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PE27.3 | Meningitis Differentiation — SDL Guide (Part 3)
Self-Assessment
The three CSF profiles below require you to apply the complete five-step interpretation method and reach a diagnostic conclusion with justification. Write out your reasoning for each step before deciding on the diagnosis and initial management. This deliberate practice is more valuable than simply matching the profile to a memorised table.
Profile 1: 18-month-old, 2-day fever and irritability, no prior antibiotics. CSF: turbid; WBC 6,800/mm³ (94% neutrophils); protein 320 mg/dL; glucose 12 mg/dL; serum glucose 90 mg/dL.
Profile 2: 10-year-old, 4-week fever and weight loss, known TB contact. CSF: clear; WBC 150/mm³ (86% lymphocytes); protein 450 mg/dL; glucose 18 mg/dL; serum glucose 80 mg/dL. Mantoux 14 mm.
Profile 3: 6-year-old, 3-day fever and headache, recent coxsackievirus in class. CSF: clear; WBC 95/mm³ (90% lymphocytes); protein 58 mg/dL; glucose 68 mg/dL; serum glucose 92 mg/dL.
Answers:
1. Bacterial meningitis (turbid, 94% neutrophils, glucose ratio 12/90 = 0.13, very high protein). Empirical: ceftriaxone + vancomycin + dexamethasone.
2. TBM Stage I (clear, lymphocytic, protein 450 mg/dL = very high, glucose ratio 18/80 = 0.23 = low; subacute tempo; TB contact; Mantoux positive). Start 2HRZE + 10HR + dexamethasone.
3. Viral meningitis (clear, lymphocytic, protein mildly elevated, glucose ratio 68/92 = 0.74 = normal; acute tempo; recent viral contact). Supportive care; no antibiotics.
SELF-CHECK
The single most useful CSF parameter for distinguishing viral meningitis from tuberculous meningitis when both present with lymphocytic pleocytosis is:
A. Total WBC count
B. CSF appearance (clear vs turbid)
C. CSF glucose (CSF:serum ratio)
D. CSF protein level
Reveal Answer
Answer: C. CSF glucose (CSF:serum ratio)
When both viral and TBM show lymphocytic pleocytosis, the CSF glucose is the most powerful single differentiator. Viral meningitis preserves CSF glucose (ratio typically >0.60) because the inflammatory response is mild and glucose transport across the BBB is maintained. TBM causes marked glucose consumption (ratio typically <0.40, often extremely low). Total WBC count and appearance overlap considerably between viral and early TBM. Protein is elevated in both, though more markedly in TBM — but is less reliable as a single differentiator than glucose.