Lumbar puncture
Check for reasons not to (raised pressure, clotting, infection at the site), position well, aim for the umbilicus at L3/4 or L4/5, and send the right bottles in the right order.
A 24-year-old woman has had fever, headache and neck stiffness for a day. GCS 15, no focal signs, no papilloedema, no rash. Your senior wants an LP. The nurse asks whether to wait for a CT and whether to hold antibiotics until the CSF is taken.
A second patient, 58, has a sudden severe headache that started 14 hours ago. CT head was normal at 8 hours.
When does a CT come first, should antibiotics wait, and what do you send in each case?
- Suspected meningitisBlood cultures, then antibiotics. Do not delay antibiotics for CT or LP.
- CT first ifGCS under 13 or falling, focal signs, papilloedema, new seizures, immunosuppressed, or very unwell.
- Check clotting and anticoagulantsPlatelets, INR, timing of the last anticoagulant dose.
- PositionLying on the side, knees to chest, back at the bed edge, shoulders and hips vertical.
- LevelL3–L4 or L4–L5 (the line between the iliac crests crosses L4).
- Opening pressure, then samples3–4 numbered tubes plus a paired blood glucose.
- AfterwardsFluids and simple analgesia. Bed rest does not prevent headache.
- Falling GCS or new focal signs
- Severe headache, leg weakness or numbness after LP (spinal haematoma)
- Opening pressure very high
- Purulent or bloody CSF
- Rash or shock in suspected meningitis
- Raised intracranial pressure before LP (coning risk)
- Coagulopathy or recent anticoagulant
- Traumatic tap mistaken for SAH
- Delaying antibiotics
- Post-LP headache
| Opening pressure (lying) | 10–20 cmH₂O (normal up to about 25 in adults) |
| CSF white cells | Under 5 per mm³ |
| CSF protein | 0.15–0.45 g/L (15–45 mg/dL) |
| CSF glucose | Over 60% of blood glucose |
| Xanthochromia | LP at least 12 hours after headache onset |
| Needle | 20–22G spinal needle, atraumatic if available |
| Volume | About 10–15 mL in adults (3–4 tubes) |
Check every dose against a current drug reference and your hospital protocol.
On this page
Indications
- Suspected meningitis or encephalitis (after antibiotics have started if needed).
- Suspected subarachnoid haemorrhage with a normal CT (especially if CT was more than 6 hours after onset).
- Guillain-Barré syndrome, idiopathic intracranial hypertension, some neurological diagnoses.
- Therapeutic: intrathecal drugs, drainage in idiopathic intracranial hypertension.
Before you start
- CT head before LP
- give antibiotics now
LP can go ahead without CT
- Signs of raised intracranial pressure or a mass lesion.
- Platelets under about 40,000/µL, INR above about 1.4, or recent anticoagulant (follow local timing).
- Skin infection at the site.
- Unstable patient (cardiovascular or respiratory).
- Spinal abnormality or previous spinal surgery (consider imaging guidance).
Technique
- ConsentExplain headache, back pain, failure, rare infection or bleeding.
- PositionLeft lateral, knees to chest, chin down, back at the bed edge, hips and shoulders vertical. Sitting forward is an alternative (opening pressure not valid).
- LandmarksLine between the top of the iliac crests crosses L4. Use L3–L4 or L4–L5 (below the end of the cord at L1–L2).
- Clean and drapeChlorhexidine, sterile gloves, gown and drape.
- AnaestheticLidocaine 1% in the skin and along the track.
- NeedleBevel parallel to the long axis of the spine, aim toward the umbilicus. Feel a give through the ligamentum flavum, then dura.
- CSFRemove the stylet; connect the manometer for opening pressure; collect 3–4 numbered tubes; replace stylet before withdrawing.
Layers passed: skin, subcutaneous fat, supraspinous ligament, interspinous ligament, ligamentum flavum, epidural space, dura and arachnoid, then subarachnoid space.
What to send
| Tube | Test |
|---|---|
| 1 | Microscopy, Gram stain, culture |
| 2 | Protein and glucose (with a paired blood glucose) |
| 3 | Cell count (compare with tube 1 if blood-stained) |
| 4 | Xanthochromia (protect from light) or extra tests: PCR (HSV, enterovirus, meningococcus, pneumococcus), AFB, TB culture, cryptococcal antigen, cytology |
Interpreting CSF
| Feature | Bacterial | Viral | TB |
|---|---|---|---|
| Appearance | Cloudy | Clear | Clear or fibrin web |
| White cells | Hundreds to thousands, neutrophils | 10–500, lymphocytes | 50–500, lymphocytes |
| Protein | High (above 1 g/L) | Normal or mildly high | High (often above 1 g/L) |
| Glucose ratio | Low (under 0.4) | Normal | Low |
| Opening pressure | High | Normal or mildly high | High |
Early bacterial and partly treated meningitis can look viral, and early TB meningitis can show neutrophils. Interpret in context.
SAH: xanthochromia (yellow bilirubin from broken-down red cells) on a sample taken at least 12 hours after onset. Red cells that fall from tube 1 to tube 4 suggest a traumatic tap, but don't rule out SAH.
Complications
| Complication | Features | Action |
|---|---|---|
| Post-LP headache | Worse upright, better lying, within 72 hours | Fluids, analgesia, caffeine; blood patch by anaesthetics if persistent |
| Back pain | At the site | Simple analgesia |
| Nerve root irritation | Shooting leg pain during the procedure | Withdraw slightly and redirect |
| Spinal haematoma | Severe back pain, leg weakness, numbness, bladder problems | Urgent MRI and surgeons |
| Infection | Fever, back pain, meningism days later | Investigate, antibiotics |
| Coning | Falling GCS, pupil changes | Resuscitate, senior, neurosurgery |
Treatment
See meningitis and headache.
Common mistakes
- Waiting for CT or LP before giving antibiotics in suspected meningitis.
- Skipping CT in a patient with focal signs or falling GCS.
- Forgetting the paired blood glucose.
- Sending xanthochromia from an LP done 4 hours after a thunderclap headache.
- Telling patients to lie flat for hours to prevent headache (it doesn't work).
- Going above L3 in adults.
Frequently asked questions
When is a CT needed before lumbar puncture?
When there is GCS under 13 or falling, focal neurology, papilloedema, new seizures, immunosuppression or shock.
At what level is a lumbar puncture done?
L3–L4 or L4–L5. The line joining the top of the iliac crests crosses L4.
What does CSF show in bacterial meningitis?
Cloudy fluid, high neutrophil count, high protein, low glucose ratio and raised opening pressure.
Why wait 12 hours for xanthochromia?
Red cells need time to break down to bilirubin, which gives the yellow colour. Earlier samples can be falsely negative.
Test yourself
Exam-style questions on this guide, from our two question banks. Choose an answer to see why each option is right or wrong.
1. A febrile patient with neck stiffness has GCS 11 and a new seizure. What before the LP?
Answer: CT head. Reduced GCS and seizure suggest raised pressure; antibiotics now, CT before LP.
- Nothing; LP now: Risk of herniation.
- EEG: No.
- MRI spine: No.
- Wait 24 hours: Antibiotics must not wait.
2. Which CSF pattern suggests bacterial meningitis?
Answer: Neutrophils, high protein, low glucose. Bacterial: cloudy, neutrophils, high protein, glucose ratio below 0.4.
- Lymphocytes, normal glucose: Viral.
- Normal cells and protein: Normal.
- Red cells only: Traumatic tap or SAH.
- Lymphocytes, very high protein, low glucose: TB.
3. At which level is an adult lumbar puncture done?
Answer: L4/L5. The cord ends around L1/L2; the iliac crest line crosses L4.
- T12/L1: Cord may be present.
- L1/L2: Too high.
- S2: No.
- C7: No.
4. A 19-year-old has fever, neck stiffness and photophobia. CSF shows turbid fluid, 1800 white cells/µL (90% neutrophils), protein 2.1 g/L and glucose ratio 0.2. What is the most likely cause?
Answer: Bacterial meningitis. Bacterial meningitis: turbid CSF, very high neutrophil count, high protein (> 1 g/L) and low glucose (CSF:plasma < 0.4). Viral: clear, lymphocytes, mildly raised protein, normal glucose. TB: lymphocytes, very high protein, low glucose, cobweb clot, subacute onset. Give IV ceftriaxone immediately (add ampicillin if > 50 years/immunosuppressed for Listeria), with dexamethasone before or with the first dose. Don’t delay antibiotics for imaging.
- Viral meningitis: Viral meningitis has lymphocytes and normal glucose.
- TB meningitis: TB meningitis has lymphocytes and a slower onset.
- Fungal meningitis: Fungal meningitis has lymphocytes and occurs in the immunosuppressed.
5. After a head injury, a man has clear watery fluid dripping from one nostril, worse when he leans forward, tasting salty. How is it confirmed?
Answer: β2-transferrin test. CSF rhinorrhoea: traumatic (anterior cranial fossa/cribriform plate, frontal/ethmoid/sphenoid sinus fractures, after sinus or skull base surgery) or spontaneous (raised ICP, obese women, empty sella). Features: unilateral clear watery discharge, positional (leaning forward, straining), salty taste, "halo" (double ring) sign on linen if mixed with blood, headache. Confirmation: β2-transferrin (specific to CSF) or β-trace protein; glucose testing is unreliable. Localisation: high-resolution CT skull base, CT/MR cisternography. Management: most post-traumatic leaks stop within 7–10 days with bed rest, head elevation, avoid nose-blowing/straining, stool softeners, ± lumbar drain; persistent leaks → endoscopic repair. Meningitis risk (pneumococcus) — pneumococcal vaccination; prophylactic antibiotics are not routinely recommended.
- Glucose dipstick: Glucose dipsticks are unreliable.
- Nasal swab culture: Culture does not confirm CSF.
- Pack the nose tightly: Tight packing does not confirm CSF and risks infection.
6. In a healthy adult about 500 mL of CSF is produced each day, yet the volume stays at about 150 mL. Through which structures does most CSF return to the blood?
Answer: Arachnoid granulations in dural sinuses. Arachnoid granulations (villi) are outpouchings of arachnoid that project through the dura into the dural venous sinuses, especially the superior sagittal sinus. CSF passes through them into venous blood in one direction, driven by the pressure difference between CSF and venous blood, so the whole volume is turned over three to four times a day. Some CSF also drains along cranial and spinal nerve sheaths to lymphatics. Blocked granulations, as after meningitis or haemorrhage, cause communicating hydrocephalus.
- Choroid plexus of the lateral ventricles: Produces CSF rather than absorbing it.
- Ependymal lining of the third ventricle: Lines the ventricles; not the main absorption site.
- Capillaries of the pia mater: Not the main route of CSF return.
- Central canal of the spinal cord: A tiny channel that does not absorb CSF.
7. An 8-month-old boy has a rapidly enlarging head, a tense fontanelle and vomiting. MRI shows a lobulated, intensely enhancing mass in the trigone of the left lateral ventricle with all four ventricles dilated and no obstruction to CSF flow seen at the foramina, aqueduct or fourth ventricle outlets. There is no history of meningitis or prematurity. Which of the following mechanisms best explains his hydrocephalus?
Answer: Overproduction of CSF by the tumour. A choroid plexus papilloma is a benign tumour of the choroid plexus epithelium. In infants it typically arises in the trigone of a lateral ventricle, and in adults in the fourth ventricle. The tumour secretes CSF at a high rate, so production exceeds absorption and all the ventricles enlarge even without a block, although blood or protein may also impair absorption. Complete surgical removal often cures the hydrocephalus.
- Block of the foramen of Monro: Would dilate only the lateral ventricles.
- Aqueduct compression: Would spare the fourth ventricle.
- Failure of the fourth ventricle exits: Possible with fourth ventricle tumours, not a lateral ventricle mass.
- Venous sinus thrombosis: Not suggested by the imaging.
References and dose sources
- Lidocaine (lignocaine) dosing: Drugs.com: Lidocaine dosage (see also our lidocaine (lignocaine) dose page)
- Ceftriaxone dosing: Drugs.com: Ceftriaxone dosage (see also our ceftriaxone dose page)
- Paracetamol dosing: Drugs.com: Acetaminophen (paracetamol) dosage (see also our paracetamol dose page)
International guidance is linked where it reflects good practice. Where your hospital’s protocol differs, follow your hospital.