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Tests & Procedures : Lab Tests

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Glucose (CSF)

Does this test have other names?

CSF glucose

What is this test?

This test measures the amount of glucose, or blood sugar, is in your cerebrospinal fluid (CSF), the fluid that surrounds the brain and spinal cord. People with serious infections that have reached the brain or spinal cord usually have lower glucose levels in their CSF than healthy people do.

This test is usually part of an overall look at CSF to help diagnose a central nervous system disorder or infection. Normally, your brain is protected from any germs in your bloodstream by a thin barrier. But when you're sick, this blood-brain barrier can become leaky, allowing bacteria and other substances to pass through.  Bacteria, viruses, and other germs can then reach your spinal cord and brain, which could cause brain swelling and nervous system infection. 

Why do I need this test?

You are likely to have this test if you have symptoms of a possible brain or central nervous system infection. These include:

  • Severe headache

  • Nausea

  • Vomiting

  • Irritability

  • Sensitivity to light

  • Confusion

  • Changes in consciousness

  • Neck so stiff that it's difficult to bend forward

Babies also often have their CSF tested if a doctor suspects they have a serious infection that could cause swelling and damage the brain. 

What other tests might I have along with this test?

Your doctor may also order other tests on the CSF sample, depending on what condition you have. These tests include:

  • Bacterial Gram stain and culture. The laboratory grows a culture from a CSF sample to find out whether you have an infection.

  • Viral culture of CSF

  • CSF-blood glucose ratio

  • CSF protein concentration

  • CSF leukocyte , or white blood cell, count. It's usually high if you have an infection.

  • Blood culture 

What do my test results mean?

Many things may affect your lab test results. These include the method each lab uses to do the test. Even if your test results are different from the normal value, you may not have a problem. To learn what the results mean for you, talk with your health care provider.

Results are given in milligrams per deciliter (mg/dL) or millimoles per liter (mmol/L). Normal levels of glucose in the CSF are 50 to 80 mg/dL, or 2.8 to 4.4 mmol/L. This level should be about 60 percent of the glucose level in your blood. If it is not, you may have an infection.

Your doctor will look at your CSF glucose level along with the other CSF tests, and possibly other tests, to better understand what your results mean.

How is the test is done? 

This test requires obtaining a sample of your cerebrospinal fluid. Your doctor will take the sample through a lumbar puncture, using a thin needle. The needle will be pushed into your lower back, and fluid will be removed.

Does this test pose any risks? 

A lumbar puncture, or LP, carries these possible risks:

  • Headaches

  • Infection

  • Bleeding

  • Tumors

  • Numbness   

What might affect my test results?

Some medications, foods, and beverages may affect the results.

How do I prepare for the test?

Your doctor may ask you not to eat or drink for four hours before the test. Be sure your doctor knows about all medicines, herbs, vitamins, and supplements you are taking. This includes medicines that don't need a prescription and any illicit drugs you may use.