In spinal tap, a thin needle is inserted through the lower (lumbar) region of the back and into the spinal canal to measure the pressure and obtain a sample of the cerebrospinal fluid (CSF). CSF is a clear fluid that cushions the brain and spinal cord, transports many vital nutrients through the central nervous system, and carries out waste products. The fluid sample is then sent to a laboratory for analysis.

Purpose of the Lumbar Puncture

  • To measure the pressure of the CSF and help to detect obstruction of CSF circulation.
  • To aid in the diagnosis of disorders affecting the brain or spinal cord, including infections such as meningitis or encephalitis, inflammation, tumors, bleeding in the brain, neurosyphilis, Guillain Barré syndrome, multiple sclerosis, neuropathy or recurrent seizures
  • To administer chemotherapy medications
  • Used therapeutically to administer drugs into the spinal canal or to lower the pressure of the CSF.

Who Performs It

  • A doctor, sometimes with the assistance of a nurse or a technician

Special Concerns

  • In some cases—for example, in people who cannot assume the curled position required for lumbar puncture or those with an infection near the standard puncture site—an alternative procedure may be performed, in which CSF is drawn from the upper spinal canal via a needle in the back of the neck (cisternal puncture).
  • Lumbar puncture is not appropriate for individuals with increased pressure within the skull (intracranial pressure) or severe arthritis in the lumbar spine.

Before the Lumbar Puncture

  • Prior to lumbar puncture, you must undergo a CT scan to detect increased pressure within the skull, which may make the procedure too dangerous to perform. You will also be given a basic neurologic assessment to evaluate your strength, sensation, and movement (especially in your legs).
  • Do not drink or eat anything after midnight prior to the procedure.
  • You will be advised to empty your bladder and bowels before the test.
  • You will be asked to disrobe and put on a hospital gown.
  • Inform your doctor if you are or think that you may be pregnant.
  • Inform your doctor if you are taking a blood-thinning medicines like Heparin, Warfarin (Coumadin), clopidogrel (Plavix) and Lovenox and over-the-counter pain relievers such as naproxen, ibuprofen or aspirin.

What You Experience

  • You will either lie on your side on a bed or table, with your knees drawn to your abdomen and your chin on your chest (someone may help you maintain this position), or the procedure is done while you are seated and leaning forward at the waist.
  • The skin at the puncture site is cleansed and draped, and the area is numbed with a local anesthetic. You may feel a brief stinging as it is injected.
  • It is important to lie very still during the procedure. You should relax and take deep, slow breaths through your mouth.
  • The doctor inserts a long, thin needle through your lower back and into the subarachnoid space, which is located between membranes that surround the spinal cord. You may feel some brief discomfort as the needle is inserted and repositioned. If you do, tell your doctor. He or she may be able to move the needle so you are more comfortable.
  • Once the needle is properly positioned, a device called a manometer may be attached to it in order to read the pressure of the CSF. You may be asked to straighten your legs before the pressure is read.
  • A sample of CSF is collected (which takes about 5 minutes). After measuring the CSF pressure again, the doctor withdraws the needle. Pressure is placed on the site and a small adhesive bandage is applied.
  • Lumbar puncture takes 20 to 30 minutes.

Risks and Complications

  • Lumbar puncture is generally safe. The most common side effect is a headache, which typically subsides with bed rest. In rare cases (less than 1%), a severe headache develops and you must stay at the testing facility until the pain subsides. In addition, transient back pain or tingling in the legs may occur.
  • Rare but serious complications include persistent CSF leak, inadvertent introduction of bacteria into the CSF leading to meningitis, herniation (rupture) of the brain due to decreased CSF pressure, accidental puncture of the spinal cord or a major blood vessel and allergic reaction, especially to the numbing medicine.

After the Lumbar Puncture

  • You may leave the testing facility. Avoid strenuous exercise or activity for 24 hours after the test.
  • Drink plenty of fluids during the 24 hours after the test to replace the CSF lost by your body.
  • If a headache develops, it is usually relieved by bed rest.
  • Call your doctor immediately if you develop a fever, stiff neck, persistent headache, numbness, or weakness—which may indicate infection or another serious reaction.


  • In some cases, your doctor will be able to immediately identify abnormalities in your CSF based on its appearance. For example, bloody fluid may indicate bleeding in the brain or spinal cord, while cloudiness may be a sign of an infection such as meningitis. High CSF pressure may suggest swelling, bleeding, or a tumor, while low pressure may indicate a blockage in the spinal canal above the puncture site.
  • At the laboratory, your CSF sample will be analyzed for any abnormality, such as blood, bacteria, antibodies, cancer cells, or excessive protein or white blood cells.
  • If a definitive diagnosis can be made, appropriate treatment will be initiated.
  • In some cases, further imaging studies, such as skeletal CT scan or MRI, may be needed to clarify abnormal results.


The Johns Hopkins Consumer Guide to Medical Tests

Simeon Margolis, M.D., Ph.D., Medical Editor

Updated by Remedy Health Media

Publication Review By: the Editorial Staff at

Published: 25 Jan 2012

Last Modified: 23 Oct 2014