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PE27.13-14 | CSF Analysis and Lumbar Puncture — Summary & Reflection
KEY TAKEAWAYS
This module has built the two competencies required by NMC CBUC 2024 for PE27.13 and PE27.14:
Lumbar puncture technique (PE27.14):
• LP is indicated for suspected meningitis (bacterial/viral/TB/fungal), SAH, idiopathic intracranial hypertension, and intrathecal therapy
• Contraindications: raised ICP with focal signs/papilloedema (→ CT first), coagulopathy (platelets <50,000 or INR >1.5), local infection, cardiorespiratory instability
• Safe interspace: L3-L4 or L4-L5, identified by Tuffier's line (iliac crest = L4); below the conus medullaris (L1-L2 adults, L2-L3 neonates)
• Position: lateral decubitus knee-chest (standard); sitting for neonates
• Technique: aseptic → two resistances (ligamentum flavum, dura) → subarachnoid space → manometry (normal 70-180 mmH2O) → sequentially numbered tubes → replace stylet before withdrawal
• Commonest complication: positional PLPHA — managed by hydration, analgesics, and epidural blood patch if refractory
CSF interpretation (PE27.13):
| Parameter | Normal | Bacterial | Viral | TB | Fungal |
|---|---|---|---|---|---|
| Appearance | Clear | Turbid | Clear | Clear/xanthochromic | Clear |
| Pressure (mmH2O) | 70-180 | ↑↑ (>300) | Normal/slight↑ | ↑ | ↑↑ |
| WBC/mm³ (diff) | <5 (lymph) | 100s-1000s (PMN) | <500 (lymph) | <500 (lymph) | <500 (lymph) |
| Protein (mg/dL) | 20-45 | >100 | 50-100 | >100-500 | Elevated |
| Glucose ratio | ≥0.6 | <0.4 | ≥0.6 (normal) | <0.4 | <0.4 |
| Special | — | Gram stain/culture | PCR enterovirus/HSV | AFB, pellicle, MTB PCR | India ink, CrAg |
- Traumatic tap: decreasing RBC tube 1→3, clear supernatant (vs SAH: uniform RBC, xanthochromic supernatant)
- Always send simultaneous blood glucose with CSF; interpret ratio, not absolute CSF glucose alone
REFLECT
Kolb's experiential learning cycle asks you to move from experience to reflection to conceptualisation to active experimentation. After completing this module, take 5 minutes to answer these questions in writing:
- Concrete experience: Have you observed an LP being performed during your paediatric posting? What did you notice about the child's positioning and the clinician's actions? If you have not yet observed one, describe what you anticipate will be the most technically challenging step.
- Reflective observation: In the hook scenario (4-year-old with anisocoria and suspected meningitis), a junior resident immediately prepared the LP tray. What was the error in their reasoning, and what would you have done differently?
- Abstract conceptualisation: The CSF glucose ratio cutoff of 0.4 is central to distinguishing bacterial from viral meningitis. Why does the ratio matter more than the absolute CSF glucose value alone? When might a 'normal' absolute glucose of 50 mg/dL actually represent a low glucose?
- Active experimentation: After this module, what one change will you make in how you evaluate a child with suspected meningitis before deciding on LP? Commit to a specific clinical habit — e.g., 'I will always check for papilloedema and pupil symmetry before LP and document this in my notes.'