By obtaining a sample of cerebrospinal fluid (CSF) by lumbar puncture. CSF should be sent to the laboratory for chemistry, microscopic examination for cells and bacteria, and for culture. As many children with meningitis also have septicaemia, the bacterial cause can often also be identified on a blood culture.
Do not do a lumbar puncture if there is reduced level of consciousness, focal neurological signs or features of meningococcal meningitis. With bacterial meningitis the CSF protein is raised (normal 0.15–0.4 g/l) and the glucose is low (normal 2–4 mmol/l) with many polymorphonuclear cells. Bacteria may be seen on a stained spun deposit or may be cultured.