CSF analysis from a 45-year-old immunocompetent patient with bacterial meningitis shows: WBC 2800 cells/µL (90% PMN), protein 350 mg/dL, glucose 20 mg/dL (simultaneous blood glucose 100 mg/dL — CSF:blood glucose ratio 0.2). Gram stain shows Gram-positive diplococci. Which test on CSF most rapidly confirms the specific aetiology without waiting for culture?
- A Latex agglutination test for bacterial antigens ✓
- B India ink preparation
- C CSF adenosine deaminase (ADA) level
- D Oligoclonal band electrophoresis
Explanation
Latex agglutination tests (LAT) on CSF detect polysaccharide antigens of common bacterial meningitis pathogens — Streptococcus pneumoniae, Neisseria meningitidis (groups A, B, C, Y, W135), Haemophilus influenzae type b, and Streptococcus agalactiae (Group B) — within 15–30 minutes, allowing rapid aetiological identification especially after antibiotics have been started (rendering culture negative). India ink is used to visualize Cryptococcus neoformans capsule (not bacterial meningitis). CSF ADA is elevated in tuberculous meningitis. Oligoclonal bands assess demyelinating disease. The Gram-positive diplococci pattern strongly suggests S. pneumoniae.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.