How to rule out meningitis?

How to Rule Out Meningitis

Perform a lumbar puncture (LP) immediately in all patients without contraindications, and if bacterial meningitis is suspected clinically, administer empiric antibiotics within one hour regardless of whether imaging or LP has been completed. 1

Clinical Assessment for LP Contraindications

Before proceeding with LP, assess for the following contraindications that mandate CT imaging first 2, 1:

• Glasgow Coma Scale ≤12 2
• Focal neurological deficits (including dilated nonreactive pupil, abnormal ocular motility, gaze palsy, arm/leg drift, cranial nerve signs) 2
• New-onset seizures (within 1 week of presentation) 2
• Papilledema (though inability to visualize the fundus is NOT a contraindication) 2
• Severe immunocompromised state (HIV/AIDS, transplant recipients, immunosuppressive therapy) 2
• History of CNS disease (mass lesion, stroke, focal infection) 2
• Signs of severe sepsis or rapidly evolving rash 2
• Respiratory or cardiac compromise 2
• Coagulopathy or thrombocytopenia (platelet count <40 × 10⁹/L may be acceptable depending on clinical context) 2

Critical pitfall: Do not delay antibiotics while awaiting CT or LP results—start empiric treatment immediately if meningitis is suspected 1.

Clinical Diagnosis When LP Cannot Be Performed

If LP is contraindicated or cannot be performed, diagnose meningitis clinically based on the presence of two or more of the following 2:

• Severe headache
• Altered mental status (inability to answer two consecutive questions or follow two consecutive commands)
• Meningeal signs (Kernig sign, Brudzinski sign, jolt accentuation test, nuchal rigidity, photophobia, meningismus)
• Other neurological symptoms/signs (recent seizures, focal signs, papilledema)

CSF Analysis to Rule Out Bacterial Meningitis

When LP is performed, the following CSF parameters have high negative predictive value for ruling out bacterial meningitis 1:

• CSF/blood glucose ratio >0.36 (sensitivity 93%) 2, 1
• CSF glucose >2.6 mmol/L 1
• CSF protein <0.6 g/L 2, 1
• CSF lactate <2 mmol/L (or <35 mg/dL) effectively rules out bacterial disease 3, 1

Characteristic CSF Findings by Etiology

Parameter	Bacterial	Viral	Tuberculous	Fungal
Opening Pressure	Raised (>20 cm H₂O) [2]	Normal/mildly raised [3]	Raised [4]	Raised [3]
Appearance	Turbid/cloudy [3]	Clear [3]	Clear/cloudy [4]	Clear/cloudy [3]
WBC (cells/μL)	>100 (typically >2000) [3]	5-1000 [3]	5-500 [4]	5-500 [3]
Cell Type	Neutrophils [3]	Lymphocytes* [3]	Lymphocytes [4]	Lymphocytes [3]
Protein (g/L)	Raised (usually >0.6) [2]	Mildly raised (<0.6) [3]	Markedly raised (>1) [4]	Raised [3]
CSF/Plasma Glucose	Very low (<0.36) [2]	Normal/slightly low (>0.36) [3]	Very low (<0.5) [4]	Low [3]

*Neutrophils may predominate in early viral meningitis, especially enteroviral, but total WBC unlikely to exceed 2000 cells/μL 2, 3

Microbiological Testing

Order the following tests on all CSF samples 2, 1:

• Gram stain (sensitivity 50-99%, specificity 97-100%) 1
• CSF culture (gold standard, sensitivity 70-85% in untreated patients) 1
• CSF opening pressure (unless LP performed sitting) 2
• Cell count with differential 2
• Glucose with simultaneous blood glucose 2
• Protein 2
• PCR for HSV-1/2, VZV, and enteroviruses (identifies 90% of viral cases) 3

Blood cultures should be obtained before antibiotics (positive in 40-90% of bacterial meningitis cases) 1. Even if antibiotics have been started, blood cultures remain valuable as they were positive in 71% of cases in one series 5.

Timing of LP After Antibiotics

If antibiotics have already been started, LP is still diagnostically useful if performed within 4 hours 6:

• LP within 4 hours of antibiotics: 73% culture-positive 6
• LP after 4-8 hours: 11% culture-positive 6
• LP after 8 hours: 0% culture-positive 6

However, PCR testing remains positive for several days after antibiotic initiation 2, so molecular diagnostics should always be requested even with delayed LP.

Critical Pitfalls to Avoid

• Never delay antibiotics for imaging or LP—administer empiric treatment within one hour if meningitis is suspected 1
• Do not exclude meningitis based on normal CSF white cell count alone—10% of bacterial meningitis cases have <100 cells/mm³, and rare cases have no pleocytosis 2, 5. In such cases, mortality is 31% for bacterial causes 5
• Do not assume lymphocytic predominance rules out bacterial meningitis—Listeria and partially treated bacterial meningitis can present with lymphocytic pleocytosis 2
• Avoid unnecessary CT scans—67% of LP delays are due to CT scans, yet only 20% of these patients had actual contraindications 6
• Request molecular diagnostics (PCR) in all cases, as they remain positive after antibiotics and have higher sensitivity than culture 6, 5
• If initial CSF is normal in suspected viral CNS infection, repeat LP in 24-48 hours as characteristic abnormalities will likely appear 3