My doctor said I need to have a spinal tap. What is this test, and why do I need it?

Lumbar puncture. Although you do not specify your symptoms, a spinal tap is employed to help diagnose multiple sclerosis, increased intracranial pressure, various infections such as meningitis, and to evaluate a number of peripheral nerve disorders (e.g., cidp). A needle is inserted into the spinal canal and fluid is collected (sort of like blood drawing). Safe, but headaches may occur after.
Spinal fluid test. A spinal tap is a test in which a small amount of spinal fluid is withdrawn from the small of the back. It is done under local anesthesia, and usually takes no more than a few minutes. Analysis of the fluid can be used to diagnose ms, inflammatory neuropathies, infections of the nervous system, and many other diseases.