Be sure to set up all the necessary equipment on your tray before you get started to avoid having to interrupt the procedure.

Identify the iliac crests with the midfingers of both hands and have your thumbs meet in the posterior midline, on an imaginary straight line between the iliac crests.

Here you will feel the space between the L4 and L5 spinous processes.

Palpate this space for confirmation and mark it with a marker pen.

Ask the patient to lie in the lateral decubitus position with their back positioned at the edge of the bed. Ensure that they are facing away from you.

With the help of an assistant, place the patient in a knee-chest position with the neck flexed, in order to facilitate maximal flexing of the lumbar spine. This widens the space between the spinous processes and facilitates entry of the spinal needle.

Place the patient's head on a pillow to ensure that the entire cranio-spinal axis is parallel to the surface of the bed. Also consider placing a pillow between the patient's legs.

Wash your hands and wear sterile gloves.

Clean the previously marked area with antiseptic solution, using gauze swabs/sponges held with sponge-holding forceps.

Start cleaning centrally and move progressively outwards in concentric circles, until an area of roughly 15 x 15 cm has been disinfected.

Apply a fenestrated sterile drape or individual drapes in such a way as to leave the marked/disinfected area exposed.

Take universal precautions.

Use an aseptic technique.

Infiltrate the local anesthetic into the skin and subcutaneous tissue in the pre-marked L4-L5 interspinous space. Pull back on the plunger before injecting to ensure that local anesthetic is not given into a vessel.

Introduce the spinal needle into the skin at the level of the pre-marked interspinous space with the bevel of the needle facing up, to minimize the theoretical risk of the needle cutting through fibres of the cauda equina. (The position of the outcropping on the needle introducer corresponds with the position of the bevel of the needle.)

Angle the needle in a slightly cranial direction (ie, aiming roughly at the patient's umbilicus) and advance it slowly.

Continue until a slight  "give" is felt. Remove the stylet to confirm backflow of the cerebrospinal fluid (CSF).

It is normal to encounter some resistance when passing the needle through the interspinous ligament.  However when bone is hit (ie, the needle is angled incorrectly), you will not be able to advance the needle. In such a case, pull the needle back all the way to the level of the skin and advance in a slightly different direction.

Connect the 3-way stopcock to the manometer with the tap closed towards yourself (ie, the tap pointing towards you). This will allow the CSF to flow up into the manometer. Make sure that the markings on the manometer are facing you.

Remove the stylet and attach the stopcock with the manometer to the needle. Do this swiftly to limit the loss of the CSF.

Allow the CSF to flow upwards into the manometer. Measure the CSF opening pressure at the level at which the CSF stops rising. At this level, you will notice the CSF gently rise and fall in relation to the patient's respiration and pulse.

Ask your assistant to uncap the specimen tubes and collect the CSF specimens while you keep control of the needle and stopcock with manometer. The assistant should not allow the specimen tubes to touch the sterile field or instruments.

Turn the stopcock tap towards the patient and allow the CSF within the manometer to run into a specimen tube.

Once the manometer has been emptied, remove it together with the stopcock and allow the CSF to run from the spinal needle into a specimen tube.

When the necessary amount of CSF has been collected, reinsert the stylet.

You need only a few milliliters for basic studies (eg, protein, glucose). You need to collect larger amounts (eg, 10 mL in a single tube) for a specialized test that requires concentration of the CSF (eg, cell count, specific antibody studies).

CSF specimens should still be collected if the opening pressure is found to be high, because any change in intracranial dynamics caused by the LP has already occurred. Premature removal of the needle without collecting the CSF will not change this situation.
 
If the CSF appears to be bloody, collect several specimens. If the blood clears in successive tubes then the blood was probably traumatic in origin - at least in part. (This sign is not fully reliable; in some traumatic taps the amount of blood may actually increase in subsequent tubes).

If the clarity of the CSF is in doubt, compare a tube of the CSF with a tube of water against a well-lit background. Keep in mind that a hazy CSF appearance can be produced by either red or white blood cells.

When sufficient fluid has been obtained, withdraw the needle and apply a dry sterile dressing to the puncture site.

Postprocedure considerations