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.