[ Tests & Procedures]

Lumbar Puncture (LP)

What is a lumbar puncture (LP)?

A lumbar puncture (an LP) is the insertion of a needle into the fluid within the spinal canal. It is termed a "lumbar puncture" because the needle goes into the lumbar portion (the "small") of the back.

Other names for a lumbar puncture (an LP) include spinal tap, spinal puncture, thecal puncture, and rachiocentesis.

Why is a lumbar puncture done?

An LP is most commonly done for diagnostic purposes, namely to obtain a sample of the fluid in the spinal canal (the cerebrospinal fluid) for examination.

An LP can also be done for therapeutic purposes, namely as a way of administering antibiotics, cancer drugs, or anesthetic agents into the spinal canal. Spinal fluid is sometimes removed by LP for the purpose of decreasing spinal fluid pressure in patients with uncommon conditions (such as, for examples, normal-pressure hydrocephalus and benign intracranial hypertension).

How is the LP performed?

The patient is typically lying down sideways for the procedure. Less often, the procedure is performed while the patient is sitting up. LPs in infants are often done upright.

After local anesthesia is injected into the small of the back (the lumbar area), a needle is inserted in between the nearby bony building blocks (vertebrae) into the spinal canal. (The needle is usually placed between the 3rd and 4th lumbar vertebrae).

What is done next?

Spinal fluid pressure can then be measured and cerebrospinal fluid (CSF) removed for testing.

What is the cerebrospinal fluid (CSF)?

The CSF circulates around the brain and spinal cord (the central nervous system). This "water bath" acts as a support of buoyancy for the brain and spinal cord. The support of the CSF helps to protect the brain from injury.

The normal CSF contains various chemicals such as protein and sugar (glucose), and few if any cells. The spinal fluid also has a normal pressure when first removed.

What is normal CSF?

Normal values for spinal fluid examination are as follows:

Protein (15-45 mg/dl)

Glucose (50-75 mg/dl)

Cell count (0-5 mononuclear cells)

Initial pressure (70-180 mm)