Meningitis laboratory tests

Gram stain and culture of the CSF are the most important laboratory tests performed for bacterial meningitis. When performed before antibiotic therapy is initiated, Gram stain is both rapid and sensitive and can confirm the diagnosis of bacterial meningitis in 75% to 90% of cases. 

The major concern of the emergency department physicians was the lethargy, hypotonia, and seizure activity. Initial laboratory studies revealed that the child had a normal complete blood count and smear. Other blood tests revealed metabolic acidosis with a bicarbonate concentration of 11 mEq/L (normal is 20-25 mEq/L) and an anion gap of 22 mEq/L (normal is < 15 mEq/L). His serum glucose, calcium, and magnesium concentrations were normal. To exclude the diagnosis of meningitis, a spinal tap was performed. The cell counts and chemistries of the cerebrospinal fluid were normal. The physicians considered that the child might have sepsis and administered antibiotics and intravenous fluids. Prior to administration of antibiotics, blood, urine, and cerebrospinal fluid were sent for bacterial culture. 

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