What cerebrospinal fluid (CSF) tests should be done for a patient suspected of having bacterial meningitis?

CSF Testing for Suspected Bacterial Meningitis

For a patient with suspected bacterial meningitis, obtain CSF opening pressure, cell count with differential, glucose and protein levels, Gram stain, and bacterial culture as the essential initial tests. 1, 2

Essential Core Tests (Order on All Patients)

Opening Pressure Measurement

• Measure and document opening pressure at the time of lumbar puncture 1, 2
• Elevated pressure is typical in bacterial meningitis 1

Cell Count with Differential

• Total white blood cell count >2,000 cells/mm³ or neutrophil count >1,180 cells/mm³ predicts bacterial meningitis with 99% certainty 1
• Neutrophilic predominance strongly suggests bacterial etiology 1, 2
• A traumatic tap can falsely elevate counts; use a 1:1000 correction factor for red blood cells 1, 2

Glucose and Protein

• CSF glucose <34 mg/dL or CSF-to-blood glucose ratio <0.23 indicates bacterial meningitis with high specificity 1
• Obtain simultaneous blood glucose for accurate ratio calculation 1
• CSF protein >220 mg/dL strongly suggests bacterial meningitis 1
• CSF protein >50 mg/dL has 88% sensitivity and 72% specificity for bacterial meningitis 3

Gram Stain

• Sensitivity ranges from 60-80% in untreated patients, dropping to 40-60% after antibiotic administration 1
• Specificity approaches 97-100% 1, 4
• Cytospin centrifugation can increase diagnostic yield 1
• Remains highly accurate even with prior antibiotics 4

Bacterial Culture

• Gold standard for diagnosis with specificity near 100% 1
• Sensitivity decreases significantly after antibiotic administration 1
• Should never delay antibiotic therapy 1

Additional Tests Based on Clinical Context

PCR Testing (When Available)

• Broad-range bacterial PCR has 100% sensitivity and 98.2% specificity for excluding bacterial meningitis 1
• Particularly valuable when antibiotics were given before lumbar puncture 1, 5, 6
• Can detect S. pneumoniae, N. meningitidis, H. influenzae, S. agalactiae, and L. monocytogenes 1
• Less affected by prior antibiotic therapy compared to culture 4, 5

CSF Lactate (If Antibiotics Not Yet Given)

• Lactate >35 mg/dL has 93% sensitivity and 96% specificity for bacterial meningitis 1
• High negative predictive value useful for ruling out bacterial disease 1
• Sensitivity drops below 50% after antibiotic administration 1

Tests NOT Recommended

Do not routinely order bacterial antigen testing (latex agglutination) - it does not modify treatment decisions and has false-positive results 1

• May have limited utility only in patients pretreated with antibiotics who have negative Gram stain and culture 1

Do not order Limulus lysate assay - insufficient sensitivity, doesn't distinguish specific organisms, and rarely influences treatment 1

Critical Timing Considerations

When to Delay Lumbar Puncture

• Severe sepsis or rapidly evolving rash 1
• Glasgow Coma Scale ≤12 1
• Focal neurological signs or papilledema 1, 2
• Continuous seizures 1
• Signs of increased intracranial pressure 1, 7

If Lumbar Puncture Delayed

• Obtain blood cultures immediately (at least 2-4 sets) 1, 8
• Start empirical antibiotics without waiting for imaging or LP 1, 2, 8
• Perform CT imaging if indicated, then proceed with LP once safe 1, 2

Special Populations

Immunocompromised Patients

• Add viral PCR testing (HSV, VZV, CMV, JC virus, enterovirus) 1
• Consider fungal studies including cryptococcal antigen 1, 9
• Maintain high suspicion even with normal cell counts 1

Patients with Intracranial Devices

• Aspirate CSF from the reservoir 1
• Consider lumbar puncture as well if flow is obstructed 1
• Remove and culture ventriculostomy catheter tip if meningitis develops 1