yes you can have SFN and still have a negative skin punch biopsy, if your degree of damage doesnt fit between the arbitrary 5-95% values. If your SFN is relatively recent and you dont have enough damage yet to equal the lower value it may well be classified as negative. If you have another skin punch biopsy down the road in time it may well be positive by then. the same holds true for long fiber neuropathy and Electromyography/ Nerve Conduction Studies.