I would agree with the above post about using a test for diagnostic purposes in relation to a sural nerve biopsy since it leaves permanent numbness, sometimes pain and a scar. The nerve biopsy is useful if certain causes such as vasculitis are suspected but for confirmation that you have PN there are better methods. As far as a spinal tap goes, i disagree with the above post because there are many things that could show up in a spinal tap ( bacteria, meniningitis, autoimmune markers, lyme markers, white blood cell count and others) and may give a good indication as to which way the search for a cause should proceed. The spinal tap is also not that big of a deal , it is invasive but not to anywhere near the degree of a sural nerve biopsy. I had one done after an exhaustive search and testing for any indications of a cause. I had mri's, blood tests for autoimmune markers, paraneoplastic cancers, vascular testing, epidurals,etc. Nothing came back positive except for the emg/ncs itself. If you need to do a spinal tap to find a cause or to give yourself peace of mind that you did an exhaustive search for a cause i would not hesitate. I would do another one myself without trepidation if it was needed.