Paper I — 2021 May (Supplementary) (2010 Scheme) — Q3
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Paper I
2021 May (Supplementary) (2010 Scheme) · 40 marks · 120 min
Question
Laboratory diagnosis of bacterial meningitis
Q35 marksShort Essays
Answer
Laboratory diagnosis of bacterial (pyogenic) meningitis relies mainly on CSF examination:
CSF examination: typically shows raised opening pressure, turbid/cloudy appearance, neutrophilic pleocytosis, raised protein, and markedly reduced glucose (low CSF:blood glucose ratio) — distinguishing it from the lymphocytic pleocytosis with normal glucose typical of viral meningitis.
Gram stain of CSF (after centrifugation/cytospin) — rapid presumptive identification: Gram-positive diplococci (Streptococcus pneumoniae), Gram-negative diplococci (Neisseria meningitidis), or small Gram-negative coccobacilli (Haemophilus influenzae); Gram-positive bacilli (Listeria monocytogenes) or Gram-negative bacilli (E. coli) in neonates.
CSF culture — the definitive diagnostic test; CSF plated on blood agar and chocolate agar, incubated in a CO2-enriched atmosphere; allows antibiotic susceptibility testing.
Latex agglutination test — rapid detection of bacterial capsular antigen directly in CSF, particularly useful when the patient has already received antibiotics (culture may be falsely sterile).
Blood culture — often positive concurrently, since bacterial meningitis is usually preceded by bacteraemia.
PCR — increasingly used for rapid, sensitive detection, particularly useful after antibiotics have been started.
Sample handling: CSF must be transported and processed promptly (some organisms, e.g., N. meningitidis, are fastidious and lose viability quickly outside the host); if delay is unavoidable, CSF for culture should be kept at room temperature rather than refrigerated (unlike urine).
