Diagnostics & Procedures · Topic 14 of 16

Lumbar Puncture & CSF Interpretation

Procedures · lumbar puncture and CSF

Lumbar Puncture & CSF Interpretation

Never delay antibiotics for the tap, image first only when herniation risk exists, transfuse platelets below 20,000, then read pressure, cells, protein and glucose.

Key numbers

Bacterial: neutrophils, high protein, low glucoseglucose < 50% of blood

Normal opening pressure 10–20 cm H₂OOP 10–20 cm H₂O

Transfuse platelets below 20,000 before LPplt < 20,000/mcL

Hold DOACs 72 hours (high dose)high-dose DOAC: ≥ 72 h

01 · The ruleCT first: focal deficit, papilledema, seizure, immunocompromise

Image before lumbar puncture for immunocompromise, known CNS disease (mass, stroke, focal infection), seizure within 1 week, papilledema, abnormal consciousness or a focal deficit; CT cannot always predict herniation.

02 · Defined byBacterial: neutrophils, high protein, low glucose

Bacterial meningitis CSF: high opening pressure, turbid fluid, neutrophil-predominant pleocytosis (100–10,000/µL), protein usually > 100 mg/dL and glucose under 50% of blood glucose; a ratio under 0.23 strongly suggests bacteria.

03 · The numberNormal opening pressure 10–20 cm H₂O

Normal CSF: clear and colorless, opening pressure 10–20 cm H₂O (100–200 mm H₂O, lateral decubitus), up to 5 lymphocytes/µL, protein about 20–45 mg/dL, and glucose about 60% of blood glucose.

Diagnosis & monitoring

Work-up

Transfuse platelets below 20,000 before LP must knowTransfuse platelets before a lumbar puncture when the count is below 20,000/mcL (2025 AABB/ICTMG, strong recommendation); correct coagulopathy or hold anticoagulants first — spinal epidural hematoma is the feared complication.

Viral: lymphocytes with normal glucoseViral meningitis CSF: lymphocytic pleocytosis (neutrophils may predominate early), normal glucose and normal or mildly raised protein; confirm with CSF PCR (enterovirus, HSV).

Lymphocytes with low glucose: TB, fungus, cancerLymphocytic pleocytosis with LOW glucose and high protein suggests tuberculous, fungal (eg, cryptococcal) or carcinomatous meningitis — collect 30–40 mL of CSF for acid-fast and fungal studies and cytology.

Guillain-Barré: high protein, normal cell countGuillain-Barré syndrome gives albuminocytologic dissociation — raised protein (often > 100 mg/dL) with a normal cell count — but it may take up to 1 week to appear and is absent in 10%.

Do right now

In this order

First moves

Antibiotics before CT when meningitis is suspected

→

CT first: focal deficit, papilledema, seizure, immunocompromise

→

Transfuse platelets below 20,000 before LPplt < 20,000/mcL

→

L3–L4 or L4–L5, lateral decubitus for pressure

Why each step

Antibiotics before CT when meningitis is suspectedWhen bacterial meningitis is suspected and imaging must come first, draw blood cultures and give dexamethasone and empiric antibiotics before the CT — never delay treatment for imaging or the lumbar puncture.

CT first: focal deficit, papilledema, seizure, immunocompromiseImage before lumbar puncture for immunocompromise, known CNS disease (mass, stroke, focal infection), seizure within 1 week, papilledema, abnormal consciousness or a focal deficit; CT cannot always predict herniation.

Transfuse platelets below 20,000 before LPTransfuse platelets before a lumbar puncture when the count is below 20,000/mcL (2025 AABB/ICTMG, strong recommendation); correct coagulopathy or hold anticoagulants first — spinal epidural hematoma is the feared complication.

L3–L4 or L4–L5, lateral decubitus for pressureEnter at L3–L4 or L4–L5, below the cord's end near T12–L1 (L4 lies on the iliac-crest line); lateral decubitus is required for opening pressure, and sitting helps when landmarks are hard to feel.

Red flags & don'ts

Avoid harm

Post-LP headache: caffeine, then blood patch complicationPost-LP headache (about 10%) is worse upright and starts hours to 1–2 days later; treat with recumbency, fluids, caffeine and analgesics, and if it persists after a day, an epidural blood patch.

New back pain after LP: exclude hematoma complicationPersistent or worsening back pain in the days after a lumbar puncture needs prompt evaluation to exclude a spinal epidural hematoma.

The rules

Know these

Hold DOACs 72 hours (high dose)ASRA 2025 neuraxial holds: high-dose apixaban, rivaroxaban or edoxaban ≥ 72 hours; low-dose apixaban ≥ 36 hours, low-dose rivaroxaban ≥ 24 hours; high-dose dabigatran ≥ 72 hours (CrCl ≥ 50) or 120 hours (CrCl 30–49).

Time heparin, LMWH and warfarin holdsHeparins and warfarin (ASRA 2025): stop IV heparin 4–6 hours with normal coagulation; prophylactic SC heparin 4–6 hours; low-dose LMWH 12 hours, high-dose LMWH 24 hours; warfarin 5 days with a normal INR.

Aspirin is fine; stop P2Y12 inhibitors firstAspirin and other NSAIDs alone need no hold before a neuraxial procedure; stop clopidogrel 5–7 days, prasugrel 7–10 days and ticagrelor 5 days beforehand (ASRA 2025).

Atraumatic needle prevents headache; bed rest doesn'tA small (22-gauge) noncutting pencil-point needle reduces post-LP headache; bed rest afterward and the volume of CSF removed do not change its incidence.

Bacterial or viral meningitis CSF?

Commonly confused

Opening pressure	High
White cells	100–10,000/µL, neutrophils
Glucose	Low — under 50% of blood glucose
Protein	Usually > 100 mg/dL

Commonly confused

Tell apart

Clearing RBCs = traumatic tap; xanthochromia = SAHA traumatic tap shows red cells falling from the first to the last tube, no xanthochromia and fresh red cells; subarachnoid hemorrhage gives uniformly bloody CSF with xanthochromia once several hours have passed.

More detail

Reference

Show 6 reference facts

Compare red cell counts in the first and last tubes to separate a traumatic tap from hemorrhage, send Gram stain, culture and chemistry, and draw a simultaneous blood glucose for the CSF:blood ratio.

Infection at or near the needle site is the absolute contraindication to LP; bacteremia has not been shown to cause meningitis after LP and is not a contraindication.

CSF obtained within about 4 hours of the first antibiotic dose still usually grows the organism.

Idiopathic intracranial hypertension shows a raised opening pressure with otherwise normal CSF, and LP also lowers the pressure; removing CSF serves as a therapeutic test for normal pressure hydrocephalus.

After fibrinolytic or thrombolytic therapy, ASRA recommends no neuraxial needle placement for at least 48 hours, with documented normalization of clotting studies including fibrinogen.

Turbid CSF means at least about 300 cells/µL; CSF protein above 500 mg/dL occurs with purulent or advanced tuberculous meningitis, spinal block or a bloody tap.

Pearl

Read the glucose: low with neutrophils is bacteria; low with lymphocytes is tuberculosis, fungus or cancer; normal with lymphocytes is usually a virus.

Sources · Merck Manual Professional — How To Do Lumbar Puncture (Feb 2026) · Merck Manual Professional — Lumbar Puncture (CSF findings table) · Merck Manual Professional — Acute Bacterial Meningitis (Sept 2026) · Tunkel AR et al. Practice guidelines for the management of bacterial meningitis (IDSA). Clin Infect Dis 2004;39:1267–84 · Smith L. Management of Bacterial Meningitis: New Guidelines from the IDSA. Am Fam Physician 2005;71:2003–8 · Kopp SL et al. Regional anesthesia in the patient receiving antithrombotic or thrombolytic therapy: ASRA Pain Medicine Evidence-Based Guidelines (fifth edition). Reg Anesth Pain Med 2025 · Guideline Central — ASRA Guidelines for Regional Anesthesia with Antithrombotic or Thrombolytic Therapy, 5th Edition: summary of recommendations · Metcalf RA et al. Platelet Transfusion: 2025 AABB and ICTMG International Clinical Practice Guidelines. JAMA 2025;334(7):606–617 (abstract via Europe PMC) · Merck Manual Professional — Post-Lumbar Puncture and Other Low-Pressure Headaches · Merck Manual Professional — Guillain-Barré Syndrome (GBS) · Starter sheet built from the verified AGACNP fact map (facts reviewed 2026-10-15).