Diagnosis of Bacterial Meningitis
The diagnosis of bacterial meningitis requires immediate CSF analysis including leukocyte count with differential, protein, glucose (with simultaneous blood glucose), Gram stain, and culture—but do not delay empiric antibiotics while awaiting lumbar puncture or imaging studies. 1, 2
Clinical Presentation: Recognize the Variable Picture
The classic triad of fever, neck stiffness, and altered mental status appears in only 41-51% of adult patients with bacterial meningitis. 1 Individual symptoms occur with the following frequencies:
- Fever (>38°C): 77-97% of cases 1, 3
- Headache: 58-87% of cases 1, 3
- Neck stiffness: 65-83% of cases 1, 3
- Altered mental status: 30-69% of cases 1, 3
- Petechial/hemorrhagic rash: 20-52% of cases (>90% indicative of meningococcal infection when present) 1, 3
Critical pitfall: Traditional meningeal signs have poor sensitivity—neck stiffness (31%), Kernig sign (11%), and Brudzinski sign (9%)—meaning their absence cannot exclude bacterial meningitis. 1 Therefore, bacterial meningitis should never be ruled out based solely on absent classic symptoms. 1
Immediate Diagnostic Algorithm
Step 1: Stabilize and Obtain Blood Cultures
- Document Glasgow Coma Scale score immediately 4, 3
- Obtain blood cultures within the first hour before antibiotics 1, 2, 4
- Blood cultures are positive in 40-90% of cases depending on organism (75% for pneumococcal, 50-90% for H. influenzae, 40-60% for meningococcal meningitis) 1, 3
Step 2: Determine Need for CT Before Lumbar Puncture
Perform cranial CT before LP only if any of the following are present: 1, 2, 4
- Glasgow Coma Scale score ≤10 (or <12 per some guidelines) 1, 2, 4
- Focal neurologic deficits (excluding cranial nerve palsies) 1, 2
- New-onset seizures 1, 2
- Severely immunocompromised state 1, 2
- Papilledema 2
Critical pitfall: Cranial imaging causes substantial delays in antibiotic initiation, which worsens outcomes. 1 If CT is indicated, start empiric antibiotics immediately after blood cultures—do not wait for imaging results. 2, 4
Step 3: Initiate Empiric Antibiotics Within 1 Hour
Antibiotics must be administered within one hour of hospital arrival, regardless of whether CT or LP has been performed. 2, 4, 3 This timing priority supersedes all diagnostic procedures.
Essential CSF Analysis Parameters
Mandatory CSF tests (Grade A recommendation): 1
- CSF leukocyte count with differential: Best single diagnostic parameter (AUC 0.95) 4
- CSF protein concentration
- CSF glucose with simultaneous blood glucose
- CSF Gram stain
- CSF culture
Diagnostic Thresholds with High Negative Predictive Value
The following CSF parameters effectively rule out bacterial meningitis when all are present: 2
- CSF/blood glucose ratio >0.36 (sensitivity 92.9%, specificity 92.9%, AUC 0.97) 2, 5
- CSF glucose >2.6 mmol/L (>45 mg/dL) 2, 6
- CSF protein <0.6 g/L (<50 mg/dL) 2, 6
Key finding: The CSF/blood glucose ratio is the single most precise indicator, maintaining accuracy even after antibiotic pretreatment. 2, 5
Additional Diagnostic Indicators
- CSF neutrophil predominance ≥50% of total CSF leukocytes suggests bacterial etiology 6
- CSF protein >50 mg/dL has 78% diagnostic accuracy with 88% sensitivity and 72% specificity 6
Microbiological Diagnostics
CSF Gram Stain
- Excellent specificity (97-100%) but variable sensitivity (50-99%) depending on organism 1, 3, 7
- Sensitivity by organism: 75% for pneumococcal, 50-90% for H. influenzae, 40-60% for meningococcal 1
- Yield decreases slightly with antibiotic pretreatment 1
CSF Culture
- Positive in 60-90% of bacterial meningitis cases (gold standard) 1, 3
- Antibiotic pretreatment decreases yield by 10-20% 1
PCR Testing (Grade A Recommendation)
In patients with negative CSF culture and Gram stain, PCR has significant additive value for pathogen identification. 1 PCR demonstrates 59% sensitivity and 97% specificity, particularly useful after antibiotic pretreatment. 7 PCR is especially valuable for detecting S. pneumoniae and N. meningitidis when cultures are negative. 7
Limited Value Tests
- Latex agglutination: Little incremental diagnostic value 1
- Immunochromatographic antigen testing: Unclear incremental value, though potentially useful 1
- Serum CRP and procalcitonin: Cannot definitively diagnose bacterial meningitis but may help differentiate from viral meningitis (procalcitonin threshold 1-2 ng/mL has good discriminatory value) 1, 3
Diagnostic Algorithms: Use with Caution
Multiple validated algorithms exist (Boyer, Oostenbrink, Bacterial Meningitis Score, Hoen, Spanos, etc.), but none achieves 100% sensitivity, meaning bacterial meningitis cases will be missed if algorithms are used in isolation. 1 These tools may guide management in individual patients, but clinical judgment must remain paramount when deciding to initiate empiric therapy. 1
Critical Pitfalls to Avoid
Never delay antibiotics for any reason once bacterial meningitis is suspected—not for imaging, not for LP, not for any diagnostic test 2, 4, 3
Do not rely on absence of classic symptoms to exclude bacterial meningitis—up to 59% of patients lack the classic triad 1
Do not skip blood cultures, but never delay antibiotics to obtain them—draw cultures and immediately start treatment 1, 2, 4
Antibiotic pretreatment reduces but does not eliminate diagnostic yield—still perform LP and cultures, and add PCR testing for culture-negative cases 1, 7
CSF/blood glucose ratio is more reliable than CSF glucose alone, especially after antibiotic administration 2, 5