Increased sensitivity of anal cytology in evaluation of internal compared with external lesions

Objective: To investigate the impact of keratin on the accuracy of internal and external anal brush sampling of known lesions. Study Design: A group of 46 human immunodeficiency virus (HIV)-seropositive patients underwent external and internal anal brush sampling before biopsy of known lesions. Results: A total of 92 ThinPrep (46 external, 46 internal) and 211 biopsies were examined. The sensitivity and specificity for internal lesions positive and negative for anal squamous intraepithelial lesion (ASIL) was 91.1% and 42.8%, respectively; and for external lesions was 79.4% and 100%, respectively. Low cellularity on cytology and markedly thickened keratin on biopsy were significantly more common in external compared with internal lesions (p < 0.0001). Conclusion: We conclude that hyperkeratosis interferes with adequate sampling and accurate grading of external anal lesions by brush sampling.