Why is the arachnoid membrane the site for a lumbar puncture?

The arachnoid mata is not the site of a lumbar puncture. The site of a lumbar puncture is between the level of L4 and L5 vertebra in the lumbar section of the spine. This is used because at this point and below, there is no spinal cord to damage.

The LP draws a liquid known as cerebrospinal fluid (CSF) from the meninges surrounding the spinal cord in order to examine for infection risk mainly.