CSF glucose

Definition

CSF glucose is a test to measure the amount of sugar (glucose) in the cerebrospinal fluid (CSF). CSF is a clear fluid that circulates in the space surrounding the spinal cord and brain. Abnormal levels of glucose in CSF suggest pathology in the central nervous system.

How the test is performed

The test is usually performed on a CSF specimen collected by lumbar puncture (spinal tap). The cerebrospinal fluid is analyzed in the lab.

How the test will feel

The patient is asked to either sit up or lie down on his side with back parallel to the bed, chin tucked under, and legs curled in the fetal position. The healthcare provider will use the bony protrusions of the hip to determine the most appropriate location to remove fluid. This is usually at the L3-L4 vertebral level.

Next, iodine is used to clean off the area and a sterile sheet is draped over the surrounding area. In infants and small children, a local anesthetic cream is used to numb the skin and is applied about 1 hour prior to the procedure. In adults, lidocaine is first injected right below the skin, and then deeper such that tissue beneath the skin will also be numbed.

A thin, 20-gauge needle is then inserted into the interspace between the vertebrae (spine bones). Depending on how much subcutaneous tissue there is, this may require shallow or deeper penetration of the needle. There may be an audible pop when the needle penetrates the dura (membrane that surrounds the spinal cord and encases the fluid).

Fluid will then flow out of the needle to be collected in plastic containers. Under normal circumstances, the opening pressure will be measured with a manometer. About 20-30 cc of spinal fluid is collected and sent for evaluation under the microscope.

In experienced hands there should not be any pain. There may be a feeling of pressure when the needle is inserted but with appropriate anesthetic, this should not hurt. Occasionally, some people may feel numbness shooting down the leg. This may be due to irritation of a nerve root and will go away when the needle is withdrawn.

Following the procedure, the patient will be asked to remain lying down for an hour or two to prevent low-pressure headaches. The entire procedure takes approximately 20 minutes.

Why the test is performed

Abnormalities of CSF glucose levels suggest bacterial or fungal infections, tumor or an inflammatory process in the central nervous system. A finding of elevated or decreased CSF glucose can help diagnose the problem in the context of other tests.

Normal Values

The glucose levels should be between 50 to 80 mg/100 ml. Levels may vary slightly depending on the amount of glucose in the blood, however, the level in the CSF should be about 2/3 the level in the blood.

Note: mg/ml = milligrams per milliliter

What abnormal results mean

Abnormal results include increased and decreased glucose levels. The finding of an abnormal glucose in the CSF is useful in diagnosing tumor, infection, inflammation, delirium, and other neurological and medical conditions.

What the risks are

• Risks of lumbar puncture include:
  • hypersensitivity (allergic) reaction to the anesthetic
  • discomfort during the test
  • headache after the test
  • bleeding into the spinal canal
• Brain herniation (if performed on a person with increased intracranial pressure), resulting in brain damage and/or death. This test is not performed in patients with CT/MRI or clinical findings suggestive of increased intracranial pressure.
• Damage to the spinal cord does not occur because the needle is inserted below the level that the spinal cord ends. The needle may irritate a nerve root which results in transient numbness or tingling in the leg. This goes away when the needle is re-adjusted.

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