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TBM Negative PCR — SCE Infectious Diseases MCQ

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ModerateTuberculosisTBM Negative PCRSCE Infectious Diseases

A 48-year-old man presents with chronic meningoencephalitis. CSF shows: WCC 120/µL (lymphocytic), protein 3.2 g/L, glucose 0.6 mmol/L (serum 5.8). CSF AFB smear is negative. CSF TB PCR is negative. CSF adenosine deaminase (ADA) is 12 U/L (reference <6). CT chest shows miliary infiltrates. What is the most likely diagnosis despite negative CSF PCR?

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Correct answer: C — Tuberculous meningitis — CSF TB PCR sensitivity is only 50–80%; negative PCR does not exclude TBM in a compatible clinical picture

TB meningitis (TBM) remains the most likely diagnosis given: lymphocytic CSF with very low glucose, high protein, elevated ADA, and miliary chest infiltrates. CSF GeneXpert/TB PCR has sensitivity of only 50–80% for TBM (much lower than for pulmonary TB) due to the paucibacillary nature of CSF in TBM. CSF volume (>6 mL), repeated lumbar punctures, and centrifuged specimens improve yield. A negative PCR does NOT exclude TBM — treatment should be started empirically if clinical suspicion is high.

Reference: NICE NG33 2016 – TB; BTS 2024 – TB meningitis; Lancet ID 2019 – TBM diagnostics