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Finding the right tests…
Glucose
Sugar (Fluid) measures glucose in a body fluid other than blood — most often cerebrospinal fluid collected by lumbar puncture, though the same measurement can be done on pleural, peritoneal, or joint fluid. Because glucose normally moves freely between blood and these fluids, a fluid glucose level well below what's expected for your blood sugar at the same time is a meaningful clue, not a routine finding. This is why the test is ordered together with a blood glucose sample taken around the same time, so the two numbers can be compared directly rather than read alone.
This test reports glucose concentration, in mg/dL, in the fluid sample your doctor has collected — commonly cerebrospinal fluid, but sometimes pleural or ascitic fluid depending on what's being investigated. Cerebrospinal fluid glucose is normally around two-thirds of the blood glucose level measured at the same time, so the lab result is read by your doctor against a same-day blood glucose sample rather than a fixed number alone.
Cerebrospinal fluid glucose is one of the standard checks run on a sample taken by lumbar puncture when a doctor suspects meningitis. In bacterial meningitis, glucose is typically driven down — both by the bacteria consuming it and by white blood cells fighting the infection — while in viral meningitis it usually stays within the normal range. This makes fluid glucose, read alongside cell counts and protein levels from the same sample, one of the clues that helps a doctor tell the two apart before culture results are back.
Normal cerebrospinal fluid glucose shifts with age — babies run higher relative to their blood glucose than adults do, and the difference narrows as we get older. Because of this, the result is always read together with a blood glucose sample taken close to the same time, rather than against one fixed cutoff for everyone.
The same glucose measurement is sometimes done on fluid drawn from other spaces in the body, such as the chest or abdominal cavity, when a doctor is investigating an unexplained fluid buildup — a low glucose level there can point toward infection or, less often, a tumour.
Glucose normally moves freely between blood and fluids like cerebrospinal fluid. So a fluid glucose that's unexpectedly low compared to a same-day blood glucose is a specific clue to infection or another problem in that space.
In cerebrospinal fluid, a low glucose level relative to blood glucose is a recognised sign of bacterial meningitis, and can also occur with certain cancers or other infections. Your doctor interprets it alongside cell counts and protein levels from the same sample.
Adult cerebrospinal fluid glucose generally runs around 50 to 80 mg/dL, roughly two-thirds of the blood glucose measured at the same time, based on published reference sources. Ranges vary by lab and shift with age, so always check the range printed on your own report.
This page is general information about the test, not medical advice. Your own report's reference ranges apply — please discuss your results with your doctor.
No. The same glucose measurement can be done on fluid from other body spaces, such as the chest or abdomen, when a doctor is investigating an unexplained fluid buildup.
Cerebrospinal fluid is collected by a lumbar puncture, a procedure your doctor performs and explains beforehand. Fluid from other body spaces is collected by a separate procedure suited to that location.
Because the fluid result is interpreted as a ratio against your blood glucose from around the same time, not as a stand-alone number. Without a matched blood sample, a low fluid reading is harder to interpret correctly.
Not entirely. Viral meningitis, for example, often leaves cerebrospinal fluid glucose within the normal range, so your doctor weighs this result together with other fluid tests, your symptoms, and sometimes imaging.