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 

Patients will be asked to either sit up or lie down on their sides with the back parallel to the bed and the chin tucked under and the 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 what is known as the L3-L4 vertebral level.

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

A thin 20-gauge needle will then be inserted into the interspace between the vertebra. 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). At that point there will be fluid flowing out of the needle and this will be collected in plastic containers. Under normal circumstances, the opening pressure will be measured with a manometer and then about 20-30 cc of spinal fluid will be collected and sent for evaluation under the microscope.

Under 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.

The needle is withdrawn, the sensation subsides and is not permanent. Following the procedure, the patient is advised 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:       o hypersensitivity (allergic) reaction to the anesthetic       o discomfort during the test       o headache after the test       o bleeding into the spinal canal  
  • Brain herniation (if performed on a person with increased intracranial pressure), and 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.

Johns Hopkins patient information

Last revised: December 7, 2012
by Sharon M. Smith, M.D.

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