Expected CSF Values in Aseptic Meningitis
In aseptic meningitis, the cerebrospinal fluid (CSF) typically shows a mild to moderate lymphocytic pleocytosis with normal glucose levels and mildly elevated protein, distinguishing it from bacterial meningitis which shows more marked abnormalities. 1
CSF Parameters in Aseptic Meningitis
Opening Pressure
- Normal to mildly raised (typically within 12-20 cm CSF) 1
- Not significantly elevated as seen in bacterial meningitis
Appearance
- Clear 1
- Unlike the turbid, cloudy appearance commonly seen in bacterial meningitis
Cell Count and Differential
- White blood cell count: Typically 5-1000 cells/μL 1
- Predominant cell type: Lymphocytes (>50% lymphocytes or mononuclear cells) 1, 2
- Important temporal pattern: Early in disease (first 24 hours), neutrophils may predominate with mean absolute neutrophil count of 182 cells/mm³ at <12 hours and 164 cells/mm³ at 12-24 hours, shifting to lymphocytic predominance after 24 hours 3
Biochemistry
- Protein: Mildly raised (typically <0.6 g/L) 1, 4
- Glucose: Normal or slightly low 1
- CSF/plasma glucose ratio: Normal or slightly low (typically >0.6) 1
- CSF lactate: <35 mg/dL (3.0 mmol/L) 1, 4, 5
Distinguishing Features from Bacterial Meningitis
Key Differentiating Parameters
- Cell count: Bacterial meningitis typically has higher counts (>1000 cells/μL) 1
- Cell type: While aseptic meningitis is predominantly lymphocytic after 24 hours, bacterial meningitis maintains neutrophil predominance (>80%) 1, 3
- CSF protein: Values >0.6 g/L suggest bacterial rather than aseptic etiology 1, 4
- CSF glucose: Values >2.6 mmol/L make bacterial meningitis unlikely 1, 4
- CSF lactate: Most discriminatory single test with 93% sensitivity and 96% specificity using 35 mg/dL cutoff 1, 4, 5
Diagnostic Pitfalls to Avoid
- Early sampling confusion: Within the first 24 hours of symptom onset, aseptic meningitis may present with neutrophil predominance, mimicking bacterial meningitis 3
- Low cell count bacterial meningitis: Up to 10% of bacterial meningitis cases may have fewer than 100 cells/mm³ 1
- Lymphocytic bacterial meningitis: Approximately 32% of bacterial meningitis cases with CSF WBC <1000/mm³ may present with lymphocyte predominance 6
- Prior antibiotic therapy: Can alter typical CSF findings, particularly reducing the sensitivity of lactate testing and cultures 1, 4
Additional Diagnostic Considerations
- PCR for viral pathogens (particularly enterovirus, HSV) is essential for confirming aseptic etiology 4
- Immunocytochemical studies in aseptic meningitis typically show T-cell predominance 2
- HMGB1 and Hsp72 levels in CSF are significantly lower in aseptic meningitis compared to bacterial meningitis 7