Cerebral spinal fluid (CSF) collection is a test to sample and measure pressure in the fluid that surrounds the brain and spinal cord. Cerebral spinal fluid acts like a cushion, protecting the brain and spine from injury. The fluid is normally clear.

See also CSF culture.

There are different ways to get a sample of spinal fluid. Lumbar puncture, commonly called a spinal tap, is the most common method. The test is usually performed in the following manner:

• The patient must lay on his or her side, with the knees pulled up toward the chest, and the chin tucked downward. Sometimes the test is done with the person sitting up, but bent over. 
• 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 between the 3rd and 4th lumbar vertebrae.
• Once the needle is properly positioned, spinal fluid pressure is measured, and fluid is collected.
• The needle is removed, the area is cleaned, and a bandage is placed over the needle site. The patient will need to remain flat for 20 minutes to 1 hour after the test.

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 in in the event of a back deformity or 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.

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.

The patient (or guardian) must sign a consent for. The patient must remain in the hospital for at least several hours after the test for observation.

The position may be uncomfortable, but it is imperative that the patient stays in the bended position to avoid moving the needle and possibly injuring the spinal cord.

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 (meninges). 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 fluid collection only take a few minutes.

This test is done to measure pressures within the cerebrospinal fluid and to collect  a sample of the fluid for further testing. CSF can be used to diagnose certain neurologic disorders, particularly infections (such as menigitis) and brain or spinal cord damage.

See also:

• CSF coccidioides complement fixation
• CSF oligoclonal banding 
• CSF smear
• CSF VDRL test

Nathan, BR. Cerebrospinal Fluid and Intracranial Pressure. In: Goetz, CG, ed. Textbook of Clinical Neurology, 2nd ed. Philadelphia, Pa:WB Saunders Company; 2003:511-524.