Koilocytosis in HPV Infection Koilocytosis, also known as koilocytic atypia or koilocytotic atypia, is the presence of koilocytes in a specimen. When a squamous epithelial cell undergoes several structural changes as a result of HPV infection, it is known as koilocyte. It is characteristic histological finding in a suspected specimen of HPV infection. It is a cytopathic effect which includes several cellular changes such as • nuclear enlargement up to 2-3 times the original size • irregular raisinoid appearance of the nuclear membrane • hyperchromasia i.e. dark staining pattern • occurrence of perinuclear halo or cytoplasmic vacuolization.