The cerebrospinal fluid (CSF) glucose test measures the amount of glucose, or sugar, 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 (CNS). The fluid delivers nutrients to and carries waste away from the brain and spinal cord. The choroid plexus in the brain continually produces it and your bloodstream continually absorbs it. Your body completely replaces CSF every few hours.

The glucose test helps in diagnosing certain conditions of the CNS, 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 diagnose these conditions because CSF is in direct contact with the brain and spine.

The most common method of collecting cerebral spinal fluid is a lumbar puncture, sometimes called a spinal tap or a glucose measurement of cerebrospinal fluid.

Your doctor will perform this test to diagnose certain medical conditions. These conditions may include:

  • infections
  • tumors
  • inflammation of the CNS
  • neurological conditions, such as delirium

Depending on your medical history, your doctor may order a CSF analysis when the following symptoms are present:

  • a severe headache
  • a stiff neck
  • hallucinations
  • confusion
  • dementia
  • seizures
  • flu-like symptoms that persist or intensify
  • fatigue
  • lethargy
  • muscle weakness
  • changes in consciousness
  • severe nausea
  • a fever
  • a rash
  • light sensitivity
  • numbness
  • tremor
  • dizziness
  • difficulty speaking
  • difficulty walking
  • poor coordination
  • mood swings
  • depression

You’ll need to sign a form stating that you understand the risks of the procedure.

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, your doctor will ask you to empty your bowels and bladder.

Lumbar puncture usually takes less than 30 minutes. The doctor who performs it is trained to collect CSF safely. Doctors usually withdraw CSF from the lower back area.

You should remain still during the procedure to avoid incorrect needle placement or trauma to your spine and surrounding tissues. You’ll either sit with your spine curled forward or lie on your side with your spine curved and your knees drawn up to your chest.

Curving your spine helps your doctor to find the correct space to insert a thin spinal needle between the bones in your lower back. These bones are called vertebrae. Sometimes, fluoroscopy, which is a type of X-ray, can help your doctor guide the needle safely between the vertebrae.

When you’re in position, a healthcare provider will clean your back with a sterile solution such as iodine. Your doctor will maintain a sterile area throughout the procedure to lower your risk of infection. Your doctor may apply numbing cream to your skin before injecting an anesthetic solution, or they may inject an anesthetic without first applying a numbing cream. When the site is numb, your doctor will insert the spinal needle.

Once the needle is in, CSF pressure, or opening pressure, is usually measured using a manometer, or pressure gauge. High CSF pressure can be a sign of certain conditions and diseases, including:

  • meningitis, which is inflammation of the brain or spinal cord
  • intracranial hemorrhage, or bleeding in the brain
  • tumors

Your doctor may also order pressure at the end of the procedure. This is called closing pressure.

Your doctor then allows CSF to drain slowly into a vial that they’ll send to a lab. Depending on the required tests, your doctor may take several vials of fluid. When fluid collection is complete, your doctor will remove the needle from your back. The puncture site is cleaned again with sterile solution, and they’ll apply a bandage. You must remain lying down for about an hour to avoid a headache, which is a common side effect of the procedure.

Rarely, if you have a back deformity, infection, or possible brain herniation, which is when structures of the brain have shifted out of place, it’s necessary to use more invasive CSF collection methods. These methods usually require hospitalization. They include ventricular puncture. During this procedure, your doctor drills a hole into your skull and inserts a needle directly into one of the ventricles of the brain. They may also perform a cisternal puncture. During this procedure, a doctor inserts a needle below the base of the skull.

The risks of lumbar puncture include:

  • bleeding from the puncture site into the spinal fluid, or traumatic tap
  • discomfort during and after the procedure
  • an allergic reaction to the anesthetic
  • an infection at the puncture site
  • a 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.

Additional risks exist if you have a brain mass, such as a tumor or abscess, which is pus surrounded by inflamed tissue. In these cases, a lumbar puncture can cause brain herniation, which is when part of the brain shifts out of the correct position. Brain herniation results from high intracranial pressure. This shift in brain tissue can lead to compression of certain parts of the brain, which can eventually cut off the supply of blood to the brain. This can cause brain damage, or even death. If your doctor suspects you have a brain mass, they won’t perform a lumbar puncture.

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 has severe risks for people who have low platelet count, called thrombocytopenia, or other blood clotting problems.

Abnormal results

This test measures the amount of glucose, or sugar, in the CSF.

No true normal range for glucose in the CSF exists. Your doctor must compare it to the level of glucose 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 CNS conditions can cause lower CSF glucose levels. Infections, especially bacterial infections, and tumors are the most common causes of low CSF glucose levels.

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, or digest, the glucose. This may result in lower-than-normal levels.

Abnormally low glucose may mean you have:

  • a bacterial infection
  • a fungal infection
  • inflammation of the CNS
  • a tumor
  • chemical meningitis
  • a subarachnoid hemorrhage, or bleeding in the space between your brain and the thin tissue that covers it
  • hypoglycemia, or low blood sugar

Researchers haven’t found a specific disease or condition that causes elevated glucose in the CSF. It only occurs if blood glucose levels are elevated.

Normal results

Normal results mean that your CSF glucose levels are within normal range.

However, normal results don’t rule out the possibility of infection. Glucose is often normal in people with viral infections and bacterial meningitis. Your doctor may order tests if they’re concerned that you may have an infection.

If your doctor finds abnormalities in CSF glucose measurement, you may need further tests. Your doctor will provide treatment based on the cause of your symptoms.

Bacterial meningitis is a medical emergency. This infection is hard to diagnose because its symptoms are similar to viral meningitis, which is a less life-threatening illness. Therefore, prompt treatment is essential. The doctor may give you broad-spectrum antibiotics, which are medications that treat a variety of bacteria types. You may receive these antibiotics while your doctor conducts additional tests to find the cause of your infection.