Diagnosis of Bacterial Meningitis

Lumbar puncture

• Opening pressure
  The opening pressure of CSF is measured with an air-water manometer.  The opening pressure is normally elevated in cases of bacterial meningitis, with values of >600 mm H₂O.¹
  
• Appearance of CSF
  Appearance of CSF is normally clear and colourless, but may appear cloudy or turbid in patients with elevated concentrations of WBCs (>200/mm³), red blood cells (>400/mm³), bacteria (>105 colony-forming units/ml) or protein.¹  Specific CSF results may vary depending upon the infectious agent involved.
  
• WBC count
  Elevated CSF concentrations of WBCs are often seen in cases of bacterial meningitis.  This disease can be diagnosed if there is a WBC count of >1000 cells/mm3 (<3 cells/mm³) in normal CSF).  However, normal CSF WBC counts have been observed in patients with meningococcal meningitis, so a Gram stain and culture should be performed even if CSF WBC levels are normal.  Patients with very low CSF WBC counts (0–20/mm³) despite high CSF bacterial concentrations tend to have a poorer prognosis than those with high WBC counts.¹ 
  
• Protein concentration
  CSF protein concentration may be elevated in patients with bacterial meningitis, presumably because of disruption to the blood-brain barrier.  Lumbar CSF protein concentrations of >220 mg/dL are considered abnormal.  However, elevated protein concentration is among the most common and least specific of all CSF parameters and may be observed in other disorders of the central nervous system.¹ 
  
• Glucose concentration
  CSF glucose concentrations may be abnormally low (hypoglycorrhachia) in patients with meningitis.  The normal CSF:serum glucose ratio is approximately 0.6 and a ratio of <0.23 is considered abnormal.  CSF glucose concentrations of <40 mg/dL are also considered abnormal.¹
  
• Bacterial culture and Gram stain
  Bacterial cultures are important in determining which agent is the cause of the disease.  Gram staining of CSF enables rapid and accurate identification of the bacterium responsible for the disease in 60–90% of patients, with a specificity of nearly 100%.  However, Gram staining may not identify the agent responsible if bacterial concentrations are very low (<103 colony forming units/ml).
  
• Other tests
  C-reactive protein and procalcitonin concentrations may be elevated in patients with bacterial meningitis and may be useful in distinguishing whether the infection is bacterial or viral.  Polymerase chain reaction may also be used to identify meningeal pathogens.

Reference:
1. Mandell GL, Bennett JE, Dolin R. Mandell, Douglas, and Bennett’s Principles and Practices of Infectious Diseases. Elsevier 6th ed. USA: 2005:1083–1126.