What are the indications for a lumbar puncture (LP)?

Indications for Lumbar Puncture

Lumbar puncture is primarily indicated for diagnosing central nervous system infections (meningitis, encephalitis) and subarachnoid hemorrhage when neuroimaging is normal or unavailable. 1, 2

Primary Diagnostic Indications

CNS Infections

• Suspected bacterial meningitis remains the most critical indication, requiring urgent LP to identify pathogens and guide antimicrobial therapy 1, 3
• Suspected viral encephalitis, particularly HSV encephalitis, where CSF PCR is essential for diagnosis and treatment decisions 1, 3
• Fungal meningitis evaluation in immunocompromised or high-risk patients 3
• Meningococcal sepsis when clinical presentation suggests CNS involvement 1

Subarachnoid Hemorrhage

• Diagnosis of subarachnoid hemorrhage when CT scan is normal but clinical suspicion remains high, looking for xanthochromia and red blood cells 2, 4

Other Neurological Conditions

• Acute severe headache of unclear etiology after neuroimaging, which yielded abnormal results in 54.5% of cases in recent Australian data 5
• Encephalopathy of unknown cause, particularly when infection is suspected (56.3% diagnostic yield) 5
• Suspected CNS inflammatory conditions including Guillain-Barré syndrome, multiple sclerosis, or other demyelinating diseases 2
• Idiopathic intracranial hypertension to measure opening pressure and provide therapeutic relief 2

Mass-Casualty and Special Circumstances

Anthrax Exposure

• Suspected anthrax meningitis in the context of bioterrorism or mass-casualty incidents, as meningitis carries 89% mortality and requires specific antimicrobial regimens 1
• Under conventional standard of care, LP should be performed unless contraindicated by CT findings or clinical evaluation 1
• Under contingency conditions with resource constraints, LP is prioritized for patients with ≥2 meningeal signs (severe headache, altered mental status, meningeal signs, neurological symptoms) 1

Important Clinical Context

High-Yield Scenarios

The diagnostic yield varies significantly by indication 5:

• Acute severe headache and encephalopathy have the highest yields (>50% abnormal results)
• Fever alone in hospitalized non-surgical patients has extremely low yield (0% in one study) and LP may be unnecessary without headache or meningeal signs 6

Nosocomial vs. Community-Acquired Infections

• Community-acquired meningitis has substantially higher diagnostic yield (14% positive) compared to suspected nosocomial meningitis in non-surgical patients (0% positive) 6
• LP for suspected nosocomial meningitis in hospitalized medical patients without headache or meningeal signs is generally not indicated 6

Absolute Contraindications

Before performing LP, assess for these contraindications 1, 7:

Clinical Contraindications

• Moderate to severe impairment of consciousness (GCS ≤10 or fall in GCS >2) 1, 7
• Focal neurological signs including unequal/dilated/poorly responsive pupils, cranial neuropathies, or limb weakness 1, 7
• Papilledema indicating raised intracranial pressure 1, 7
• Systemic shock or hemodynamic instability 7
• Local skin infection at the puncture site 1, 7

Hematologic Contraindications

• Platelet count <50 × 10⁹/L for diagnostic LP (some guidelines suggest <100 × 10⁹/L as safer threshold) 7, 8
• Active anticoagulation without reversal 7
• Coagulopathy with abnormal clotting parameters 1, 7

Management Algorithm When Contraindications Present

• If clinical contraindications exist, obtain urgent CT scan before LP 7
• If CT shows significant brain shift, tight basal cisterns, or raised ICP, defer LP 7
• For anticoagulated patients, consider reversal with protamine, vitamin K, or fresh frozen plasma before proceeding 7
• Clinical assessment, not CT alone, should determine LP safety, as CT may not accurately detect all cases of increased intracranial pressure 7

Critical Pitfalls to Avoid

• Do not delay LP in suspected bacterial meningitis if no contraindications exist—empiric antibiotics should be given immediately if LP is delayed, as blood cultures remain positive for several days after antibiotic initiation 1
• Do not rely on fever and altered mental status alone in hospitalized non-surgical patients without headache or meningeal signs, as yield is extremely low 6
• Do not assume normal initial CSF excludes HSV encephalitis—5-10% of proven HSV encephalitis cases have normal initial CSF, and repeat LP may be necessary if clinical suspicion persists 1, 3, 7
• Do not perform LP based solely on normal CT in patients with clinical signs of herniation risk 7