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Spinal tap gives clues on infection

Tue., May 15, 2012

DEAR DOCTOR K: My doctor wants me to have a spinal tap to help confirm a diagnosis. What happens during a spinal tap?

DEAR READER: A spinal tap is also called a lumbar puncture, or “LP” for short. During this procedure, fluid known as cerebrospinal or spinal fluid is removed from the space surrounding your spinal cord. The term “spinal tap” sounds scary, but it is only briefly painful and mostly risk-free.

If a person has an infection of the brain or spinal cord, signs of infection (such as the presence of white blood cells) can be seen in the spinal fluid. The fluid can help identify the specific germ causing an infection. It can distinguish a life-threatening bacterial infection (bacterial meningitis) from milder viral infections. If a person has a brain hemorrhage, red and white blood cells will be in the spinal fluid.

An area on your lower back will be cleaned with an antiseptic solution. Then your doctor will use a tiny needle to inject numbing medicine just below the skin. This will numb the area where the spinal needle will be inserted. This medicine causes very brief stinging, but then the area is numb when the larger spinal needle is inserted.

The doctor will insert the thin spinal needle into your lower back, slowly pushing the needle forward until it reaches the spinal canal.

At this point, fluid from the spinal canal will begin to drip out of the spinal needle into a sterile test tube. Usually less than 3 tablespoons of spinal fluid are collected.

Because of the numbing medicine, you should experience only a little pressure when the fluid is removed. Let your doctor know if you feel any pain.

The whole lumbar puncture takes 30 to 45 minutes. The spinal needle will be in place for about one minute.

Dr. Komaroff is a physician and professor at Harvard Medical School. To send questions, go to, or write: Ask Doctor K, 10 Shattuck St., Second Floor, Boston, MA 02115.


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