On histology, HSIL is diagnosed when abnormal cells with increased nuclear to cytoplasmic ratio replace 50% or more of the epithelial thickness.  LSIL is diagnosed when 20-25% is replaced or koilocytes are present.  As in cervical cytology, ASCUS refers to cells found on cytology that are difficult to categorize as either normal or abnormal [1].  LSIL and ASCUS frequently regress, while HSIL rarely regresses and is considered a pre-cancerous lesion.  In one study of MSMs, 62% of LSIL lesions in HIV positive men progressed to HSIL compared to     36 % in HIV negative men[3].  Though anal cytology may be as sensitive as a cervical PAP smear, biopsy remains the gold standard since cytologic grade often does not correlate with final histological grade [9].

            The terminology used to describe the epithelial changes observed with HPV associated anal disease are very similar to those used for cervical cytology.  Anal cytologies are read as either normal or abnormal.  In the abnormal group there are three possible subgroupings: atypical squamous cells of undetermined significance (ASCUS), low-grade squamous intraepithelial lesions (LSIL), and high-grade squamous intraepithelial lesions (HSIL).