Examqo › Examqo library › Neurology › Localisation and diagnostics · IMLE
Lumbar puncture and CSF interpretation
Lumbar puncture is a core diagnostic test for infections, subarachnoid haemorrhage and inflammatory diseases — and every interpretation rests on 4 numbers: opening pressure, cells (and their type), protein, and glucose relative to blood glucose.
Why it matters in the exam
Questions present CSF results and ask you to identify bacterial, viral, tuberculous or fungal meningitis, GBS, subarachnoid haemorrhage or multiple sclerosis. They also test when CT must precede the puncture (immunosuppression, central nervous system disease, new seizure, papilloedema, reduced consciousness, focal deficit), contraindications (mass effect, coagulopathy, anticoagulants, skin infection at the site), and treatment of post-puncture headache. A classic distractor is delaying antibiotics in suspected meningitis until after CT and puncture — blood cultures and antibiotics come first. Another point is a "traumatic" tap, in which the red cell count falls from the first tube to the last, unlike subarachnoid haemorrhage. Further CSF tests by suspicion: Gram stain and culture, viral PCR, cryptococcal antigen, oligoclonal bands and antibodies. Opening pressure is measured with the patient lying on the side with legs extended, because sitting or flexion changes the value.
Key points
- Interpret by 4 numbers: pressure, cells, protein and glucose.
- Bacterial: neutrophils, high protein and low glucose; viral: lymphocytes and normal glucose.
- Tuberculosis and fungus: lymphocytes, very high protein and low glucose.
- CT before puncture with immunosuppression, new seizure, papilloedema, reduced consciousness or focal deficit.
- Antibiotics are never delayed for CT or puncture.
Summary table
| Condition | Cells | Protein | Glucose (ratio to blood) |
|---|---|---|---|
| Normal | Up to 5 per cubic millimetre, lymphocytes | 15 to 45 mg/dL | About two thirds of blood glucose (ratio above 0.6); opening pressure 10 to 20 cm water |
| Bacterial meningitis | Hundreds to thousands, neutrophils | High | Low (ratio below 0.4) |
| Viral meningitis | Tens to hundreds, lymphocytes | Normal or slightly raised | Normal |
| Tuberculosis or fungus | Tens to hundreds, lymphocytes | Very high | Low |
| GBS | Normal (under 50) | High | Normal |
| Subarachnoid haemorrhage | Red cells that do not fall between tubes | High | Normal; xanthochromia |
In the full version for subscribers
- A simple algorithm
- Memory aid
- Exam traps
- 5 review cards
- Enough for IMLE
The full version is open to subscribers of: Neurology, Medical licensing exam (IMLE), Internal medicine, Emergency medicine, Pediatrics.
Subscriber? Read the full topic Plans and prices 25 free questions
Sources
- IDSA Practice Guidelines for the Management of Bacterial Meningitis, 2004 — CT before lumbar puncture and CSF interpretation
- Post-Dural Puncture Headache: Consensus Practice Guidelines (Uppal et al., JAMA Network Open), 2023 — Prevention and treatment
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.