there are many tests for PN that should be done before you have a spinal tap but especially a nerve biopsy. a nerve biopsy is a last resort and only if there is an indication of certain conditions such as vasculitis. nerve biopsies lead to permanent damage and numbness in the area that the biopsy is taken from. It can also lead to long term pain. it also at best just confirms that you have pn not the cause.

lumbar puncture i would have less of reserve about doing, actually i have had one done. it wasnt that bad, but that was only after exhaustive testing before that. ruled out a number of possible causes and conditions.