Lumbar puncture (or spinal tap as it is more commonly called), is a medical test which requires a small sample of cerebrospinal fluid. Cerebrospinal fluid (CSF) is a colorless, clear liquid that acts as a cushion for the brain and the spinal cord. Lumbar punctures are performed to detect or rule out suspected illnesses or conditions. Testing the CSF involves looking for possible infections by examining the white blood count, protein, bacteria, glucose level, and any abnormal cells that indicate specific diseases in the central nervous system.
Most often, lumbar puncture procedures are ordered to test for meningitis. CSF testing is usually done when life-threatening illnesses like subarachnoid hemorrhage and meningitis are suspected. A lumbar puncture should only be performed after a neurological examination indicates the need for one. Other conditions detected through CSF testing include bleeding in the brain, multiple sclerosis, and Guillain-Barré syndrome.
Once the puncture procedure is explained to you and the doctor has answered any questions you have, you will need to sign a consent form stating that you understand the procedure and that you have authorized it.
While the doctor should have your medical history, it is best that you inform the doctor once more regarding any antibiotics you are taking or have taken in the recent past. Apart from this, a lumbar puncture test requires no other preparation.
In order to successfully collect the CSF sample, you will be asked to either lie on a bed sideways with your knees drawn as far up towards your chest as possible, or you will be asked to sit at the edge of a chair and lean your head and chest forward towards your knees. Both these positions will widen the space between the bones of your lower spine and will help insert the needle into your spine easily.
The doctor will mark the puncture site on your lower back with a pen. This area is then cleaned with a special soap and patted dry with sterile towels. A numbing agent (local anesthetic) will be applied to that area. The area should be numb in a few seconds. Your doctor will then insert a long, thin needle into the spinal canal. Once the needle is in place, the stylet, i.e. the central solid core of the needle, is removed. If the insertion is in the right place, a few drops of CSF will drip from the end of the needle. If not, the stylet is reattached and the needle is moved to a different spot or angle.
When the needle is inserted into the spinal canal, a manometer is attached to the needle to determine the CSF pressure. Your doctor will check the opening pressure and also whether the fluid collected is clear or bloody. Once the sample is collected, the doctor will also make a note of the closing pressure. After the needle is withdrawn, the site is cleaned and bandaged. A lumbar puncture procedure takes about half an hour.
Lumbar puncture results can be available as early as 60 minutes. However, if the sample has to be put in a specific culture to check for any bacterial or fungal growth, it could take longer.
A normal result implies that the CSF sample is clear and colorless, and the pressure readings, protein, glucose, and cell counts are all within the specified range. A normal result also means that no abnormal or harmful cells were detected.
An abnormal result is determined by a CSF sample that is bloody. It can also be the result of high pressure readings or abnormal protein, glucose, and/or blood cell counts.
Headaches after a lumbar puncture are very common. Due to this, you will be advised to lie flat on your back for two to four hours after the test. This will allow the body to generate new CSF and replenish the amount taken, thereby reducing the severity of the headache. You can also consume extra fluids to reduce the impact of headaches.