Lumbar Puncture and CSF Interpretation
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Direct answer
Lumbar puncture is the sampling of cerebrospinal fluid from the lumbar subarachnoid space, and interpretation rests on the pattern of cell count, protein and glucose taken together. Bacterial meningitis gives a neutrophil-predominant pleocytosis with high protein and a CSF glucose below 40 percent of the blood value, while viral meningitis gives lymphocytes with normal glucose. Tuberculous and cryptococcal meningitis produce lymphocytosis with low glucose, so they can mimic viral disease, and special stains and tests decide between them.
What you must remember
- Normal CSF: clear, fewer than 5 lymphocytes per cubic mm, protein roughly 15 to 45 mg/dL, and glucose about two-thirds of the blood value; normal opening pressure is about 6 to 20 cm of water.
- Bacterial pattern: neutrophil-predominant pleocytosis, often in the hundreds to thousands per cubic mm, with raised protein and low glucose; Gram stain and culture of CSF accompany blood cultures.
- Viral pattern: lymphocytic pleocytosis, mildly raised protein and normal glucose, typical of enterovirus; herpes simplex encephalitis needs separate PCR testing.
- Tuberculous pattern: lymphocytosis, often high protein, low glucose and a fibrin web on standing; CBNAAT or PCR and adenosine deaminase support the diagnosis, and a negative smear does not exclude it.
- Cryptococcal pattern: lymphocytic pleocytosis with low glucose; serum or CSF cryptococcal antigen and India ink staining are the quick tests.
- Contraindications: raised intracranial pressure with mass effect, focal neurological signs, platelets below about 50,000 per microlitre or uncorrected coagulopathy, and local skin infection at the site.
A worked case: fever, neck stiffness and a clear CT
A 28-year-old man presents with fever, headache and neck stiffness, and the non-contrast CT is normal without mass effect. The lumbar puncture shows 1,200 cells per cubic mm with 90 percent neutrophils, protein of 180 mg/dL and CSF glucose of 20 mg/dL against a blood glucose of 120 mg/dL, a ratio near 0.17.
- Read the pattern: A neutrophil-predominant picture with a very low glucose ratio is bacterial until proven otherwise.
- Take cultures: Blood cultures are taken, and empirical therapy begins without waiting for culture.
- Add steroids: Dexamethasone is given with or before the first antibiotic dose.
A contrasting case is a 40-year-old woman with three weeks of low-grade fever and headache, whose CSF shows 150 lymphocytes, protein of 90 mg/dL and glucose of 35 mg/dL against a blood value of 110. A lymphocytic pattern with that low ratio should trigger acid-fast bacilli staining, CBNAAT, adenosine deaminase and cryptococcal antigen testing before any viral label is accepted.
A third case shows why the sequence matters: a patient with a space-occupying lesion and focal weakness needs imaging and expert review before any puncture, and blood cultures with empirical antibiotics come first.
Where the examiner sets the trap
Several traps recur in CSF questions.
- Trap: Reading a partially treated bacterial meningitis as viral. Correct: After a few doses of antibiotics the CSF can turn lymphocytic with normal glucose, so the clinical story still points to bacteria.
- Trap: Mistaking a traumatic tap for subarachnoid haemorrhage. Correct: Red cells fall across successive tubes in a traumatic tap, whereas subarachnoid haemorrhage gives persistent blood and xanthochromia that appears after several hours.
- Trap: Dismissing tuberculous meningitis because the acid-fast smear is negative. Correct: The bacillary load in CSF is often low, so a negative smear does not exclude it.
- Trap: Judging CSF glucose by its absolute value. Correct: Glucose depends on the blood level at the time of the tap, which is why the ratio matters more than the absolute value.
Frequently asked questions
What CSF pattern suggests bacterial meningitis?
A neutrophil-predominant pleocytosis with high protein and low glucose, giving a CSF to blood glucose ratio under 0.4, suggests bacterial meningitis. Gram stain and culture of the CSF, together with blood cultures, complete the initial workup.
Why is a lymphocytic CSF with low glucose not always viral?
Tuberculous and cryptococcal meningitis produce a lymphocytic pleocytosis with low glucose and sometimes high protein, so they can mimic viral meningitis. Acid-fast bacilli staining, CBNAAT, adenosine deaminase and cryptococcal antigen testing help separate them.
What are the main contraindications to lumbar puncture?
Raised intracranial pressure with mass effect, focal neurological signs suggesting a mass, and significant coagulopathy or low platelet counts are the main contraindications to lumbar puncture. Local skin infection at the puncture site also precludes the procedure, and imaging is needed first when these concerns arise.
Why does xanthochromia matter in CSF interpretation?
Xanthochromia, the yellow colour of CSF from bilirubin, persists after subarachnoid haemorrhage for several hours, whereas a traumatic tap clears across successive tubes. Its presence therefore helps distinguish true subarachnoid haemorrhage from blood introduced by the puncture.