The “anal pap” is the screening tool used in at risk populations to identify individuals who have pre-malignant cytologic changes in their anal epithelium.  The sample is acquired blindly, without visualization of the squamocolumnar transition of the anus using a water-moistened Dacron swab.  This swab is inserted 1 to 1.5 inches into the anal canal and is rotated firmly as it is being pulled out of the canal [13].  The squamocolumnar transition zone is about 2 cm (1 inch) from the anal verge.  The swab is then smeared across a glass slide, immersed in 95% ethanol, stained with Papanicolaou stain, and then sprayed with a preservative prior to cytopathologist examination.  Alternatively, the swab sample may also be sent for automated ThinPrep slides.  ThinPrep smears are less prone to drying artifact, but initial studies reveal that the traditional smear technique yields similar results [14]. 

            In a study of HIV positive and negative MSM comparing cytology to biopsy as the gold standard, the sensitivity of  the anal PAP was 69% in HIV-positive men and 47% in HIV-negative men on their first smear.  The sensitivity increased to 81% and 50% respectively for all subsequent PAPs. 

           The absence of columnar cells in the smear does not reflect the validity of the sample.  The sensitivity, specificity, or predictive value do not hinge on the presence of absence of these columnar cells [15].   Some sources, however, do recommend that both squamous and columnar cells should be present in samples for adequate interpretation of slides [13].

           If the cytology is abnormal (atypical squamous cells of undetermined significance [ASCUS], LSIL, or HSIL) the patient should be sent for “high resolution anoscopy,” which is analogous to colposcopy.  3% acetic acid solution is applied during anoscopy.  This solution produces a white discoloration of abnormal tissue (the “acetowhite area” ).  Vascular changes like punctation and mosaicism can also be observed in the acetowhite area, increasing the suspicion of HSIL.    External colposcopy is also performed before and after the application of acetic acid to evaluate the perianal area for lesions.  All suspicious lesions are biopsied.  Because anal cytology grade and biopsy grade do not always correlate, it is recommended that any finding on cytology other than normal should have this evaluation [13, 15].