Lumbar Puncture in Meningitis: Interpretation and Management
When to Perform LP Without Prior CT
Perform LP immediately without CT imaging if the patient has a Glasgow Coma Scale (GCS) >12 and lacks focal neurological signs, as delaying for unnecessary CT scans reduces diagnostic yield and increases mortality. 1
Absolute Contraindications to Immediate LP
Do NOT perform LP if any of the following are present:
- GCS ≤12 1
- Focal neurological signs (excluding isolated cranial nerve VI or VII palsy) 1
- Papilledema 1
- Seizures until patient is stabilized 1
- Signs of severe sepsis or rapidly evolving rash 1
- Respiratory or cardiac compromise 1
- Coagulopathy or platelet count <40 × 10⁹/L 1
- Local infection at LP site 1
Anticoagulation Management Before LP
- Prophylactic LMWH: Wait 12 hours after last dose 1
- Therapeutic LMWH: Wait 24 hours after last dose 1
- Warfarin: INR must be ≤1.4 1
- Clopidogrel: Delay 7 days OR give platelet transfusion/DDAVP after hematology consultation 1
- Aspirin/NSAIDs: No delay needed 1
- Unfractionated heparin: Can restart 1 hour after LP 1
Critical Timing Principle
If bacterial meningitis is suspected, give antibiotics IMMEDIATELY—do not delay for LP or CT. 2, 3 Swedish data showed that starting antibiotics 1.2 hours earlier reduced mortality from 11.7% to 6.9% and sequelae from 49% to 38%. 1
However, if LP can be performed within 4 hours of starting antibiotics, CSF culture remains positive in 73% of cases, compared to only 11% if delayed beyond 4 hours. 3 No cultures were positive after 8 hours of antibiotics. 3
CSF Interpretation
Visual Assessment
- Clear and colorless: Normal 2
- Turbid: Elevated WBCs or protein, suggests bacterial meningitis 2
- Xanthochromia: Subarachnoid hemorrhage or elevated protein 2
Cell Count Analysis
- Neutrophilic predominance (>50% neutrophils): Bacterial meningitis—start empiric antibiotics immediately 2
- Lymphocytic predominance: Viral infection, tuberculosis, fungal infection, or partially treated bacterial meningitis 2
For traumatic taps: Subtract 1 WBC for every 7,000 RBCs to correct the count 4. A WBC:RBC ratio ≤1:100 (0.01) has 100% specificity for excluding bacterial meningitis. 5
Biochemical Parameters
- CSF glucose <40 mg/dL or <50% of serum glucose: Highly suggestive of bacterial meningitis, tuberculosis, or fungal infection 2
- CSF lactate <2 mmol/L: Helps rule out bacterial disease 2, 4
- Elevated protein: Seen in bacterial meningitis, viral infections, TB, fungal infections 2
Microbiological Studies
- Gram stain: Identifies bacteria in 60-90% of untreated bacterial meningitis 2
- Culture: Gold standard but takes time; positive in 73% if LP done within 4 hours of antibiotics 3
- PCR for HSV: Sensitivity >95% for viral encephalitis 2
- Pneumococcal and meningococcal PCR: Should be sent on EDTA blood sample 1
Opening Pressure
- Normal: 10-20 cm H₂O (must be measured in lateral recumbent position only) 2, 6
- >25 cm H₂O: Suggests increased intracranial pressure, meningitis, or venous sinus thrombosis 2
Common Pitfalls to Avoid
Sending patients for unnecessary CT scans: 67% of patients in one study had unnecessary CT scans, causing diagnostic delays 3. CT does not reliably detect raised intracranial pressure and should only be done if clinical contraindications exist. 1
Delaying antibiotics for LP: If meningitis is suspected, give antibiotics first, then perform LP within 4 hours if possible 2, 3
Using diagnostic scoring systems: These are not recommended as they lack validation and practical utility 1
Misinterpreting traumatic taps: Use the WBC:RBC ratio ≤1:100 or observed-to-predicted WBC ratio ≤0.01 to exclude meningitis with 100% specificity 5
Failing to document LP position: Pressure readings are invalid if LP performed sitting up 6
If LP Cannot Be Performed Initially
Review the situation every 12-24 hours and perform LP when safe to do so. 1 Continue empiric antibiotics in the interim. If initial LP is non-diagnostic, repeat LP 24-48 hours later. 1