Skin Biopsy for Rabies - Most reliable diagnostic test during first week of illness - Results are 50% positive within the first week of illness - Procedure; Collect full-thickness punch biopsy specimen from the nape of neck which includes hair follicles as well. In a sterile container, place a saline-soaked sterile gauze. Now place the specimen in the container and store it at 70 C. Ship it to the laboratory.