Performance of anal cytology in a clinical setting when measured against histology and high-resolution anoscopy findings.

OBJECTIVES: The success of cervical cytology in screening for cervical neoplasia has led to the concept of anal cytology screening for anal neoplasia. Our objective is to study the performance of anal cytology as a screening tool.
DESIGN: We assessed anal cytology against histology and high-resolution anoscopy in a clinical setting.
METHODS: Anal pap test was obtained prior to high-resolution anoscopy examinations and biopsies. The results were analysed against a number of patient variables.
RESULTS: From 395 individuals (93% men), 584 anal pap tests were obtained. HIV status was positive in 212 (54%) and negative in 156 (39%) individuals. On the basis of 288 histology results, the sensitivity of anal cytology to detect disease was 70% [95% confidence interval (CI) 64-75], whereas the specificity was 67% (95% CI 38-88). For high-grade disease (anal intraepithelial neoplasia 2/3), sensitivity of anal cytology was 81% (95% CI 70-90), and the negative predictive value was 85% (95% CI 76-92). Sensitivity was dependent on the area of disease (86% for two or more quadrants vs. 69% for one or more quadrants, P = 0.002) and HIV positivity (76% in HIV positive vs. 59% in HIV negative, P = 0.009). Amongst HIV-positive patients, the sensitivity was 90% when CD4 cell count was 400 cells/microl or less compared with 67% when CD4 cell count was above 400 cells/microl (P = 0.005).
CONCLUSION: Anal cytology performs similar to cervical cytology in a clinical setting. Sensitivity of anal smear is dependent on the area (quadrants) of disease present. Sensitivity of anal cytology is enhanced when CD4 cell count is less than 400 cells/microl in HIV-positive men. Our results may explain the variable sensitivity reported in the literature.