Comprehensive CSF Analysis Components
A comprehensive CSF analysis after lumbar puncture should include: opening pressure measurement, cell count with differential, glucose (with concurrent plasma glucose), protein, Gram stain, bacterial culture, and PCR testing for common pathogens—with adequate volume (at least 10 mL, ideally 22 mL) collected to enable all necessary tests. 1
Essential Core Components
Physical Measurements
- Opening pressure: Must be documented unless LP performed in sitting position (artificially elevated in that position). Normal pressure is typically <20 cm H₂O; bacterial meningitis often shows >20 cm H₂O 1
Biochemical Analysis
- CSF glucose with concurrent plasma glucose: Critical for interpretation—CSF/serum ratio <0.4 is 80% sensitive and 98% specific for bacterial meningitis in children >12 months; ratio <0.6 abnormal in neonates 2
- CSF protein: Elevated in virtually all bacterial meningitis cases; typically >220 mg/dL in bacterial meningitis 3, 2
- CSF lactate: Useful for differentiating bacterial from viral causes; <2 mmol/L essentially rules out bacterial disease 1, 4
Cellular Analysis
- Total and differential white cell count: Bacterial meningitis typically shows 1,000-5,000 cells/mm³ with 80-95% neutrophils, though 10% may show lymphocyte predominance 1, 2
- Red cell count: Important for detecting hemorrhage or correcting for traumatic tap 4
- Cytological staining: Should be evaluated whenever pleocytosis is found or when leptomeningeal metastases suspected 5
Microbiological Studies
- Gram stain: Immediate diagnostic value 6, 3
- Bacterial culture: Positive in 70-85% of untreated bacterial meningitis 2
- PCR testing: For HSV-1/2, VZV, and enteroviruses—identifies 90% of viral encephalitis cases 4
- Additional pathogen-specific testing: Pneumococcal and meningococcal PCR from blood (EDTA sample) 1
Critical Volume Requirements
Collect at least 10 mL of CSF, ideally 22 mL or more to avoid inadequate samples that limit diagnostic testing. CSF is produced at approximately 15 mL/hour, making larger volumes safe to remove 1, 7. The first tube has highest contamination risk and should not be sent for microbiology 6.
Tube Distribution Strategy
Distribute CSF into 3-4 sterile tubes 6, 5:
- Tube 1 (0.5-1 mL minimum): Gram stain and bacterial culture
- Tube 2 (1-2 mL): Glucose and protein
- Tube 3 (2-2.5 mL): Cell counts and differential
- Tube 4 (2 mL): Virology/PCR and storage for future testing
For mycobacteria and fungi, larger volumes (5-10 mL) significantly increase sensitivity 6.
Timing and Transport
CSF must be analyzed immediately after collection—within 2 hours maximum 6, 7. Send specimens to laboratory in sterile containers at room temperature immediately 6. If bacterial meningitis suspected and LP delayed for any reason (including imaging), start empirical antibiotics after blood cultures obtained 3.
Additional Considerations for Specific Scenarios
When Intracranial Devices Present
Obtain CSF from the reservoir; if CSF flow obstructed, also sample lumbar space 3
When Initial LP Non-Diagnostic
Perform second LP at 24-48 hours—particularly important as 5-10% of HSV encephalitis cases show normal initial CSF 4
Correction for Traumatic Tap
Subtract 1 WBC per 700 RBCs and 0.1 g/dL protein per 100 RBCs to approximate true values 4
Common Pitfalls to Avoid
- Insufficient volume: Most common error limiting diagnostic yield 1
- Missing concurrent plasma glucose: Makes CSF glucose interpretation impossible 1, 4
- Sending first tube for culture: Highest contamination risk 6
- Delaying transport: Cell counts deteriorate rapidly; analysis must occur within 2 hours 7, 5
- Not measuring opening pressure: Provides critical diagnostic information 1