What is a Lumbar Puncture Test?
A cerebrospinal fluid (CSF) test (also known as a lumbar puncture or a spinal tap) is the most common method of collecting a sample of a patient’s spinal fluid. The cerebrospinal fluid, normally clear and colorless, surrounds the brain and spinal cord and acts as a buffer. Any changes in the fluid’s color, consistency or quantity may indicate a neurological disease or disorder.
Bacteria or increased numbers of white blood cells in the CSF will make it appear cloudy and may indicate an infection in the brain or spinal cord such as meningitis or Lyme disease. High protein levels are often a sign of a spinal cord tumor or an acute peripheral nerve disorder such as Guillain-Barré syndrome  (GBS) or chronic inflammatory demyelinating polyneuropathy  (CIDP). Abnormal antibodies may suggest multiple sclerosis. A variety of diseases, including meningitis and brain tumors, can increase the fluid’s pressure.
Purpose of a Lumbar Puncture Test
To identify the presence of an autoimmune disorder , such as GBS or chronic inflammatory demyelinating polyneuropathy (CIDP), a sample of the patient’s cerebrospinal fluid often must be analyzed. In a patient with GBS or CIDP, the fluid will contain more protein than usual, with no increase in the number of white blood cells or pressure.
How is a Lumbar Puncture Test Performed?
In the procedure, the patient is positioned lying on his/her side. A small anesthetic is injected into the skin in the middle of the lower back. A long, thin needle is inserted into this region and in the spinal canal. The CSF pressure is measured and fluid is collected for testing. After the sample is collected, the needle is removed and the patient must remain flat, or nearly flat, for an hour to two after the test.
A lumbar puncture with fluid collection may also be part of another procedure, particularly a myelogram (a CAT scan  after dye has been inserted into the CSF).
Does a Lumbar Puncture Test Hurt?
Overall, discomfort from the procedure is minimal. The entire procedure usually takes about 30 minutes. The actual pressure measurement and fluid collection take only a few minutes. Some patients experience headaches, hours or days after the test. These headaches almost always disappear spontaneously over time.