CSF blood glucose provides a significant hint towards a possible bacterial meningitis by dropping to a level lower than 45mg/dL.
But if a patient has high blood sugar state (e.g. not properly controlled blood sugars), then it’s expected that some of this high blood sugar will be reflected in the CSF sugar levels because blood sugar will cross blood brain barrier and finally be in CSF too. Cells in the blood brain barrier transports glucose actively. This can make the CSF sugar level drop less significant to point towards a bacterial meningitis. In such hyperglycaemic scenario, a ratio of the blood and CSF sugar will be helpful.
If the blood and CSF ratio is less than 0.4 than it will hint towards a bacterial aetiology for meningitis. It can also suggest fungal or tubercular aetiology too.