What samples are required for the diagnosis and management of meningitis?

Essential Samples for Diagnosis and Management of Meningitis

Blood and cerebrospinal fluid (CSF) samples are required for the diagnosis and management of meningitis, with blood samples collected before antibiotics whenever possible and CSF obtained through lumbar puncture unless contraindicated. 1

Blood Samples Required

• Blood cultures should be obtained prior to antibiotic administration whenever possible, or within the first hour of hospital arrival if antibiotics were given in the community 1
• Pneumococcal and meningococcal PCR (EDTA sample) to identify causative organisms even after antibiotics have been started 1
• Blood glucose measurement (to compare with CSF glucose) 1
• Lactate measurement 1
• Procalcitonin (if available) - helps differentiate bacterial from viral meningitis 1, 2
• Full blood count, urea, creatinine, electrolytes, liver function tests and clotting screen 1
• Storage sample for potential serological testing if a cause is not identified (with convalescent sample 4-6 weeks later) 1

CSF Samples Required

• CSF opening pressure measurement (unless lumbar puncture is performed in sitting position) 1
• CSF glucose with concurrent plasma glucose - low CSF:blood glucose ratio suggests bacterial meningitis 1, 3
• CSF protein - elevated in bacterial meningitis, moderately elevated in viral meningitis 1, 3
• CSF cell count and white cell differential - polymorphonuclear predominance suggests bacterial infection, lymphocytic predominance suggests viral infection 3, 4
• CSF Gram stain - can provide rapid diagnosis in 70-85% of bacterial meningitis cases 1, 5
• CSF culture - gold standard for bacterial identification 1, 4
• CSF PCR for common pathogens:
  • Enterovirus, HSV-1, HSV-2, and VZV for suspected viral meningitis 1
  • Bacterial PCR (including 16S ribosomal RNA) if culture negative 1
• Storage sample for additional testing if needed 1

Additional Samples Based on Clinical Suspicion

• Nasopharyngeal swabs for meningococcal isolation (especially if patient has received antibiotics) 1
• Stool and/or throat swabs for enterovirus PCR in suspected viral meningitis 1
• Additional CSF testing for specific pathogens based on risk factors:
  • Acid-fast stain and mycobacterial culture for suspected TB meningitis 4
  • India ink preparation and cryptococcal antigen for suspected fungal meningitis 4

Timing and Procedural Considerations

• If lumbar puncture is delayed due to contraindications (e.g., focal neurological findings, signs of increased intracranial pressure), blood samples should be collected and antibiotics administered before neuroimaging 1, 6
• When performing lumbar puncture, use of smaller gauge needles (22G practical minimum) and atraumatic needles can reduce post-LP headache risk 1
• Replacement of the stylet before needle withdrawal may reduce post-LP headache risk 1

Interpretation of Results

• Combined analysis of CSF parameters (cell count, protein, glucose) with serum biomarkers (especially procalcitonin) significantly improves differentiation between bacterial and viral meningitis 2
• Multiplex PCR panels (like FilmArray ME) can provide rapid diagnosis with high sensitivity (94.2%) and specificity (98.2%) 7
• Latex agglutination tests for bacterial antigens can provide rapid results but have been largely replaced by PCR testing 1, 5

Important Caveats

• Delay in obtaining samples should never postpone antibiotic administration in suspected bacterial meningitis, as this increases mortality 1, 6
• CSF findings may be altered by prior antibiotic administration, but will likely still provide evidence for or against bacterial meningitis 3
• Lumbar puncture is contraindicated in patients with signs of increased intracranial pressure (e.g., decerebrate posturing) until pressure is controlled 6
• Large volumes of CSF (up to 40-50mL) may be required for diagnosis of tuberculous or fungal meningitis, unless contraindicated by increased intracranial pressure 4