What is the typical cerebrospinal fluid (CSF) profile in acute bacterial meningitis?

CSF Picture in Bacterial Meningitis

In acute bacterial meningitis, the classic CSF profile shows elevated opening pressure (>20 cm H₂O), turbid/cloudy appearance, marked pleocytosis (typically 1,000-5,000 cells/mm³ with 80-95% neutrophil predominance), elevated protein (>0.6 g/L), low glucose (<40 mg/dL in 50-60% of cases), and a CSF/plasma glucose ratio <0.4. 1

Opening Pressure

• Elevated opening pressure is characteristic, typically in the range of 200-500 mm H₂O (or >20 cm CSF), though values may be lower in neonates and infants 1

Appearance

• CSF appears turbid, cloudy, or purulent depending on the concentration of white blood cells, red blood cells, bacteria, and protein 1

White Blood Cell Count and Differential

Typical Findings

• WBC count is typically elevated in the range of 1,000-5,000 cells/mm³, though the range can be quite broad (100 to 110,000 cells/mm³) 1
• Neutrophil predominance of 80-95% is the hallmark finding 1

Important Exceptions and Pitfalls

• Approximately 10% of patients present with lymphocyte predominance (≥50% lymphocytes or monocytes), particularly with Listeria monocytogenes or partially treated bacterial meningitis 1
• Some patients may have minimal or no pleocytosis, especially early in illness or in immunocompromised patients—one study showed 10% had fewer than 100 cells/mm³ 1
• In pneumococcal meningitis specifically, 5% of patients have CSF WBC counts <10 cells/mm³, and 17% have <100 cells/mm³ 1
• Neonates frequently have atypical CSF findings—in culture-proven neonatal meningitis, 10% had fewer than 3 WBC/mm³, with a median of only 6 cells/mm³ 1

Timing Considerations

• Early viral meningitis may show neutrophil predominance, particularly with enteroviral disease, though total WBC count is unlikely to exceed 2,000 cells/mm³ 1
• In bacterial meningitis, the absolute neutrophil count and percentage remain elevated and sustained beyond 24 hours, unlike aseptic meningitis where neutrophils decline after the first 24 hours 2

Glucose

Absolute Values

• CSF glucose <40 mg/dL occurs in approximately 50-60% of bacterial meningitis cases 1
• CSF glucose >2.6 mmol/L (approximately 47 mg/dL) makes bacterial meningitis unlikely 1
• CSF glucose <34 mg/dL predicts bacterial meningitis with 99% certainty when combined with other parameters 3

CSF/Plasma Glucose Ratio

• A ratio <0.4 is 80% sensitive and 98% specific for bacterial meningitis in children ≥2 months of age 1, 3
• A ratio <0.36 has 93% sensitivity and specificity for diagnosing bacterial meningitis 1
• A ratio <0.23 predicts bacterial etiology with 99% certainty when combined with other CSF parameters 3
• The CSF/plasma glucose ratio is more diagnostically useful than absolute CSF glucose, especially when serum glucose is abnormal 4

Special Population Considerations

• In term neonates, a ratio of 0.6 (not 0.4) is considered abnormal because they have higher normal CSF/blood glucose ratios 1, 3

Protein

• CSF protein is elevated, typically >0.6 g/L in bacterial meningitis 1
• A CSF protein <0.6 g/L makes bacterial meningitis unlikely 1
• Bacterial meningitis tends to have higher CSF protein than viral meningitis, though there is overlap between different etiologies 1

CSF Lactate

• CSF lactate has 93% sensitivity and 96% specificity for distinguishing bacterial from viral meningitis when antibiotics have not been given beforehand 1
• A CSF lactate cutoff of 35 mg/dL (3.9 mmol/L) has the best sensitivity for distinguishing bacterial from viral meningitis 1
• Critical limitation: If patients have received antibiotics, sensitivity drops to less than 50% 1
• The high negative predictive value makes it useful to rule out bacterial meningitis and provide reassurance to withhold antibiotics when done prior to antibiotic administration 1
• CSF lactate diagnostic accuracy is better than CSF WBC count according to meta-analyses 1

Microbiological Studies

Gram Stain

• Sensitivity ranges from 50-99% depending on organism and prior antibiotic use, with specificity of 97-100% 1
• Cytospin centrifugation of CSF can increase the yield 1

Culture

• CSF culture is the gold standard, diagnostic in 70-85% of cases when prior antibiotics have not been given 1
• CSF sterilization may occur within 2 hours for meningococci and within 4 hours for pneumococci after antibiotic administration 1
• Culture positivity varies by organism: 96% positive in some series 1

PCR

• PCR has sensitivities of 79-100% for S. pneumoniae, 91-100% for N. meningitidis, and 67-100% for H. influenzae, with specificity of 95-100% 1
• PCR has incremental value compared to CSF culture and Gram stain, particularly after antibiotic administration 1

Clinical Algorithm for Interpretation

When CSF shows the following combination, bacterial meningitis can be predicted with 99% certainty: 3

• CSF glucose <34 mg/dL
• CSF/blood glucose ratio <0.23
• Protein >120 mg/dL
• WBC >12,000/mm³, OR
• Neutrophils >11,000/mm³

When considering serum inflammatory markers (CRP, procalcitonin): Use these when the CSF/serum glucose ratio is 0.23-0.36, especially if Gram stain is negative 3

Key Pitfalls to Avoid

1. Do not rely solely on WBC count—approximately 10% of bacterial meningitis cases have minimal pleocytosis 1
2. Do not dismiss bacterial meningitis based on lymphocyte predominance—10% of bacterial meningitis presents this way, particularly Listeria 1
3. Prior antibiotic administration does not greatly alter CSF WBC count according to one study in children, though it does affect culture yield and CSF lactate accuracy 1
4. Neonates require different interpretation—normal CSF parameters differ significantly in this population 1
5. Calculate the CSF/plasma glucose ratio rather than relying on absolute CSF glucose alone, as serum glucose abnormalities can be misleading 4