What cerebrospinal fluid white‑blood‑cell count cutoff is used to consider meningitis?

CSF White Blood Cell Count Cutoffs for Meningitis

The CSF WBC cutoff to define pleocytosis varies by age: ≥5 cells/µL in adults and older children, ≥10 cells/µL for infants 29–90 days old, and ≥25 cells/µL for neonates 0–28 days old. 1, 2

Age-Specific Thresholds for CSF Pleocytosis

The definition of abnormal CSF white blood cell count is age-dependent and represents the threshold for considering meningitis:

• Adults and children >90 days: CSF WBC >5 cells/µL defines pleocytosis 2, 1
• Infants 29–90 days: CSF WBC ≥10 cells/µL defines pleocytosis 2
• Neonates 0–28 days: CSF WBC ≥25 cells/µL defines pleocytosis 2

These thresholds represent the minimum values to consider meningitis as a diagnostic possibility, but pleocytosis alone does not distinguish bacterial from viral meningitis. 2

Critical Interpretation Caveats

Normal CSF Does Not Exclude Bacterial Meningitis

Approximately 10% of patients with culture-proven bacterial meningitis have CSF WBC counts <100 cells/mm³, and some have no pleocytosis at all. 2 This is particularly common in:

• Neonates: 10% of neonates with culture-proven meningitis had <3 WBC/mm³ in CSF, and 6% of neonates with Streptococcus agalactiae meningitis had completely normal CSF 2
• Immunocompromised patients: May lack CSF pleocytosis despite active infection 2
• Early presentation: Very early in the disease course before significant inflammatory response develops 2
• Specific pathogens: Listeria monocytogenes meningitis shows atypical CSF findings in 26% of cases 2

Pleocytosis Does Not Equal Bacterial Meningitis

CSF pleocytosis is common in non-bacterial conditions and should not trigger automatic antibiotic treatment without additional supporting evidence. 2

• Enterovirus infections: Approximately 50% of enterovirus-positive infants <90 days have CSF pleocytosis despite absence of bacterial disease 2, 1
• Urinary tract infections: Can cause CSF pleocytosis without meningitis 2
• Lymphocytic predominance in bacterial meningitis: 32% of bacterial meningitis cases with CSF WBC ≤1,000/mm³ show >50% lymphocytes, mimicking viral meningitis 3

Distinguishing Bacterial from Non-Bacterial Meningitis

When pleocytosis is present, additional CSF parameters help differentiate bacterial from viral etiologies:

CSF/Plasma Glucose Ratio (Most Discriminatory)

A CSF/plasma glucose ratio <0.36 has 93% sensitivity and 93% specificity for bacterial meningitis and is the single best predictor. 4, 1

• Ratio <0.36: Strongly suggests bacterial meningitis 4, 2
• Ratio 0.36–0.5: Intermediate zone; consider tuberculous or fungal meningitis 1, 5
• Ratio >0.66: Normal; favors viral meningitis 2

Always measure simultaneous plasma glucose to calculate this ratio—absolute CSF glucose values alone are misleading. 2, 1

CSF Protein Level

• CSF protein <0.6 g/L: Bacterial meningitis unlikely 2
• CSF protein >1 g/L: Strongly favors tuberculous or fungal meningitis over bacterial 2, 1, 5

CSF Lactate

CSF lactate >35 mg/dL (>3.9 mmol/L) indicates bacterial meningitis with 93% sensitivity and 96% specificity. 1, 5 Conversely, CSF lactate <2 mmol/L effectively rules out bacterial disease. 5

Cell Count Thresholds for Bacterial Meningitis

While no absolute cutoff is definitive, higher CSF WBC counts increase the likelihood of bacterial etiology:

• CSF WBC >321 cells/µL: 80.6% sensitivity and 81.4% specificity for bacterial meningitis in children 6
• CSF WBC >1,000 cells/mm³: Typical for bacterial meningitis, though viral meningitis rarely exceeds 2,000 cells/mm³ 2
• Neutrophil predominance (>80%): Strongly suggests bacterial meningitis, but 32% of bacterial cases with low cell counts show lymphocytic predominance 3, 2

Correction for Traumatic Lumbar Puncture

When CSF contains significant RBCs from a traumatic tap, use a correction ratio of 500 RBCs:1 WBC to adjust the white cell count. 2 The International Encephalitis Consortium formula is: True CSF WBC = actual CSF WBC – (WBC in blood × RBC in CSF)/RBC in blood. 2

However, these correction formulas have not been well validated and should be interpreted cautiously. 2

Clinical Decision-Making Algorithm

When evaluating CSF results:

1. Apply age-appropriate pleocytosis threshold (5,10, or 25 cells/µL) 2, 1
2. If pleocytosis present, immediately calculate CSF/plasma glucose ratio 4, 2
3. If ratio <0.36, treat as bacterial meningitis 4
4. If ratio 0.36–0.5 with lymphocytic predominance and protein >1 g/L, consider tuberculous/fungal meningitis 1, 5
5. If ratio >0.66 with lymphocytic predominance, viral meningitis most likely 2
6. Never exclude bacterial meningitis based solely on normal or low CSF WBC count in ill-appearing patients 2

Special Populations Requiring Lower Threshold for Treatment

Start empiric antibiotics despite normal or minimally elevated CSF in:

• Ill-appearing neonates (even with <25 WBC/µL) 2
• Immunocompromised patients 2
• Patients with prior antibiotic exposure (CSF may be partially sterilized) 2
• Clinical presentation strongly suggesting bacterial meningitis regardless of CSF findings 2