empiric treatment for an adult with suspected bacterial meningitis?

Initial Evaluation and Empiric Treatment of Suspected Bacterial Meningitis in Adults

Administer empiric antibiotics within 1 hour of hospital arrival—immediately after obtaining blood cultures—without waiting for lumbar puncture or imaging results, as delays beyond this window significantly increase mortality and neurological sequelity. 1, 2

Immediate Actions (Within First Hour)

Stabilization and Assessment

• Prioritize airway, breathing, and circulation stabilization as the first intervention upon presentation. 1
• Document Glasgow Coma Scale (GCS) to assess severity and guide need for intensive care; patients with GCS ≤12 require urgent critical care assessment and should be considered for intubation. 1, 2
• Record vital signs using an early warning score system; aggregate scores of 5-6 or single-parameter scores of 3 warrant urgent senior review, while scores ≥7 require critical care team assessment. 1
• Assess for signs of meningococcal sepsis or shock: rapidly evolving rash, hypotension, altered mental status, or seizures indicate high-risk disease requiring immediate fluid resuscitation with 500 mL crystalloid bolus. 1

Blood Cultures and Antibiotic Timing

• Obtain blood cultures immediately upon arrival and within 1 hour of presentation, but never delay antibiotics to acquire them. 1, 2
• Initiate empiric antibiotics within 1 hour of hospital entry; this timing is critical because delays are strongly associated with progression to high-risk clinical severity and irreversible neurological injury. 1, 2

Indications for CT Before Lumbar Puncture

Perform urgent head CT before lumbar puncture only if any of the following high-risk features are present: 1, 2

• Focal neurological signs (gaze palsy, facial weakness, limb drift)
• Papilledema on fundoscopic examination
• Continuous or uncontrolled seizures
• GCS ≤12
• History of CNS disease (mass lesion, stroke, focal infection)
• Age ≥60 years
• Immunocompromise (HIV/AIDS, immunosuppressive therapy, malignancy)

If any CT indication is present: Give antibiotics immediately after blood cultures, then obtain CT, then perform LP only if no mass effect or elevated intracranial pressure is seen. 2

If no CT indications: Perform LP within 1 hour of arrival, then start antibiotics immediately after CSF is obtained. 1

Empiric Antibiotic Regimens

Adults 18-50 Years (Immunocompetent)

Ceftriaxone 2 g IV every 12 hours (or 4 g IV once daily) PLUS Vancomycin 15-20 mg/kg IV every 8-12 hours (target trough 15-20 µg/mL). 2, 3

• Rationale: Covers Streptococcus pneumoniae (including penicillin- and cephalosporin-resistant strains) and Neisseria meningitidis, the predominant adult pathogens. 2
• Alternative cephalosporin: Cefotaxime 2 g IV every 4-6 hours may substitute for ceftriaxone. 2

Adults ≥50 Years or Immunocompromised (Any Age)

Ceftriaxone 2 g IV every 12 hours PLUS Vancomycin 15-20 mg/kg IV every 8-12 hours PLUS Ampicillin 2 g IV every 4 hours. 2, 3

• Rationale: Ampicillin adds essential coverage for Listeria monocytogenes, which cephalosporins cannot treat and which accounts for significant mortality in older and immunocompromised patients. 2, 3
• Listeria risk factors include: age >50 years, diabetes mellitus, immunosuppressive drugs, cancer, chronic liver disease, and other immunocompromising conditions. 2, 3

Regional Resistance Considerations

In areas with high pneumococcal penicillin or cephalosporin resistance (or recent travel to such regions), the vancomycin-cephalosporin combination is mandatory. 2, 3

• Animal model data demonstrate that ceftriaxone combined with vancomycin achieves superior CSF sterilization compared with ceftriaxone alone in resistant pneumococcal meningitis. 3

Adjunctive Dexamethasone Therapy

Administer dexamethasone 10 mg IV every 6 hours for 4 days, given with or 10-20 minutes before the first antibiotic dose. 2, 3

• Evidence of benefit: Reduces unfavorable outcomes (15% vs 25%; P=0.03) and mortality (7% vs 15%; P=0.04) in adults with bacterial meningitis, with greatest effect in pneumococcal disease (unfavorable outcomes 26% vs 52%; deaths 14% vs 34%). 2
• Timing is critical: If not given initially, dexamethasone may still be started up to 4-12 hours after antibiotics, but efficacy diminishes with delay. 2, 3
• Discontinue dexamethasone if Listeria monocytogenes is confirmed, as adjunctive steroids are associated with increased mortality in neurolisteriosis. 3
• Important interaction: Dexamethasone may reduce vancomycin CSF penetration; consider adding rifampicin 300 mg IV every 12 hours to the empiric regimen when dexamethasone is used in suspected pneumococcal meningitis. 2

Lumbar Puncture Timing and CSF Analysis

When to Perform LP

In patients with suspected meningitis without shock or severe sepsis: Perform LP within 1 hour of arrival if safe to do so (no CT indications present), then start antibiotics immediately after CSF is obtained. 1

In patients with predominantly sepsis or rapidly evolving rash: Give antibiotics immediately after blood cultures; do not perform LP at this time—defer until patient is stabilized. 1

If LP cannot be performed within 1 hour: Start antibiotics immediately after blood cultures, then perform LP as soon as possible thereafter, preferably within 4 hours of antibiotic initiation, as culture yield drops rapidly after that window. 1

Expected CSF Findings in Bacterial Meningitis

Parameter	Typical Finding	Clinical Significance
Opening pressure	200-500 mm H₂O	Indicates raised intracranial pressure [2]
WBC count	1,000-5,000 cells/µL (range 100-110,000)	Reflects intense inflammation [2]
Differential	Neutrophils 80-95% (≈10% may be lymphocyte-predominant)	Supports bacterial etiology [2]
Glucose	<40 mg/dL in 50-60% of cases	Hypoglycorrhachia strongly suggests bacterial infection [2,4]
CSF/serum glucose ratio	<0.4	Distinguishes bacterial from viral meningitis [2]
Protein	Elevated (often >2.2 g/L)	Marker of blood-brain barrier disruption [1,4]

Gram stain sensitivity: Overall 60-90%, with pathogen-specific yields of 90% for S. pneumoniae, 86% for H. influenzae, 75% for N. meningitidis, 50% for Gram-negative bacilli, and 33% for Listeria. 2

Critical Care Transfer Criteria

Transfer to ICU within the first hour if any of the following are present: 1, 2

• Rapidly evolving rash (suggests meningococcemia)
• GCS ≤12 or deteriorating consciousness
• Cardiovascular instability or shock (hypotension, capillary refill >2 seconds)
• Hypoxia or respiratory compromise
• Continuous seizures
• Need for specific organ support or intensive monitoring

Common Pitfalls to Avoid

• Never delay antibiotics for imaging or LP—give empiric therapy first if any delay is anticipated beyond 1 hour. 1, 2
• Never omit ampicillin in patients ≥50 years or immunocompromised—failure to cover Listeria can be fatal. 2, 3
• Never use vancomycin monotherapy—it must be combined with a third-generation cephalosporin for adequate coverage. 2
• Never administer dexamethasone more than 12 hours after antibiotics—timing determines efficacy. 2
• Never underdose antibiotics—meningitis requires high-dose regimens to achieve adequate CSF penetration. 2
• Never be falsely reassured by low early warning scores—patients with meningitis can deteriorate rapidly despite initially stable vital signs. 1
• Never perform LP in patients with shock or rapidly evolving rash—stabilize first, defer LP until safe. 1

Pathogen-Specific Therapy After Identification

Once cultures identify the causative organism, narrow therapy based on susceptibility results: 2, 3

• Penicillin-sensitive S. pneumoniae: Switch to penicillin G or ampicillin; continue for 10-14 days
• Penicillin-resistant S. pneumoniae: Continue vancomycin plus ceftriaxone/cefotaxime; 10-14 days
• N. meningitidis: Penicillin G or ceftriaxone; 5-7 days total
• Listeria monocytogenes: Ampicillin 2 g IV every 4 hours; 21 days total
• H. influenzae: Ceftriaxone or cefotaxime; 10 days total

Discontinue dexamethasone if Listeria is confirmed. 3