What is the proper interpretation and management of cerebrospinal fluid (CSF) on lumbar puncture (LP)?

Proper Interpretation and Management of CSF on Lumbar Puncture

Proper interpretation of CSF findings from lumbar puncture requires systematic analysis of cell count, glucose, protein, and opening pressure, with specific patterns distinguishing bacterial from viral meningitis and other neurological conditions. 1, 2

When to Perform LP and Contraindications

Indications for LP:

• Suspected central nervous system infections (meningitis/encephalitis)
• Subarachnoid hemorrhage when CT is negative
• Suspected inflammatory or demyelinating conditions

Contraindications (delay LP if present):

• Signs of severe sepsis or rapidly evolving rash
• Respiratory or cardiac compromise
• Anticoagulant therapy/thrombocytopenia (<40-50 × 10⁹/L)
• Infection at the LP site
• Focal neurological signs (requires neuroimaging first)
• Papilledema (requires neuroimaging first)
• Continuous or uncontrolled seizures
• GCS ≤12 (requires neuroimaging first) 1, 2

Important: Never delay antimicrobial therapy in suspected meningitis while awaiting CSF collection. LP may still be diagnostically useful even after several days of treatment. 1, 2

Procedural Considerations

Optimal Technique:

• Use atraumatic (pencil-point) needles to reduce post-LP headache (4.2% vs 11.0% with cutting needles)
• Use small-bore needles (≥22G) to balance reduced complications with procedural ease
• Position patient sitting with flexed hips or lateral recumbent position
• Orient needle bevel in transverse plane
• Replace stylet before withdrawal
• Minimize number of attempts 1, 2

CSF Collection:

• Measure opening pressure (unless in sitting position)
• Collect adequate volume (up to 22 ml can be safely removed)
• Use sequential tubes for "three tube test" when traumatic tap is suspected 1, 3

CSF Interpretation

Key Parameters to Analyze:

1. Opening pressure:

   • Normal: <20 cm H₂O
   • Elevated in bacterial meningitis and other conditions with increased ICP

2. Cell count and differential:

   • Normal: <5 WBC/mm³
   • Bacterial meningitis: Typically >1000 WBC/mm³, neutrophil predominance
   • Viral meningitis: Typically 100-1000 WBC/mm³, lymphocyte predominance (though early viral may show neutrophils)
   • Note: 10% of bacterial meningitis cases may have <100 cells/mm³ 1

3. Glucose:

   • Normal: >2.6 mmol/L (>45 mg/dL)
   • CSF:plasma glucose ratio normally ~0.6
   • Bacterial meningitis: Typically <35 mg/dL, ratio <0.23
   • Always measure concurrent plasma glucose 1, 2

4. Protein:

   • Normal: <0.45 g/L (<45 mg/dL)
   • Bacterial meningitis: Typically >220 mg/dL
   • Viral meningitis: Mildly elevated, usually <0.6 g/L 1, 2

5. Lactate:

   • Bacterial meningitis: >35 mg/dL (high sensitivity and specificity if antibiotics not yet given)
   • Useful negative predictor to rule out bacterial meningitis 1

Distinguishing Patterns:

Parameter	Bacterial Meningitis	Viral Meningitis	Normal
WBC	>1000/mm³ (PMN)	100-1000/mm³ (lymphocytes)	<5/mm³
Glucose	<35 mg/dL	Normal/slightly low	>45 mg/dL
CSF:plasma glucose	<0.23	>0.5	~0.6
Protein	>220 mg/dL	<150 mg/dL	<45 mg/dL
Lactate	>35 mg/dL	<35 mg/dL	<20 mg/dL

Special Considerations

Traumatic LP vs. Subarachnoid Hemorrhage:

• Use "three tube test" - clearing of RBCs in sequential tubes suggests traumatic tap
• Check for xanthochromia (yellow discoloration) - present in true SAH after 12 hours
• Measure RBC:WBC ratio - should remain constant in true SAH 3

Ventricular vs. Lumbar CSF:

• EVD samples typically have lower WBC counts and protein than LP samples
• Consider repeating LP before discontinuing antibiotics when monitoring via EVD 4

Post-LP Headache Management:

• Not prevented by bed rest, hydration, or reduced CSF volume
• Treatment: hydration, caffeine, analgesics
• For severe cases: epidural blood patch 1, 2

Common Pitfalls to Avoid

1. Delaying antibiotics to obtain CSF in suspected meningitis
2. Misinterpreting partially treated bacterial meningitis (may show lymphocyte predominance)
3. Failing to measure concurrent plasma glucose for accurate interpretation
4. Collecting inadequate CSF volume for complete testing
5. Missing early bacterial meningitis with minimal cell count elevation
6. Assuming EVD samples are equivalent to LP samples when monitoring treatment response 1, 2, 4

Remember that CSF findings must always be interpreted in the context of the clinical presentation, as no single parameter provides absolute diagnostic certainty.