Nothing, glucose is present at all times, all though the presencié of LOW glucose, can occur because bacteria are present (meningitis)
Cerebrospinal fluid is a clear fluid surrounding the brain and the spinal cord. It is collected via a lumbar puncture under sterile conditions. CSF is analyzed to confirm certain diseases. Common tests performed on CSF include protein and glucose levels, cell counts and differential count, microscopic examination, stains, and cultures. CSF may also be tested for glutamine, lactate dehydrogenase, lactic acid, tumor markers and C-reactive proteins. At the time of the tap, the CSF glucose level is usually around 60% of the peripheral blood glucose level. A simultaneous blood glucose test is advised (particularly if the CSF glucose level is expected to be low). Glucose levels decrease when cells like bacteria, white blood cells or cells shed by tumors, which are not usually present use up the glucose. Bacterial infection is frequently linked to a low CSF glucose level (probably due to enzymatic inhibition rather than to actual bacterial consumption of the glucose). Even if cytologic results are negative, this feature is observed in tumour infiltration and may be one of the characteristics of meningeal carcinomatosis. A high CSF glucose level is usually a result of an increased blood glucose level and has no diagnostic relevance.
Normal glucose levels in CSF is 40-70 mg/dl If glucose decreases the. We can consider presence of bacteria which consumes glucose.