There are different ways to get a sample of CSF. Lumbar puncture, commonly called a spinal tap, is the most common method. The test is usually done like this:
The patient lies on his or her side, with knees pulled up toward the chest, and chin tucked downward. Sometimes the test is done with the person sitting up, but bent forward.
After the back is cleaned, the health care provider will inject a local numbing medicine (anesthetic) into the lower spine.
A spinal needle is inserted, usually into the lower back area.
Once the needle is properly positioned, CSF pressure is measured and a sample is collected.
The needle is removed, the area is cleaned, and a bandage is placed over the needle site. The person is often asked to lie down for a short time after the test.
Occasionally, special x-rays are used to help guide the needle into the proper position. This is called fluoroscopy.
Lumbar puncture with fluid collection may also be part of other procedures, particularly a myelogram (x-ray or CT scan after dye has been inserted into the CSF).
Alternative methods of CSF collection are rarely used, but may be necessary if the person has a back deformity or an infection.
Cisternal puncture uses a needle placed below the occipital bone (back of the skull). It can be dangerous because it is so close to the brain stem. It is always done with fluoroscopy.
Ventricular puncture is even more rare, but may be recommended in people with possible brain herniation. This test is usually done in the operating room. A hole is drilled in the skull, and a needle is inserted directly into one of brain's ventricles.
CSF may also be collected from a tube that's already placed in the fluid, such as a shunt or a ventricular drain. These sorts of tubes are usually placed in the intensive care unit.
How to prepare for the test
The patient (or guardian) must give the health care team permission to do the test.
Afterward, you should plan to rest for several hours, even if you feel fine. You won't be required to lie flat on your back the entire time, but rest is advised to prevent additional leakage of CSF around the site of the puncture.
How the test will feel
The test is usually done with you curled up on your side with knees pulled up and chin to chest. Sometimes, CSF is collected with the person seated and bent forward over a table or chair. Holding the position may be uncomfortable, but it is extremely important to stay in this bent position to avoid moving the needle and possibly injuring the spinal cord. The person doing the test may ask you to straighten out slightly after the needle is in place, in order to accurately measure the CSF pressure, called the "opening pressure."
The anesthetic will sting or burn when first injected. There will be a hard pressure sensation when the needle is inserted, and there is usually some brief pain when the needle goes through the tissue surrounding the spinal cord. This pain should stop in a few seconds.
Overall, discomfort is minimal to moderate. The entire procedure usually takes about 30 minutes, but it may take longer. The actual pressure measurements and CSF collection only take a few minutes.
Why the test is performed
This test is done to measure pressures within the cerebrospinal fluid and to collect a sample of the fluid for further testing. CSF analysis can be used to diagnose certain neurologic disorders, particularly infections (such as meningitis) and brain or spinal cord damage.
CSF glucose: 50 - 80 mg/100 mL (or greater than 2/3 of blood sugar level)
CSF cell count: 0 - 5 white blood cells (all mononuclear), and no red blood cells
Chloride: 110 - 125 mEq/L
Note: mg/mL = milligrams per milliliter; mEq/L = milliequivalents per liter
Note: Normal value ranges may vary slightly among different laboratories. Talk to your doctor about the meaning of your specific test results.
The examples above show the common measurements for results for these tests. Some laboratories use different measurements or may test different specimens.
What abnormal results mean
If the CSF looks cloudy, it could mean there is an infection or a build up of white blood cells or protein.
If the CSF looks bloody or red, it may be a sign of bleeding or spinal cord obstruction. If it is brown, orange, or yellow, it may be a sign of increased CSF protein or previous bleeding (more than 3 days ago). Occasionally, there may be blood in the sample that came from the spinal tap itself. This makes it harder to interpret the test results.
Hypersensitivity (allergic) reaction to the anesthetic
Infection introduced by the needle going through the skin
There is an increased risk of bleeding in people who take blood thinners.
Brain herniation may occur if this test is done on a person with a mass in the brain (such as a tumor or abscess). This can result in brain damage or death. This test is not done if an exam or test reveals signs of a brain mass.
Damage to the nerves in the spinal cord may occur, particularly if the person moves during the test.
Cisternal puncture or ventricular puncture carry additional risks of brain or spinal cord damage and bleeding within the brain.
This test is particularly dangerous for people with:
A tumor in the back of the brain that is pressing down on the brain stem
Blood clotting problems
Low platelet count (Thrombocytopenia)
Griggs RC, Jozefowicz RF, Aminoff MJ. Approach to the patient with neurologic disease. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier. 2007: chap 418.
David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine; Kevin Sheth, MD, Department of Neurology, University of Maryland School of Medicine, Baltimore, MD. Review provided by VeriMed Healthcare Network. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.