The cerebrospinal fluid (CSF) glucose test is one in a group of laboratory procedures performed to analyze a patient’s CSF. This test measures the amount of glucose (sugar) that is present in the fluid.
CSF is a clear liquid that cushions and surrounds the brain and spinal cord, which make up your central nervous system. The fluid delivers nutrients and carries away waste. It is continually produced by the choroid plexus in the brain and absorbed into the blood stream. The body completely replaces the CSF every few hours.
The glucose test helps in diagnosing certain conditions of the central nervous system, such as bacterial infection or tumors. Although obtaining a spinal fluid sample is difficult, testing a CSF sample is one of the best ways to correctly diagnose these conditions because CSF is in direct contact with the brain and spine.
The most common method of collecting cerebral spinal fluid is lumbar puncture, sometimes called spinal tap or glucose measurement of cerebrospinal fluid.
This test is done to assist in diagnosing medical conditions, such as infections, tumors, and inflammation of the central nervous system, and neurological conditions such as delirium.
Depending on your medical history, your doctor may order a CSF analysis when the following symptoms are present:
- severe headache
- stiff neck
- hallucinations, confusion, or dementia
- flu-like symptoms that persist or intensify
- fatigue, lethargy, or muscle weakness
- changes in consciousness
- severe nausea
- fever or rash
- light sensitivity
- numbness or tremor
- speaking difficulties
- trouble walking or poor coordination
- mood swings or depression
A lumbar puncture requires a signed release stating that you understand the risks of the procedure.
Be sure to tell your doctor if you take any blood-thinning medications such as warfarin (Coumadin). You may need to stop taking them for a couple of days before the procedure.
Before the procedure, you’ll be asked to empty your bowels and bladder.
Lumbar puncture usually takes less than 30 minutes and is performed by a physician specially trained to safely collect CSF. CSF is usually withdrawn from the lower back area.
It is very important to remain completely still to avoid incorrect needle placement or trauma to the spine. You will either be seated with your spine curled forward or lie on your side with your spine curved and your knees drawn up to your chest.
Curving the spine allows the doctor to find ample space to insert a thin spinal needle between the bones in the lower back (vertebrae). Sometimes fluoroscopy (X-ray) is used to guide the needle safely between the vertebrae.
When you are in position, the doctor or a nurse will clean your back with a sterile solution such as iodine. A sterile area is maintained throughout the procedure to lower the risk of infection. Numbing cream is applied to the skin and the site is injected with an anesthetic (pain-killing) solution. When the site is numb, the doctor inserts the spinal needle.
Once the needle is in, CSF pressure (opening pressure) is usually measured using a manometer (pressure gauge). High CSF pressure can be a sign of certain conditions and diseases, including:
- meningitis (inflammation of the brain and/or spinal cord)
- intracranial hemorrhage (bleeding in the brain)
Pressure may also be measured at the end of the procedure (closing pressure).
The doctor then takes fluid samples up through the needle and into an attached syringe. Several vials of fluid may be taken. When fluid collection is complete, the doctor takes the needle out of your back. The puncture site is cleaned again with sterile solution and a bandage is applied.
You must remain lying down for about an hour to avoid headache, a common side effect of the procedure. Rarely, if you have a back deformity, infection, or possible brain herniation (when structures of the brain have shifted out of place), it is necessary to use more invasive CSF collection methods.
These methods usually require hospitalization. They include:
- ventricular puncture: a doctor drills a hole into the skull and inserts a needle directly into one of the ventricles of the brain
- cisternal puncture: a doctor inserts a needle below the base of the skull
Primary risks associated with lumbar puncture include:
- bleeding from the puncture site into the spinal fluid (traumatic tap)
- discomfort during and after the procedure
- allergic reaction to the anesthetic
- infection at the puncture site
- headache after the test
- damage to spinal cord nerves (especially if you move during the procedure)
If you take blood thinners, your risk of bleeding is higher.
There are serious additional risks if you have a brain mass, such as a tumor or abscess (puss surrounded by inflamed tissue). In these cases, lumbar puncture can cause brain herniation. Brain herniation results in high intracranial pressure, which can eventually cut off the supply of blood to the brain. This can result in brain damage or even death. If a brain mass is suspected, lumber puncture will not be performed.
Cisternal and ventricular puncture have additional risks, including:
- damage to the spinal cord or brain
- bleeding within the brain
- disturbance of the blood/brain barrier in the skull
Lumbar puncture is extremely dangerous for people who have low platelet count (thrombocytopenia) or other blood clotting problems.
This test measures the amount of glucose (sugar) in the cerebrospinal fluid.
There is no true normal range for glucose in CSF. It must be compared to the level found in a sample of blood taken within two to four hours of the lumbar puncture. In healthy adults, the ratio of glucose in CSF should be roughly two-thirds of the amount of glucose found in the blood sample.
Certain central nervous system conditions can cause lower CSF glucose levels. Infections, especially bacterial infections, and tumors are the most common cause of low CSF glucose levels (Reeves & Swenson, 2008).
Cells that aren’t usually present in your CSF—such as bacteria, cells produced by tumors, or white blood cells (present to fight inflammation)—can metabolize (digest) the glucose. This may result in lower-than-normal levels.
Abnormally low glucose may mean:
- bacterial infection
- fungal infection
- inflammation of the central nervous system
- chemical meningitis
- subarachnoid hemorrhage (bleeding in the space between the brain and the thin tissue that covers it)
- hypoglycemia (low blood sugar)
There is no known disease or condition that causes elevated glucose in the CSF. It only occurs if blood glucose levels are elevated.
Normal results mean that your CSF glucose levels were within normal range.
However, normal results do not rule out the possibility of infection. Glucose is often normal in patients with viral infections and bacterial meningitis. If there is doubt, other tests will be done.
If abnormalities are found in CSF glucose measurement, further tests may be needed. Appropriate treatment will be provided based on the condition that is found to be causing your symptoms.
Bacterial meningitis is a medical emergency. This infection is hard to diagnose because its symptoms are similar to viral meningitis, a less life-threatening illness. Therefore, prompt treatment is essential. The doctor may put you on broad-spectrum antibiotics (medication that treats a variety of bacteria types) while conducting additional tests to find the exact cause of the infection.