Impression cytology, with cellulose acetate filters, was introduced in 19771 as a minimally invasive conjunctival biopsy. It provides an alternative to conjunctival diagnostic excision biopsy or conjunctival smears made from scrapes taken with a blunt spatula. It was found that cellulose acetate filter paper pressed onto the ocular surface removed one to three cell layers of the surface epithelium, preserving its morphology and permitting the use of a limited range of histological techniques.23 Impression cytology provides a flat mount of an area as large as the size of the applied filter paper with well preserved morphology. By comparison, conjunctival smears destroy much of the morphological information and conjunctival biopsies provide information on a relatively small sample of the surface epithelium, both because of the difficulty of preparing flat mounts and because of their necessarily small size. For these reasons impression cytology is …