Diagnosing Meningitis With a Lumbar Puncture

When a doctor suspects that a patient has meningitis, a lumbar puncture, or spinal tap, is done to remove fluid and send it to the lab for testing.

By Jean Rothman

Medically reviewed by Lindsey Marcellin, MD, MPH

Proper Positioning for a Lumbar Puncture

During the procedure, the patient lies down on one side, knees drawn up close to the chest and chin tucked down, similar to a fetal position. A child may be told to curl up like a ball. 

Occasionally, the test will be performed with a patient sitting up and bent over, usually resting his or her head and arms on a table. 

When the patient is a young child, intravenous (IV) sedation may be used for the procedure. Rarely, a child will be put under under general anesthesia. 

Cleaning and Numbing the Lumbar Puncture Site

After the patient is placed in the correct position, the area in the lower portion of the spine where the lumbar puncture will be performed is cleaned with an antiseptic. Sterile cloths are carefully put around the area.

The next step is to numb the area using an anesthetic injected into the skin through a tiny needle. This may sting a little bit. 

Inserting the Lumbar Puncture Needle

When the area is numb, a hollow needle is inserted between two vertebrae — the bones that form the backbone — in the lower, or lumbar, portion of the spine. The needle is typically placed between the third and fourth lumbar vertebrae. 

Usually, the patient will feel some pressure when the needle is inserted through the skin and a brief moment of discomfort as the needle penetrates the ligament that surrounds the spinal canal. Because the needle must be properly positioned to avoid any damage to the surrounding area, the doctor will sometimes use painless X-ray guidance, called fluoroscopy. The needle is in proper position when spinal fluid is seen coming out of the needle. 

Finishing the Lumbar Puncture Procedure

At this stage of the lumbar puncture the spinal fluid pressure is measured. Then some of the spinal fluid is allowed to slowly drain from the hollow needle into a sterile container. After a sufficient sample of spinal fluid has been obtained, the needle is taken out and the point where it was inserted is bandaged. 

The spinal fluid will be examined for red and white blood cell counts, glucose (sugar) level, and the presence of any visible microorganisms, such as bacteria. (Viruses are too small to be visible.) The fluid will also be “cultured” to allow any bacteria to grow so that they can be identified later, usually within 48 hours. Other tests to identify specific viruses and bacteria may also be done. 

Usually, a lumbar puncture takes around a half hour. Afterwards, patients need to stay lying down for about an hour; getting up too soon can cause a headache. Children may play quietly in bed.

Pros and Cons of Spinal Taps

As with just about all medical procedures, there are pros and cons. Risks associated with lumbar puncture include:

• Allergic reactions. Some people have an allergic reaction to the numbing medicine.
• Bleeding. People who take blood-thinning medications have a higher risk of bleeding.
• Damage to the nerves in the spinal cord

Other side effects associated with the test include:

• Spinal headache. About 10 to 20 percent of patients undergoing lumbar puncture have a spinal headache following the test. The headache gets worse with sitting or standing.
• Pain or discomfort during the test.
• Blood in spinal canal.

Any risks should be weighed against the benefits of the test. The huge advantage of undergoing a lumbar puncture is to be able to identify a dangerous, potentially life-threatening, disease like bacterial meningitis, so that proper treatment can be started quickly. If it is bacterial meningitis, antibiotics must be given as soon as possible to prevent after-effects from the disease, and even death. 

Cost of a Lumbar Puncture

The cost of a lumbar puncture will vary depending on where it is done, whether additional medical techniques such as fluoroscopy or general anesthesia are required, and how many different tests are done on the spinal fluid. 

Talk to your doctor to find out about the specific costs of the procedure based on your situation. You should also contact your health insurance company to find out if the test is covered.