The method used to obtain CSF (cerebrospinal fluid) for tests. Usually done to confirm diagnosis of meningitis (and encephalitis).

It involves sticking a needle between either the 3rd and 4th or the 4th and 5th lumbar vertebrae, poking through skin, fascia, muscle, spinal ligaments and the dura.

The pressure of the CSF can be measured during a lumbar puncture using a manometer - a tube with graduated marks that you point upwards to see how far the CSF flows up to. Tests normally done on CSF fluid are protein, glucose, microscopy, cell count and culture. Other tests may be ordered as indicated by the situation.

Normal CSF fluid is clear and colourless. A milky solution usually indicates a fulminant infection, while a red solution suggests either blood in the CSF or a bloody tap. To tell the difference between a bloody tap and a subarachnoid haemorrhage (blood in the CSF), measure the levels of red cells in each successive bottle taken of CSF - fewer red cells in successive bottles == bloody tap. Normally, 3 samples of CSF are taken into three separate bottles, a few drops in each bottle.

LPs can cause a post-LP headache. The patient is advised to lie flat for several hours post LP.

LPs are contraindicated where there are clinical signs of increased intracranial pressure. There is a significant danger of coning if an LP is performed on someone who has, for example, a large intracranial tumour.

Sometimes also called a spinal tap.


How to do a lumbar puncture:

Start by obtaining informed consent. This is a non-routine invasive procedure, so you need to inform the patient of the nature of the procedure, the potential complications (infection), the likely side effects (headache) and instruct them to lie in bed for several hours afterward.

  1. Prepare spinal pack. Check that all equipment works.
  2. Place patient on his left side, with his back to the edge of the bed, with knees to his chin (back fully flexed). In children, it may be necessary to get an adult to restrain the child.
  3. Feel for the space between vertebrae L3 and L4 or between L4 and L5. In adults, a line drawn between the Iliac crests will cross the spine at about L4, so picking the space just below this is usually fine. Mark the site to be used with a firm indentation of a fingernail.
  4. Wash hands. Wear sterile gloves.
  5. Sterilize the back with iodine.
  6. Inject 0.5ml of 1% lignocaine subcutaneously at the marked site.
  7. Wait a couple of minutes
  8. Insert spinal needle through the mark, aiming for the umbilicus. Feel the resistance of the spinal ligaments, dura and the 'give' of the subarachnoid space.
  9. Withdraw stilette and wait for CSF.
  10. Attach manometer and measure CSF pressure.
  11. Catch CSF in 3 sequentially marked bottles. (at least 10 drops each - some say 3ml each)
  12. Remove needle.
  13. Place dressing.
  14. Send off CSF to the laboratory for tests.
A Spinal Tap is a somewhat dangerous medical procedure where a needle is inserted into the spine and spinal fluid is withdrawn. This is only done when it is an absolute medical necessity, because of the danger of damaging the spine.

It is useful in detection of certain diseases. There is an urban legend that a spinal tap can detect past LSD use. This turns out to not be true, and even if it was, spinal taps are far too dangerous to be used for drug testing.

Lumbar puncture is one of the more painful medical procedures available to the discerning practitioner. That's part of why it may be necessary for an adult to restrain the child. But lots of malpractice suits arise from failure to perform the puncture, so grab your knees and bite down on this gauze.

In the Tuskegee syphilis study (wherein the United States Public Health Service deliberately failed to treat a group of African-American men with syphilis in Alabama during a period from 1932 all the way up to 1972) one of the doctors offered patients "special free treatment" for their disease consisting of a feels-so-yummy lumbar puncture...this procedure was done purely in the interest of the doctors' medical research; there was no possibility of benefit to the patients, and a considerable risk.

I had one once. They use a very, very small, thin needle. You get a shot of anisthetic, which hurts, but you'll feel thankful for.

To have one done, you must curl up in the fetal position, meaning bring your legs up to your chin and hug them tightly.

My advice for you. Don't Move! If the doctor messes up, there is a risk that the fluid in your spine will depressurize, causing brain damage. Thus the need for a consent form, and it's a safe but serious procedure.

I advise going to the bathroom beforehand, because you should lie down for 12 hours afterward, or you could get a tremendous migraine if you sit up.

Other than having a Band-aid on your back and a small scab, there are no longterm effects.

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