Lumbar Puncture: Testing for Meningitis
A lumbar puncture is the most common method of obtaining a sample of the fluid in the spinal canal (the cerebrospinal fluid or CSF) for examination. The evaluation of the spinal fluid is usually necessary for the definitive diagnosis and to help make optimal treatment decisions (such as the appropriate choice of antibiotics).
The diagnosis is confirmed by abnormal spinal fluid results and, in the case of an infection, by identifying the organism causing the infection. In patients with meningitis, the CSF fluid often has a low glucose (sugar) level and increased white blood cell count.
What is a lumbar puncture (LP)?
A lumbar puncture (LP) is the insertion of a needle into the fluid within the spinal canal to collect and examine the fluid that surrounds the brain and spinal cord (cerebral spinal fluid). It is termed a "lumbar puncture" because the needle is placed in the lumbar portion of the back and used to puncture through tissues to enter the spinal canal.
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 performed to obtain a sample of the fluid that surrounds the brain and spinal cord. Examination of the fluid can be crucial in diagnosing diseases such as meningitis, neurologic diseases, or effects of systemic disease on the brain and spinal fluid. An LP can also be done to treat diseases, 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, normal-pressure hydrocephalus and benign intracranial hypertension, for example).
Sometimes a lumbar puncture is performed in patients with migraines to assure that no other underlying pathology exists.
How is the LP procedure performed?
The patient is typically lying down on their side for the procedure. Less often, the procedure is performed while the patient is sitting up and leaning slightly forward.
After local anesthesia is injected into the lumbar area of the back , a long needle is inserted in between the bones of the spine (vertebrae) into the spinal canal. (The needle is most commonly placed between the third and fourth lumbar vertebrae.)
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 (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 normally appears clear and contains various substances, such as protein and sugar (glucose), and few if any cells. The spinal fluid also has a normal pressure when first removed.
Medically Reviewed by a Doctor on 11/2/2015