Cerebrospinal fluid
A lumbar puncture (spinal tap) is necessary to diagnose meningitis. Cerebrospinal fluid culture is the most important study for the diagnosis of neonatal bacterial meningitis because clinical signs are non-specific and unreliable. Blood cultures may be negative in 15-55% of cases, making them unreliable as well. However, a cerebrospinal fluid to blood glucose ratio below two-thirds has a strong relationship to bacterial meningitis. A spinal tap should be done in all neonates with suspected meningitis, with suspected or proven sepsis (whole body inflammation) and should be considered in all neonates in whom sepsis is a possibility. The role of the spinal tap in neonates who are healthy appearing but have maternal risk factors for sepsis is more controversial; its diagnostic yield in these patients may be low.