When is a lumbar puncture (LP) warranted in a patient?

Indications for Lumbar Puncture in Clinical Practice

A lumbar puncture (LP) is primarily indicated when central nervous system infection (meningitis/encephalitis) is suspected, when subarachnoid hemorrhage requires confirmation after negative CT imaging, or when specific neurological symptoms suggest conditions requiring CSF analysis. 1

Primary Indications for Lumbar Puncture

Suspected Central Nervous System Infections

• Meningitis/Encephalitis:

  • Presence of fever with any of the following:
    • Headache (especially severe)
    • Altered mental status or confusion
    • Meningeal signs (neck stiffness, Kernig sign, Brudzinski sign)
    • Photophobia
    • Jolt accentuation of headache (headache worsening when head is horizontally nodded) 2
  • The classic triad of fever, neck stiffness, and altered consciousness is present in <50% of bacterial meningitis cases 3

• Suspected Anthrax Meningitis:

  • Patients with systemic anthrax should undergo LP unless contraindicated 2
  • In resource-limited settings, patients with severe headache, altered mental status, meningeal signs, or other neurological symptoms should be considered to have anthrax meningitis 2

Suspected Subarachnoid Hemorrhage

• When CT scan is negative but clinical suspicion remains high 1, 4

Other Indications

• Unexplained altered consciousness or focal neurologic signs with fever 2
• Patients with intracranial devices (e.g., ventriculostomy) who develop fever 2

Contraindications to Immediate Lumbar Puncture

Absolute Contraindications

• Local infection at the puncture site 1
• Signs of increased intracranial pressure due to mass lesion 1
• Clinical suspicion of spinal cord compression 1
• Coagulopathy (INR >1.5, PTT >60 seconds) 1
• Platelet count <100 × 10⁹/L 1
• Current anticoagulation therapy (unless appropriately reversed) 2, 1

Clinical Contraindications Requiring CT Before LP

• Moderate to severe impairment of consciousness (GCS <13) 2
• New onset seizures 2
• Focal neurological signs 2
• Papilledema 2
• Immunocompromised state 1
• Abnormal posture or posturing 2

Decision Algorithm for Lumbar Puncture

1. Initial Assessment:

   • Evaluate for signs of CNS infection
   • Check for contraindications to immediate LP

2. If No Contraindications Exist:

   • Proceed directly to LP

3. If Contraindications Exist:

   • Perform CT scan to evaluate for mass effect or brain shift
   • If CT is negative for contraindications, proceed with LP
   • If CT shows significant brain shift or tight basal cisterns, defer LP 2

4. Special Considerations:

   • In suspected bacterial meningitis with delayed LP (e.g., for CT), obtain blood cultures and start empiric antibiotics before imaging 2
   • In patients with ventriculostomies who develop stupor or signs of meningitis, remove the catheter and culture the tip 2

CSF Analysis and Interpretation

• Basic Tests:

  • Cell count and differential
  • Glucose and protein concentrations
  • Gram stain and bacterial cultures 2

• Typical Findings in Bacterial Meningitis:

  • CSF glucose <35 mg/dL
  • CSF-blood glucose ratio <0.23
  • CSF protein >220 mg/dL

  • 2,000 white blood cells/μL

  • 1,180 neutrophils/μL 2

• Additional Tests based on clinical suspicion:

  • Cryptococcal antigen
  • Fungal staining/cultures
  • Acid-fast bacillus smears/cultures
  • PCR tests for viral pathogens (HSV, CMV, etc.) 2

Important Clinical Caveats

• The yield of LP in hospitalized non-surgical patients with new-onset fever and altered mental status but without headache or meningeal signs is extremely low 5, 6

• 5-10% of adults with proven HSV encephalitis may have normal initial CSF findings; consider repeat LP after 24-48 hours if clinical suspicion remains high 1

• CT scan is not a reliable tool for diagnosing increased intracranial pressure; clinical assessment should be the primary determinant of LP safety 2, 1

• Jolt accentuation of headache has higher diagnostic value in patients with acute onset of headache and fever who have normal mental status 7

By following these guidelines, clinicians can appropriately identify patients who would benefit from lumbar puncture while minimizing unnecessary procedures and associated risks.