What are the typical cerebrospinal fluid findings for bacterial, viral, tuberculous, and fungal meningitis?

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CSF Findings in Meningitis

Bacterial Meningitis

The classic CSF profile in bacterial meningitis consists of neutrophilic pleocytosis (typically >100 cells/mm³ with 80-95% polymorphonuclear cells), very low glucose, very low CSF-to-blood glucose ratio, elevated protein, and turbid/cloudy appearance. 1

Key Parameters

  • Cell count and type: Pleocytosis with predominantly polymorphonuclear leukocytes is the most distinguishing feature separating bacterial from viral or tuberculous meningitis 1
  • Glucose: Very low CSF glucose concentration 1
  • CSF/plasma glucose ratio: Very low ratio (<0.36 suggests bacterial over tuberculous meningitis); this is more diagnostically useful than absolute glucose values alone 1, 2
  • Protein: Raised but not as markedly as in tuberculous meningitis 1
  • Opening pressure: Commonly raised 1
  • Appearance: Turbid or cloudy 1

Critical Pitfalls in Neonates

  • Classic CSF findings are frequently absent in neonates, making diagnosis particularly challenging 1
  • 6% of neonates with proven bacterial meningitis had completely normal CSF 1
  • 10% had fewer than 3 WBC/mm³ 1
  • Median CSF WBC count was only 6 cells/mm³ (range 0-90,000/mm³) 1
  • CSF WBC >21 cells/mm³ had only 79% sensitivity and 81% specificity in neonates 1

Organism-Specific Variations

  • Streptococcus pneumoniae: 5% have CSF WBC <10 cells/mm³, 17% have <100 cells/mm³ 1
  • Listeria monocytogenes: 26% show atypical CSF findings not characteristic of bacterial meningitis 1

Adjunctive Diagnostic Tests

  • CSF lactate has better diagnostic accuracy than CSF WBC count for differentiating bacterial from viral meningitis 3, 1
  • CSF lactate <2 mmol/L effectively rules out bacterial disease 1, 2
  • CSF lactate >35 mg/dL indicates bacterial meningitis with 93% sensitivity and 96% specificity 2
  • Prior antibiotic treatment reduces CSF lactate sensitivity to 49% (vs. 98% without pretreatment) 3

Viral Meningitis

Viral meningitis characteristically shows lymphocytic pleocytosis (5-1000 cells/mm³), normal or slightly low glucose, normal or slightly low CSF/plasma glucose ratio (>0.36), mildly raised protein, and clear appearance. 1, 4

Key Parameters

  • Cell count and type: Lymphocytes predominate, with WCC typically 5-1000 cells/mm³ 1, 4
  • Glucose: Normal or slightly low 1, 4
  • CSF/plasma glucose ratio: Normal or slightly low but remains >0.36 1, 2
  • Protein: Mildly raised 1, 4
  • Opening pressure: Normal or mildly raised 4
  • Appearance: Clear 1, 4

Tuberculous Meningitis

Tuberculous meningitis presents with lymphocytic pleocytosis (5-500 cells/mm³), very low glucose (<2.2 mmol/L), very low CSF/plasma glucose ratio (<0.5), markedly elevated protein (typically >1 g/L), and clear or cloudy appearance. 4, 2

Key Parameters

  • Cell count and type: Lymphocytes predominate (typically ≈98%), though neutrophils may predominate early in the disease course 4, 2
  • WCC: Typically 5-500 cells/μL 4
  • Glucose: Very low, typically <2.2 mmol/L (sensitivity 68%, specificity 96%) 4, 2
  • CSF/plasma glucose ratio: Very low, typically <0.5 (sensitivity 90%); this is the key discriminator from viral meningitis 4, 2
  • Protein: Markedly raised, typically >1 g/L (sensitivity 78%, specificity 94%) 4, 2
  • Opening pressure: Raised 4
  • Appearance: Clear or cloudy 4

Diagnostic Algorithm

  1. First, assess cell type: Lymphocyte predominance (≈98%) strongly suggests tuberculous etiology 2
  2. Second, calculate CSF/plasma glucose ratio: <0.5 is highly suggestive of TB meningitis; <0.36 points toward bacterial meningitis 2
  3. Third, evaluate protein: >1 g/L strongly favors TB meningitis, whereas <0.6 g/L makes bacterial meningitis unlikely 2
  4. Fourth, consider clinical course: Subacute course >5 days is independently predictive with 93% sensitivity 2

Microbiological Confirmation

  • TB PCR on CSF demonstrates 87-100% sensitivity and 98-100% specificity 2
  • Obtain 6 mL of CSF for AFB smear, culture, and TB PCR 2
  • MRI may reveal basal meningeal enhancement, tuberculomas, or infarcts 2

Critical Clinical Caveat

  • CSF parameters do not correlate with severity of TBM, radiological features, paradoxical worsening, or 3-month outcome 5
  • Typical CSF findings are present in only 66% of cases 5
  • Start empiric four-drug anti-tuberculous therapy immediately based on clinical suspicion without waiting for microbiological confirmation 2

Fungal Meningitis

Fungal meningitis (Cryptococcus, Histoplasma) presents similarly to tuberculous meningitis with lymphocytic pleocytosis (5-500 cells/mm³), low glucose, low CSF/plasma glucose ratio, raised protein, and clear or cloudy appearance. 4, 2

Key Parameters

  • Cell count and type: Lymphocytes predominate, WCC 5-500 cells/μL 4
  • Glucose: Low 4
  • CSF/plasma glucose ratio: Low 4
  • Protein: Raised 4
  • Opening pressure: Raised 4
  • Appearance: Clear or cloudy 4

Specific Diagnostic Tests

  • Cryptococcal meningitis: India ink staining, cryptococcal antigen (lateral flow assay), fungal culture 2, 6
  • Histoplasmosis: Complement-fixing antibodies to Coccidioides immitis if Southwest US residence 6
  • CSF WCC <25 × 10⁶/L is optimal for excluding cryptococcal meningitis 7
  • Large volumes of CSF (up to 40-50 mL) should be obtained for culture unless contraindicated by increased intracranial pressure 6

Comparative Summary Table

Parameter Bacterial Viral Tuberculous Fungal
Cell Type Neutrophils (80-95%) Lymphocytes Lymphocytes* Lymphocytes
WBC Count (cells/mm³) >100 (typically 1,000-5,000) 5-1000 5-500 5-500
Glucose Very low Normal/slightly low Very low (<2.2 mmol/L) Low
CSF/Plasma Glucose <0.36 >0.36 <0.5 Low
Protein Raised Mildly raised Markedly raised (>1 g/L) Raised
Opening Pressure Raised Normal/mildly raised Raised Raised
Appearance Turbid/cloudy Clear Clear/cloudy Clear/cloudy

*Neutrophils may predominate early in tuberculous meningitis 1, 4, 2

Key Discriminators

  • Neutrophil vs. lymphocyte predominance: Neutrophils strongly suggest bacterial; lymphocytes suggest viral, tuberculous, or fungal 1
  • CSF/plasma glucose ratio: <0.36 bacterial, 0.36-0.5 tuberculous/fungal, >0.5 viral 2
  • Protein level: Markedly elevated (>1 g/L) strongly favors tuberculous over bacterial 4, 2
  • CSF lactate: <2 mmol/L rules out bacterial; >35 mg/dL confirms bacterial 1, 2

References

1
Guideline

Diagnostic Criteria for Bacterial Meningitis

Praxis Medical Insights: Practical Summaries of Clinical Guidelines, 2026

2
Guideline

Diagnosis and Management of Tuberculous Meningitis

Praxis Medical Insights: Practical Summaries of Clinical Guidelines, 2026

3
Guideline

Guideline Directed Topic Overview

Dr.Oracle Medical Advisory Board & Editors, 2025

4
Guideline

Characteristic CSF Picture in Tubercular Meningitis

Praxis Medical Insights: Practical Summaries of Clinical Guidelines, 2025

6
Research

Approach to diagnosis of meningitis. Cerebrospinal fluid evaluation.

Infectious disease clinics of North America, 1990

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Professional Medical Disclaimer

This information is intended for healthcare professionals. Any medical decision-making should rely on clinical judgment and independently verified information. The content provided herein does not replace professional discretion and should be considered supplementary to established clinical guidelines. Healthcare providers should verify all information against primary literature and current practice standards before application in patient care. Dr.Oracle assumes no liability for clinical decisions based on this content.

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