Lumbar puncture and CSF interpretation

• A lumbar puncture (LP), or spinal tap, samples cerebrospinal fluid (CSF, the clear fluid that cushions the brain and spinal cord) from the spinal canal in the lower back.
• It is the central test for meningitis, subarachnoid haemorrhage that imaging does not settle, and many inflammatory or demyelinating diseases of the nervous system.
• The safest LPs follow a short checklist first: look for raised intracranial pressure and bleeding risk, and get a head scan when either is suggested.
• Opening pressure and basic CSF numbers (cells, protein, glucose) are interpreted together; no single value diagnoses on its own.
• The commonest complication is post-lumbar puncture headache; using a thin pencil-point (atraumatic) needle reduces it. ^[4]
• Removing 10 to 20 mL of CSF is enough for most tests; the body replaces it within about an hour.

By the end of this page you should be able to:

1. List the indications for lumbar puncture and the situations in which it should be deferred.
2. Decide when a head scan is needed before lumbar puncture, using the recognised warning signs.
3. Describe the equipment, positioning, needle insertion and manometry steps of a lumbar puncture, and the aftercare.
4. State normal CSF values and abnormal opening-pressure thresholds, and explain what each abnormality means.
5. Read a CSF table and distinguish bacterial, viral and cryptococcal meningitis, subarachnoid haemorrhage, multiple sclerosis and Guillain-Barre syndrome.
6. Name the serious and common complications of lumbar puncture and how to reduce them.

Lumbar puncture is both diagnostic and, less often, therapeutic. Diagnostic use means obtaining CSF to look for infection, blood, abnormal cells and immune markers, and measuring the pressure that the fluid is under. Therapeutic use means removing fluid to relieve pressure (as in idiopathic intracranial hypertension), delivering a drug into the fluid space (intrathecal chemotherapy, analgesics), or instilling contrast for imaging. The needle enters the spinal canal below the end of the spinal cord, so with correct technique it is one of the safest procedures in neurology. The important discipline is to prepare well and to read the CSF as a pattern rather than as isolated numbers.

The first decision is whether to do the tap at all. The second is whether imaging must come first. Both are covered before technique. Technique is then followed by interpretation, because a well taken sample is only useful if it is read in the right order.

The diagnostic indications for lumbar puncture include: ^[5]

• Suspected central nervous system infection: acute bacterial, viral or fungal meningitis, encephalitis, subacute or chronic meningitis
• Suspected inflammatory or demyelinating disease of the central nervous system, including multiple sclerosis, neuromyelitis optica, myelin oligodendrocyte glycoprotein antibody associated disease, autoimmune encephalitis and neurosarcoidosis
• Suspected spontaneous subarachnoid haemorrhage not diagnosed on neuroimaging, or when immediate neuroimaging is unavailable
• Suspected idiopathic intracranial hypertension, to measure CSF pressure with a manometer
• Guillain-Barre syndrome and other polyneuropathies, leptomeningeal metastasis (carcinomatous meningitis), and selected genetic or metabolic encephalopathies
• Administration of radiopaque contrast for myelography (now rarely needed)

Therapeutic indications include reduction of intracranial pressure in idiopathic intracranial hypertension, administration of intrathecal medications, and relief of symptoms in normal pressure hydrocephalus when a large therapeutic tap is planned. ^[5]

StatPearls lists a broader, overlapping set of indications for sampling CSF: infection, malignancy and inflammatory or demyelinating disorders; also intrathecal drug administration, myelography and cisternography, blood patch for a CSF leak, baclofen pump trials and lumbar shunt procedures. ^[1] The two lists agree on the common ones; the difference is mainly breadth.

Urgency matters as much as indication. In suspected bacterial meningitis the tap (or the first dose of antibiotics, if the tap must wait) should not be delayed: the mortality of bacterial meningitis is 14 to 25 percent in adults. ^[2]

There is one absolute contraindication: infection of the skin at or near the planned insertion site, such as cellulitis or a local abscess. ^[5] If possible, use an uninfected lumbar site. If the whole lumbar region is unavailable, a fluoroscopically guided cervical or suboccipital (cisternal) puncture is the alternative.

Relative contraindications are situations in which the LP may be dangerous or may yield nothing, and they are managed rather than obeyed blindly. ^[5]

• Suspicion of raised intracranial pressure due to an intracranial mass, especially an infratentorial one, that could cause transtentorial or cerebellar herniation after the tap. Focal neurological deficits, altered mental status, papilledema or signs of herniation suggest this; in such cases do CT or MRI before the LP.
• Coagulopathy: an international normalised ratio above 1.5 (including therapeutic anticoagulation), thrombocytopenia below 50,000 per microlitre, or a pathologically raised activated partial thromboplastin time. Correct first where possible, and balance any reversal against thrombosis risk. ^[5]
• Cardiopulmonary insufficiency or respiratory distress, which recumbent positioning can worsen.
• Anatomic abnormality at the site, such as spinal fusion, laminectomy or congenital anomalies.

Bacteraemia does not predispose to meningitis after lumbar puncture, so it is not a contraindication. ^[5] When an LP is deferred, treat suspected bacterial meningitis or subarachnoid haemorrhage immediately, taking blood cultures first in suspected meningitis. ^[5]

Imaging before puncture is the area where mistakes happen. Older teaching blamed the LP itself for causing herniation in bacterial meningitis; current consensus is that raised pressure, not the needle, is the danger, and that LP should be skipped when herniation is suspected. ^[6]

The StatPearls meningitis article lists signs of impending herniation that should make you stop and scan: Glasgow coma scale score below 11, lethargy, altered mental status, new onset seizures, or focal neurological deficit. ^[6] A normal head CT does not preclude raised pressure or impending herniation. ^[6]

The following flowchart puts the decision in one place. It is a teaching summary and does not replace local protocols or senior advice.

A separate, imaging-based review of lumbar puncture safety was published in 2026 (Mayo Clinic Proceedings) but sits outside the scope of this page. Its message is the same: the listed brain imaging findings that contraindicate LP include supratentorial mass lesions with pre-existing herniation or significant midline shift, medium to large posterior fossa masses, posterior fossa malformation, globally elevated pressure with mass effect or generalised oedema, severe brain sag and obstructive hydrocephalus.

The spinal cord ends at about T12 to L1. The needle therefore enters below L1, almost always at the L3-L4 or L4-L5 interspace and occasionally at L2-L3. The L4 spinous process is the one that lies on a line drawn between the tops of the iliac crests. Passing through the midline, the needle traverses the supraspinous and interspinous ligaments, the ligamentum flavum, the epidural space and finally the dura and arachnoid together into the subarachnoid space. ^[5] A paramedian (lateral) approach, about a finger's breadth to one side and slightly caudad, avoids calcified midline ligaments in older patients. ^[5] Puncture above L2 risks direct spinal cord injury, which is why level selection is not negotiable.

The equipment list below is taken from the MSD Manual step-by-step account. ^[5] Where prepacked kits are unavailable, assemble:

• Sterile gloves, gown, mask and cap; sterile drapes; antiseptic solution (chlorhexidine, povidone-iodine or alcohol)
• Local anaesthetic: 1 or 2 percent lidocaine without epinephrine, 20 and 25-gauge needles, 10 mL syringe ^[5]
• Spinal needle with stylet: cutting (bevelled-tip) or non-cutting (pencil-point, atraumatic) needle, 20 or 22 gauge, 9 cm for adults (6 cm children, 4 cm infants) ^[5]
• Four numbered CSF collection tubes, plus extra tubes for large volume or special tests
• Manometer and stopcock, with optional short extension tubing to absorb patient movement
• Adhesive dressing, sterile gauze

For infants, topical skin anaesthetic is standard, and a high-frequency ultrasound probe can be used in teaching centres to locate landmarks. ^[5] Four tubes of 1 to 2 mL each suffice for most tests, but tuberculosis, fungal and carcinomatous meningitis may need 30 to 40 mL. ^[5] Draw blood for glucose (and for oligoclonal band comparison) about 30 minutes before or at the time of the LP, so the CSF glucose can be compared with serum glucose.

The choice of needle matters more than most novices realise. Use of non-cutting (pencil-point) needles and the smaller 22-gauge calibre reduces post-lumbar puncture headache, as the American Academy of Neurology (AAN) concluded for adult patients (Level A recommendation) in its now retired 2005 practice parameter. ^[4] The original AAN document is retired, so its recommendations are historical; modern systematic reviews have if anything strengthened the point.

Explain the procedure, risks and aftercare to the patient (or carers) and obtain consent. ^[1] Review indications and contraindications, check platelet count and coagulation where relevant, and verify whether a scan is required. ^[1] Decide the tests in advance and confirm any unusual ones with the laboratory. Lie the patient flat and side-on before manometry; the sitting position can be used when landmarks are hard to palpate (for example in obesity), but opening pressure cannot be measured sitting. ^[5]

The steps below are written for bedside performance by a clinician using landmarks, which is still the commonest method. ^[5]

1. Position. Lateral decubitus, fetal posture, with hips, back and shoulders perpendicular to the bed; a pillow under the head aligns head with spine. Sitting, leaning forward over a pillow, helps when landmarks are difficult. In infants, an assistant holds the sitting fetal position.
2. Identify the site. Palpate the spinous processes; the one on the line between the iliac crests is usually L4, giving an L3-L4 insertion point. Mark the skin.
3. Prepare. Expand clean, allow antiseptic to dry for at least one minute, then wipe off iodine or chlorhexidine with alcohol. Drape, glove, and assemble manometer, stopcock and tubing. ^[5]
4. Anaesthetise. Raise a wheal with 25-gauge needle, then infiltrate deeper along the needle track.
5. Insert. Fully seat the stylet whenever advancing or withdrawing. If using a bevelled (cutting) needle, orient the bevel to part the dural fibres (facing up in the lateral position). Direct the needle toward the umbilicus, about 15 degrees cephalad, advancing 2 to 3 mm at a time and checking for CSF return between steps. A "pop" may be felt through ligamentum flavum and sometimes through dura; it is often absent in children. If bone is met, withdraw to subcutaneous level and aim more cephalad. If blood returns, withdraw 1 mm at a time. If no CSF is obtained despite apparent entry, rotate the needle 90 degrees.
6. Measure pressure. Attach the manometer with the stopcock open and hold it level with the needle. Read the height when CSF rises and fluctuates with respiration. Normal fluid column height is 7 to 18 cm. If flow is sluggish, straighten the legs slightly or rotate the needle.
7. Collect. Remove the stylet and let CSF drip; never aspirate. Drain the manometer contents into the first tube. Then allow about 1 to 2 mL into each of four tubes. Do cell counts on tubes 1 and 3 and chemistry and microbiology on tube 2, saving tube 4 frozen if extra unanticipated tests become necessary. ^[5]
8. Troubleshooting a bloody tap. If CSF is pink or bloody, collect 10 to 20 mL until it clears. If it does not clear, mark the tap as bloody; blood can interfere with results and the procedure may have to be repeated.
9. Conclude. Partially reinsert the stylet, withdraw the needle gently, apply a dressing.

Aftercare and fluid removal advice follow in the next sections.

Opening pressure is the pressure of the CSF at rest, read before any fluid is removed. It is the first measurement taken and should be obtained with the patient lying flat on one side, legs and neck in a neutral position, because abdominal compression from flexed hips falsely raises it. ^[2] A normal opening pressure in adults and children aged 8 years or over is 60 to 250 mm of water (about 6 to 25 cm); in children under 8 years it is 10 to 100 mm of water. ^[2] The MSD Manual gives the normal height as 7 to 18 cm, equivalent to about 70 to 180 mm, which is narrower and typical of many procedural texts; both are reproduced here so that you are not surprised when two references disagree. ^[5]

During the procedure the manometer should remain in place in a patient with a raised opening pressure, monitoring continuously, and no further fluid should be removed after the pressure falls by half. ^[2]

Interpretation:

• Raised (above 250 mm of water in adults): infection, intracranial mass, haemorrhage, or idiopathic intracranial hypertension. Opening pressure is above 300 mm of water in 39 percent of bacterial meningitis. ^[2]
• Low (below 60 mm of water in an adult): spontaneous intracranial hypotension, a CSF leak (for example after a previous puncture) or dehydration. ^[2]
• Idiopathic intracranial hypertension is diagnosed on clinical features, an opening pressure above 250 mm of water and a normal CSF composition, after neuroimaging has excluded other causes. ^[2]
• When pressure is raised and repeat taps are needed, continuous pressure monitoring and management of the underlying cause are required rather than repeated drainage alone.

Bed rest is unnecessary after an LP and does not reduce the incidence of post-lumbar puncture headache; recumbency may help relieve it once it occurs. Vigorous or strenuous exercise should probably be avoided for at least 24 hours. ^[5] Increased oral fluids have been suggested to treat post-procedure headache but are not proven; caffeine may help and increases the diuresis, so fluids should be taken with it. ^[5] Warn patients to report persistent or worsening back pain for several days afterwards, because it can indicate a spinal haematoma. ^[5] Collect about 4 to 8 mL of CSF across the four tubes for routine diagnostics, and 30 to 40 mL where indicated. ^[5]

CSF is interpreted as a pattern across several measurements: appearance, opening pressure, red and white cell counts, protein, glucose (usually expressed as CSF:serum ratio), and special tests. The normal values from the American Family Physician review are reproduced in the table. ^[2]

The companion table below summarises the patterns that every medical student must recognise. Numbers are taken from the sources cited; tuberculosis, fungal and parasitic meningitis, malignancy and idiopathic intracranial hypertension are described in text.

Bacterial meningitis carries a 14 to 25 percent mortality, so speed outweighs perfection. ^[2] The CSF is often cloudy, green or purulent, with a polymorphonuclear predominance and a markedly raised white cell count, although 6 percent of culture proven cases have a normal count and lymphocytes predominate in up to 10 percent. ^[2] White counts over 2,000 per microlitre, protein above 220 mg per dL, glucose below 34 mg per dL or neutrophils above 1,180 per microlitre each give more than 99 percent diagnostic certainty. ^[2]

Gram stain has a specificity of 97 percent but a sensitivity of only 33 to 90 percent depending on organism, technique and prior antibiotics, so a negative stain does not exclude bacterial meningitis. ^[2] CSF lactate above 35.1 mg per dL (3.9 mmol per L) is 93 to 95 percent sensitive and 94 to 99 percent specific if the sample is taken before antibiotics. ^[2] Note the European and IDSA contrast here: the Infectious Diseases Society of America (IDSA) does not recommend routine CSF lactate measurement in suspected bacterial meningitis, even though two meta-analyses support its value, particularly in neurosurgical patients. ^[2] Where the evidence and the guideline differ, trust the clinical picture and the culture.

Cultures remain definitive, and giving antibiotics before the tap reduces their yield by 10 to 20 percent. ^[2] That is why guidelines say to obtain CSF before antibiotics when this can be done without delay, but to start antibiotics first and tap second when the patient is severely ill. ^[6] The IDSA 2017 healthcare-associated ventriculitis and meningitis guideline makes a similar, slightly different point: in shunt, drain or post-neurosurgical infection the usual CSF parameters are unreliable, and a normal cell count, glucose and protein do not exclude infection; CSF cultures are the most important test and must be held for at least 10 days to recover organisms such as Cutibacterium acnes (formerly Propionibacterium acnes). ^[3] That distinction between community-acquired bacterial meningitis and healthcare-associated ventriculitis matters in practice: do not reassure yourself with a normal cell count in a shunt patient. ^[3]

Adjusting cell counts for a traumatic tap: subtract one white cell for every 500 to 1,500 red cells, or use the predicted formula, CSF white cells = CSF red cells times (blood white cells divided by blood red cells). ^[2]

Viral meningitis is the commonest form. Opening pressure is usually normal, white cells generally below 250 per microlitre, lymphocytes predominant (polymorphonuclear cells may dominate in the first hours), protein normal or mildly raised below 150 mg per dL, glucose usually normal. ^[2] Polymerase chain reaction (PCR) testing of CSF is preferred: it detects common pathogens with more than 95 percent sensitivity and specificity and allows early stopping of antibiotics, though it says nothing about antibiotic sensitivities. ^[2]

Cryptococcal meningitis mainly affects immunocompromised hosts. CSF protein is usually below 40 mg per dL, glucose usually above 40 mg per dL, red counts minimal, and opening pressure is above 250 mm of water in severe cases, sometimes requiring repeated therapeutic LPs or a shunt. ^[2]

Tuberculous and fungal meningitis, neurosyphilis and parasitic causes (with CSF eosinophilia) have their own characteristic patterns; the AAFP review sets out CSF adenosine deaminase above 10 U per L in tuberculous disease and (1-3)-beta-D-glucan for fungal disease. ^[2] These are diagnosed with culture and PCR rather than number patterns alone.

A clinically significant subarachnoid haemorrhage is apparent on non-contrast head CT in nearly 100 percent of cases within the first six hours of symptoms; thereafter sensitivity wanes and CSF analysis may be needed. ^[2] The single most characteristic CSF finding is xanthochromia, the yellow colour produced by breakdown of haemoglobin to bilirubin in the subarachnoid space. ^[2]

Xanthochromia emerges after six hours in roughly 20 percent of subarachnoid haemorrhages and is present in 90 percent of patients by 12 hours; it can persist for up to two weeks. ^[2] Because bilirubin forms only in vivo, xanthochromia favours a real bleed over a traumatic tap, which merely introduces whole red cells. ^[2] As few as 400 red blood cells per microlitre change the appearance of CSF. ^[2] A red cell count below 2,000 per microlitre without xanthochromia effectively excludes subarachnoid haemorrhage. ^[2]

Other causes of xanthochromia to keep in mind are a serum bilirubin above 10 to 15 mg per dL, CSF protein above 150 mg per dL (which can bind bilirubin), or a traumatic tap with more than 100,000 red cells per microlitre. ^[2] Red cell counts falling across successive collection tubes suggest a traumatic tap rather than a true bleed. ^[2]

At least half of patients who present with a subarachnoid haemorrhage have had an earlier minor ("sentinel") bleed two to three weeks before, which increases the chance of xanthochromia. ^[2]

In multiple sclerosis the CSF is a support to the diagnosis rather than its basis. The classic CSF finding is restricted oligoclonal bands; their inclusion in the McDonald criteria allows them to substitute for the traditional requirement of new lesions appearing over time in younger patients with a typical presentation. ^[2] CSF neurofilament light chain elevations correlate with relapsing disease. ^[2] Otherwise the fluid is near-normal: opening pressure normal, cell count at most mildly raised with a lymphocytic profile, protein normal or mildly raised, glucose normal. Oligoclonal bands are not specific to multiple sclerosis and occur in other inflammatory and infectious conditions. ^[2] Newer work on kappa free light chains in CSF is replacing oligoclonal banding in some centres, but the principle remains the same: evidence of intrathecal immunoglobulin synthesis. ^[2]

The classic CSF picture in Guillain-Barre syndrome is albuminocytologic dissociation: high protein with a normal cell count. It is present in 50 percent of patients in the first week and about 75 percent by three weeks, and the protein may not be raised at all early on, so a normal CSF does not exclude it. ^[2] In the Miller Fisher variant, which presents with ophthalmoplegia, ataxia and areflexia, the same dissociation is found in roughly 90 percent of patients at the peak of illness; only about half show it on the initial tap, and 10 percent of Guillain-Barre patients have completely normal CSF studies. ^[7] Between 15 and 20 percent have a mild rise in white cells of 5 to 50 per microlitre. ^[7] Protein concentration exceeds the reference range in 49 percent initially and 88 percent by three weeks. ^[7] Cell counts above 50 per microlitre (the Brighton criteria use fewer than 50 cells per microlitre) should make you reconsider the diagnosis and look for mimics such as infection, lymphomatous or carcinomatous disease. ^[7] CSF examination is especially valuable when the diagnosis is uncertain and when an alternative cause of weakness needs excluding. ^[7]

The commonest complication is post-lumbar puncture headache, which develops in about 10 percent of patients, usually hours to a day or two later, and can be severe. ^[5] Younger patients with small body mass are at greatest risk. It is caused by continued CSF leak through the dural hole rather than by the volume removed. Headache is best reduced by technique and equipment: use a non-cutting (pencil-point) needle, the smaller 22-gauge calibre, and bevel the cutting needle parallel to the long axis of the spinal cord. ^{[4] [5]} Bed rest and high fluid intake are not proven to prevent it, although lying down relieves it. ^[5] Severe or persistent headache is treated with an epidural blood patch. ^[4]

The complete list of complications: ^[5]

• Post-lumbar puncture headache
• Spinal haematoma from bleeding, especially with coagulopathy
• Transient local back or leg pain
• Infection (rare with sterile technique)
• Cerebral or cerebellar herniation (rare)
• Cortical blindness, cranial nerve VI palsy, transient deafness, intracranial hypotension, iatrogenic epidermoid tumour years later (all rare)
• Transient nausea or tinnitus

Herniation is the one to fear. It is not caused by taking 10 mL of fluid from a normal patient but by performing the procedure when a mass lesion has already displaced brain tissue. That is why the contraindication and CT-first sections matter. When meningitis or subarachnoid haemorrhage is suspected and the LP is deferred because herniation is a possibility, treat immediately and scan. ^[5]

• In obesity or after spinal surgery, landmarks may be unpalpable: seated position, and fluoroscopic or ultrasound guidance when available. ^[5]
• In pregnancy, idiopathic intracranial hypertension is common; the modified Dandy criteria apply, and management is specialist.
• In neonates and young children opening pressures and white counts have different normal ranges; cryptococcal and tuberculous disease behave differently in the immunocompromised.
• Where CT is unavailable, rely on the clinical signs of possible raised pressure (Glasgow coma scale below 11, lethargy, altered mental status, new seizures, focal deficit, papilledema), defer the tap, start treatment and refer. ^[6]

• Lumbar puncture samples CSF from below the end of the spinal cord, usually at L3-L4 or L4-L5, and is both diagnostic and (less often) therapeutic.
• Suspected central nervous system infection or haemorrhage is the urgent indication; defer and treat when raised intracranial pressure or coagulopathy is likely.
• Head imaging before the procedure is indicated by focal deficits, altered mental status, Glasgow coma scale below 11, papilledema, new seizures, suspected mass lesion, coagulopathy or site infection. A normal scan does not exclude raised pressure.
• Normal adult opening pressure is 60 to 250 mm of water. Above 250 mm suggests raised pressure; below 60 mm suggests a CSF leak.
• Normal CSF contains fewer than 5 white cells per microlitre, protein under 50 mg per dL and a glucose ratio of 0.44 to 0.90.
• Bacterial meningitis shows polymorphonuclear predominance, very low glucose and very high protein; viral meningitis is lymphocytic with normal glucose.
• Xanthochromia appearing from 6 hours and present in 90 percent of patients by 12 hours supports subarachnoid haemorrhage; a red cell count below 2,000 per microlitre without xanthochromia effectively excludes it.
• Multiple sclerosis is supported by a normal pressure, normal glucose and restricted oligoclonal bands; Guillain-Barre syndrome by raised protein with a near-normal cell count.
• Healthcare-associated and shunt infection: CSF cells, glucose and protein are unreliable, cultures are definitive and must be incubated for at least 10 days.
• Use a thin pencil-point 22-gauge needle, with good technique and limited CSF removal, to reduce post-lumbar puncture headache; bed rest is unnecessary.
• Treat meningitis or subarachnoid haemorrhage first and scan first when herniation is a risk.

Plain-text names of topics that pair with this page: meningitis, bacterial meningitis, subarachnoid haemorrhage, multiple sclerosis, Guillain-Barre syndrome, Miller Fisher syndrome, idiopathic intracranial hypertension, post-lumbar puncture headache, normal pressure hydrocephalus, meningeal carcinomatosis, ventriculitis, cryptococcal meningitis, tuberculous meningitis. Link them in MedGrasp once their topic pages exist.