What is the procedure for performing a cerebrospinal fluid (CSF) tap in adults?

Lumbar Puncture Procedure in Adults

The proper technique for performing a cerebrospinal fluid (CSF) tap in adults involves patient positioning in lateral recumbent position, using atraumatic needles (22G or larger), inserting at L3-L4 or L4-L5 interspace with the bevel oriented transversely, and collecting adequate CSF samples for appropriate testing while measuring opening pressure. 1

Indications and Contraindications

Indications:

• Suspected CNS infections (meningitis, encephalitis)
• Subarachnoid hemorrhage with negative CT
• Inflammatory or demyelinating conditions
• Suspected increased intracranial pressure

Contraindications:

• Signs of severe sepsis or rapidly evolving rash
• Respiratory or cardiac compromise
• Anticoagulant therapy/known thrombocytopenia (platelet count <100 × 10⁹/L)
• Infection at the LP site
• Focal neurological signs
• Papilledema
• Continuous or uncontrolled seizures
• GCS ≤12 2

Pre-Procedure Considerations

Neuroimaging:

• CT scan should be performed before LP if any contraindications are present
• Even if CT is normal, LP should be performed with caution if clinical suspicion for increased ICP exists 2

Coagulation:

• Ensure normal coagulation parameters
• Platelet count should be ≥100 × 10⁹/L
• Consider timing if patient is on anticoagulants 2

Equipment Required:

• LP tray with sterile drapes and gloves
• Antiseptic solution (chlorhexidine or povidone-iodine)
• Local anesthetic (1% lidocaine)
• Spinal needles (preferably atraumatic/pencil-point, 22G)
• Manometer for measuring opening pressure
• Collection tubes (at least 4)
• Sterile dressing

Procedure Steps

1. Patient Positioning:

• Position patient in lateral recumbent position (preferred) with knees drawn to chest and chin tucked
• Alternatively, sitting position can be used if lateral position is difficult
• Ensure spine is flexed to open intervertebral spaces 1

2. Site Identification:

• Identify L3-L4 or L4-L5 interspace (below the termination of spinal cord)
• Use the iliac crests as landmarks (line connecting tops of iliac crests crosses L4 spinous process)
• Consider ultrasound guidance for difficult cases 1

3. Aseptic Technique:

• Perform hand hygiene and don sterile gloves
• Clean the area with antiseptic solution in a circular motion from center outward
• Apply sterile drapes

4. Local Anesthesia:

• Infiltrate skin and deeper tissues with 1% lidocaine
• Allow 1-2 minutes for anesthetic effect

5. Needle Insertion:

• Insert spinal needle with stylet in place
• Orient bevel in transverse plane (perpendicular to longitudinal axis)
• Advance needle toward umbilicus
• A "pop" sensation may be felt when dura is penetrated
• Remove stylet to check for CSF flow 2

6. CSF Collection:

• Measure opening pressure using manometer (normal <20 cm H₂O)
• Collect CSF in sequential tubes (typically 4 tubes, 2-3 mL each)
• Replace stylet before withdrawing needle 2, 1

7. Post-Procedure:

• Apply sterile dressing
• Allow patient to lie flat for comfort (though not proven to prevent headache)
• Monitor for complications 1

CSF Analysis

Standard Tests:

• Opening pressure
• Cell count and differential
• Glucose (with concurrent plasma glucose)
• Protein
• Lactate
• Gram stain and culture
• PCR for viral pathogens when indicated 2, 1

Normal Values:

• Opening pressure: <20 cm H₂O
• Cell count: <5 WBC/mm³
• Glucose: >45 mg/dL (CSF:plasma ratio ~0.6)
• Protein: <45 mg/dL
• Lactate: <20 mg/dL 1

Complications and Management

Post-LP Headache:

• Occurs in 10-30% of patients
• Use of atraumatic needles reduces risk
• Treatment includes hydration, caffeine, and analgesics
• Severe cases may require epidural blood patch 1

Practices to Reduce Complications:

• Use atraumatic/pencil-point needles
• Use smaller gauge needles (22G is practical)
• Orient needle bevel transversely
• Replace stylet before withdrawal
• Limit attempts to 4 or fewer 2, 1

Practices NOT Proven to Reduce Headache:

• Reducing CSF volume
• Bed rest after procedure
• Increased hydration
• Prophylactic caffeine 2

Special Considerations

Traumatic Tap:

• If bloody CSF is obtained, collect sequential tubes to check for clearing
• Correction formula: subtract 1 WBC for every 700 RBCs and 0.1 g/dL protein for every 1000 RBCs 2

Dry Tap:

• If no CSF flows, rotate needle slightly or withdraw slightly
• Consider repositioning patient
• May occur in approximately 10% of cases 3

Timing:

• Never delay antimicrobial therapy in suspected meningitis while awaiting LP
• If initial LP is normal but clinical suspicion remains high, consider repeat LP in 24-48 hours 2, 1