Meningitis is a potentially serious infection of the meninges, the tissue covering the brain and the spinal cord. Viruses, bacteria and fungi can cause meningitis. The most important test in identifying or ruling out meningitis is analysis of the cerebrospinal fluid (CSF) collected through lumbar puncture. The CSF sample is examined for the presence and types of leucocytes, protein content and glucose and lactate levels (1). The predominant type of leucocyte and levels of biochemical markers (Table 1) indicate whether the meningitis is of bacterial, viral or fungal origin. However, in the initial phase of the disease this is not always a reliable indicator.
The concentration of glucose in CSF is normally 40 % higher than its concentration in blood. In bacterial meningitis, it is typically lower. The CSF glucose level is divided by the blood glucose (the ratio of CSF glucose to serum glucose) and a ratio ≤ 0.4 is indicative of bacterial meningitis (2).
The main protein in CSF is albumin, a large protein playing an important part in the body’s fluid balance. During bacterial infection, the protein level in the CSF goes up, due to the increased numbers of replicating bacteria and body cells fighting the infection, with both of them having a high concentration of protein.
High levels of lactate in CSF indicate a higher likelihood of bacterial meningitis. If lactate levels are below 35 mg/dL and the patient has not previously received any antibiotics then this may typically rule out bacterial meningitis (3).
Various other specialised tests may be used to distinguish between different types of meningitis. CSF Gram staining, PCR and culturing are essential components of diagnosing acute bacterial meningitis.

Table 1 CSF findings in different aetiologies of meningitis (4)