What is the procedure and management for cerebrospinal fluid (CSF) analysis?

Cerebrospinal Fluid (CSF) Analysis: Procedure and Management

The standard procedure for cerebrospinal fluid analysis requires proper lumbar puncture technique, collection of appropriate CSF volumes in sequential tubes, measurement of opening pressure, and analysis of cell count with differential, protein, glucose (with concurrent plasma glucose), and specific microbiological or molecular tests based on clinical suspicion. 1, 2

Indications for CSF Analysis

• Suspected meningitis or encephalitis
• Suspected subarachnoid hemorrhage with negative CT
• Evaluation of demyelinating diseases
• Diagnosis of neurodegenerative disorders (e.g., Alzheimer's disease)
• Evaluation of inflammatory or autoimmune CNS conditions
• Assessment of CNS malignancies

Pre-Procedure Considerations

Contraindications

• Increased intracranial pressure with risk of herniation
• Local infection at puncture site
• Coagulopathy or anticoagulant therapy
• Suspected spinal epidural abscess or mass
• Cardiorespiratory compromise

Imaging Before Lumbar Puncture

• Neuroimaging (CT or MRI) is required before LP in patients with:
  • Focal neurologic deficits
  • New seizures
  • Papilledema
  • Altered mental status
  • Immunocompromised state
  • History suggesting mass lesion 1

Procedure Technique

Patient Positioning

• Lateral recumbent position (preferred for measuring opening pressure)
• Sitting position (alternative if lateral position difficult)

Equipment

• Sterile gloves, mask, and drapes
• Antiseptic solution (chlorhexidine preferred)
• Local anesthetic (1-2% lidocaine)
• Spinal needle:
  • 22G atraumatic/non-traumatic needle recommended (reduces post-LP headache risk)
  • Needle with stylet 1

Technique Steps

1. Position patient with back flexed, knees to chest
2. Identify L3-L4 or L4-L5 interspace (below L2 to avoid spinal cord injury)
3. Mark site, prepare with antiseptic solution
4. Infiltrate skin and deeper tissues with local anesthetic
5. Insert spinal needle with bevel oriented parallel to longitudinal dural fibers
6. Advance slowly until "pop" sensation or CSF return
7. Remove stylet to check for CSF flow
8. Measure opening pressure with manometer (normal: 10-20 cmH₂O)
9. Collect appropriate CSF samples
10. Replace stylet before withdrawing needle (reduces headache risk) 1, 3

CSF Collection and Analysis

Collection

• Collect 8-15 mL total CSF in 3-4 sequential tubes:
  • Tube 1: Microbiological studies (culture, PCR)
  • Tube 2: Cell count, differential
  • Tube 3: Protein, glucose
  • Tube 4: Special studies (oligoclonal bands, specific antibodies, cytology) 1

Standard Analysis

1. Opening pressure measurement
2. Visual inspection (color, clarity)
3. Cell count with differential
4. Protein concentration
5. Glucose concentration (with concurrent plasma glucose)
6. Gram stain and culture
7. Additional tests based on clinical suspicion 1, 2

Special Tests Based on Clinical Indication

Infectious Disease Workup

• Bacterial culture and Gram stain
• PCR for herpes viruses (HSV-1/2, VZV), enteroviruses
• Cryptococcal antigen
• Acid-fast bacilli smear/culture for TB
• VDRL for neurosyphilis 1, 2

Neurological/Autoimmune Disorders

• Oligoclonal bands
• IgG index
• Myelin basic protein
• Autoimmune encephalitis antibody panel
• 14-3-3 protein (CJD) 1, 2

Alzheimer's Disease Biomarkers

• Amyloid β1-42
• Total tau
• Phosphorylated tau 1

Interpretation of Results

Normal Values

• Opening pressure: 10-20 cmH₂O
• Appearance: Clear, colorless
• WBC count: 0-5 cells/μL
• Protein: 15-45 mg/dL
• Glucose: >60% of serum glucose
• Gram stain: No organisms

Abnormal Findings and Significance

Bacterial Meningitis

• Elevated opening pressure
• Cloudy appearance
• WBC count >1000/μL (neutrophil predominance)
• Low glucose (<40 mg/dL)
• Elevated protein (>220 mg/dL)
• Positive Gram stain/culture 1

Viral Meningitis/Encephalitis

• Normal to mildly elevated opening pressure
• Clear appearance
• WBC count 5-1000/μL (lymphocyte predominance)
• Normal to slightly low glucose
• Normal to mildly elevated protein
• Positive PCR for viral pathogens 2

Subarachnoid Hemorrhage

• Elevated opening pressure
• Xanthochromic or bloody appearance
• RBCs present (do not clear in sequential tubes)
• Elevated protein
• Normal glucose 1

Post-Procedure Management

Monitoring

• Observe for immediate complications
• Monitor vital signs
• Check puncture site for CSF leak or bleeding

Complications Management

• Post-LP headache:

  • Bed rest is NOT proven to prevent headache
  • Hydration (not proven but commonly recommended)
  • Analgesics as needed
  • Caffeine may help some patients
  • Persistent severe headache: consider epidural blood patch 1, 3

• Back pain:

  • Usually self-limiting
  • Analgesics as needed

• Serious complications (rare):

  • Brain herniation
  • Spinal hematoma
  • Infection
  • CSF leak 1, 3

Special Considerations

Traumatic Tap

• RBCs present but clear in sequential tubes
• Apply correction formula for WBC count: subtract 1 WBC for every 700 RBCs 2

CSF Storage for Research

• Process within 30-60 minutes of collection
• Separate cell pellet from supernatant by centrifugation
• Store at -80°C in dedicated CSF collection tubes 1

Pitfalls and Caveats

• Never delay treatment for suspected bacterial meningitis to perform LP
• CSF analysis may be normal early in disease course
• Acellular CSF does not exclude encephalitis if clinical suspicion is high
• CSF eosinophils may be mistaken for neutrophils in automated cell counters
• Always measure concurrent blood glucose when interpreting CSF glucose
• Antibiotics given before LP may affect culture results but PCR remains reliable 1, 2