CSF Findings in Pyogenic, Tuberculous, Fungal and Carcinomatous Meningitis

Normal CSF and pre-requisites. Opening pressure 8–18 cm H O; clear and colourless; 0–5 lymphocytes/mm³; 2 protein 15–45 mg/dL; glucose 45–80 mg/dL, or at least 60% of a simultaneous blood glucose — always send a paired blood sugar, or the CSF glucose cannot be interpreted. Contraindications to lumbar puncture: raised intracranial pressure with focal signs or papilloedema, coagulopathy or anticoagulation, and local sepsis. CT brain before LP in any older, immunocompromised, seizing or focally abnormal patient.

The comparison table — the core of the answer. See table below.

Pyogenic (bacterial) meningitis. Turbid fluid at high pressure; neutrophilic pleocytosis, typically 1000– 5000/mm³; protein 100–500 mg/dL; glucose markedly reduced (<40% of blood glucose); raised CSF lactate (>3.5 mmol/L); Gram stain positive in 60–90% before antibiotics; culture and multiplex PCR. Partially treated meningitis shifts towards a lymphocytic picture with a less dramatic fall in glucose, and is a common diagnostic trap.

Tuberculous meningitis. Clear or ground-glass fluid at high pressure, forming a cobweb (spider-web) clot on standing; lymphocytic pleocytosis of 100–500/mm³ (neutrophils may predominate in the first few days); protein markedly raised, 100–500 mg/dL, and may exceed 1 g/dL with spinal block (Froin’s syndrome); glucose low; adenosine deaminase raised. AFB smear has a low yield; Xpert MTB/RIF Ultra is the recommended rapid test; culture takes weeks.

Fungal (especially cryptococcal) meningitis. Clear fluid, often at a very high opening pressure requiring therapeutic drainage; lymphocytic pleocytosis of 20–500/mm³, which may be minimal in the profoundly immunosuppressed; protein raised; glucose low; India ink positive in about 50–70%; and CSF cryptococcal antigen (CrAg) is the test of choice with sensitivity above 95%. Fungal culture confirms.

Carcinomatous (neoplastic/leptomeningeal) meningitis. Clear or xanthochromic; mononuclear pleocytosis, usually modest (<100/mm³); protein raised; and glucose low (hypoglycorrhachia) — making it the great mimic of tuberculous meningitis. Diagnosis rests on CSF cytology from repeated large-volume samples (a single sample is only about 50% sensitive; three samples approach 90%), flow cytometry for haematological malignancy, and contrast MRI of the whole neuraxis showing leptomeningeal enhancement.

Two diagnostic and therapeutic changes with direct Indian relevance. (i) WHO now recommends Xpert MTB/RIF Ultra as the initial diagnostic test on CSF for suspected tuberculous meningitis, in preference to smear microscopy and culture, and India’s NTEP has scaled up Ultra and Truenat testing accordingly. (ii) For cryptococcal disease, WHO guidance (2022, implemented through 2023–2025) recommends the CrAg lateral flow assay as the preferred rapid test and a single high-dose liposomal amphotericin B (10 mg/kg) plus flucytosine and fluconazole induction regimen, based on the AMBITION-cm trial — markedly less toxic and more feasible than 7–14 days of conventional amphotericin in resource-limited settings.