Lumbar Puncture (Spinal Tap)
Lumbar puncture (spinal tap) is performed in your lower back, in the lumbar region. During lumbar puncture, a needle is inserted between two lumbar bones (vertebrae) to remove a sample of cerebrospinal fluid — the fluid that surrounds your brain and spinal cord to protect them from injury.
A lumbar puncture can help diagnose serious infections, such as meningitis; other disorders of the central nervous system, such as Guillain-Barre syndrome and multiple sclerosis; or cancers of the brain or spinal cord. Sometimes doctors use lumbar puncture to inject anesthetic medications or chemotherapy drugs into the cerebrospinal fluid.
Why It’s Done
Lumbar puncture may be done to:
- Collect cerebrospinal fluid for laboratory analysis
- Measure the pressure of your cerebrospinal fluid
- Inject spinal anesthetics, chemotherapy drugs or other medications
- Inject dye (myelography) or radioactive substances (cisternography) into cerebrospinal fluid to make diagnostic images of the fluid’s flow
Information gathered from a lumbar puncture can help diagnose:
- Bleeding around the brain (subarachnoid hemorrhage)
- Certain cancers involving the brain or spinal cord
- Certain inflammatory conditions of the nervous system, such as multiple sclerosis and Guillain-Barre syndrome
- Serious bacterial, fungal and viral infections, including meningitis, encephalitis and syphilis
Though lumbar puncture is generally recognized as safe, it does carry some risks. These include:
- Post-lumbar puncture headache
- Back discomfort or pain
- Brainstem herniation
A computerized tomography (CT) scan or MRI prior to a lumbar puncture can be obtained to determine if there is evidence of a space-occupying lesion that results in increased intracranial pressure. This complication is uncommon.
The spinal fluid samples are sent to a laboratory for analysis. Lab technicians check for a number of things when examining spinal fluid, including:
- General appearance. Spinal fluid is normally clear and colorless. If it’s cloudy, yellow or pink in color, it may indicate infection.
- Protein (total protein and the presence of certain proteins). Elevated levels of total protein — greater than 45 milligrams per deciliter (mg/dL) — may indicate infection or another inflammatory condition. Specific lab values may vary from medical facility to medical facility.
- White blood cells. Spinal fluid normally contains up to 5 mononuclear leukocytes (white blood cells) per microliter. Increased numbers may indicate infection. Specific lab values may vary from medical facility to medical facility.
- Sugar (glucose). A low glucose level in spinal fluid may indicate infection or another condition.
- Microorganisms. The presence of bacteria, viruses, fungi or other microorganisms can indicate infection.
- Cancer cells. The presence of abnormal cells in spinal fluid — such as tumor or immature blood cells — can indicate certain types of cancer.
Lab results are combined with information obtained during the test, such as spinal fluid pressure, to help establish a possible diagnosis.