A spectrum of benign findings can be seen on anal cytology; some are similar to cervical cytology, others are different. While reactive changes, such as tight perinuclear halos and small nucleoli, are frequently seen, typical reparative changes are not (Fig. 8.6). Keratotic changes are common on anal cytology since the keratinized and nonkeratinized portions of the anal canal are juxtaposed. Cytologic samples from the keratinized portion of the anal canal and hyperkeratosis due to a variety of causes both manifest as anucleated squames and are not distinguishable on anal cytology. Parakeratosis can be seen in both reactive changes and HPV-associated lesions. Atypical parakeratosis is abnormal and may be associated with cytologic interpretations ranging from ASC-US to SIL to cancer, depending on the degree of accompanying abnormalities.

A variety of organisms can be encountered on anal cytology including viruses, protozoa, fungi, and helminthes. Some are identical to those encountered on Pap tests, such as Candida (Fig. 8.7) and herpes virus (Fig. 8.8). Others are unique to the gastrointestinal tract and are rare on gynecologic cytology. A large number of species of ameba can parasitize the human intestinal tract. Both amebic cysts and trophozoites are seen (Fig. 8.9a). All but Entamoeba histolytica are thought to be nonpathogenic commensals. The range of pathogens may be larger in immunocompromised patients who are at risk for opportunistic infections. Numerous macrophages can sometimes be seen on anal cytology, particularly in patients after ablative treatment (Fig. 8.9b). These need to be distinguished from amebic organisms. Various other intestinal parasites can be seen, including pinworms and their eggs (Fig. 8.10). The Centers for Disease Control (CDC) provides helpful information on the comparative morphology of intestinal parasites [36].