p16 is the most useful diagnostic marker for human papillomavirus (HPV)-associated anogenital lesions. In the cervix, the pattern of p16 immunoreactivity generally correlates with lesion severity. p16 expression in anal intraepithelial neoplasia (AIN) is far less studied. Whether such correlation holds true has to be determined. We correlated the degree and pattern of p16 immunohistochemistry (IHC) results with morphologic diagnoses of 1000 anal squamous and transitional zone biopsy specimens. Using the Lower Anogenital Squamous Terminology criteria, p16 IHC results were classified as block staining, partial staining, or negative. Among 150 samples without morphologic evidence of AIN, p16 was negative in 85% and partial staining in 15%. AIN 1 (n=400) revealed diverse results: 28% negative, 35% partial, and 37% block staining. Among AIN 2 (n=298), 89% were block, 9% partial staining, and 2% negative. AIN 3 (n=152) revealed block (95%) or partial staining (5%). For the detection of AIN 2/3, p16 block staining yielded 91% sensitivity, 73% specificity, 80% positive predictive value, 91% negative predictive value, and a Youden Index of 0.64. Combining block staining and partial staining slightly increased sensitivity (99%) and negative predictive value (98%), but significantly decreased specificity (43%), positive predictive value (59%) and Youden Index (0.42, P<0.001). As with the cervix, p16 immunoreactivity correlates with morphologic diagnoses of AIN. Block staining offers the optimal diagnostic value for AIN 2/3. Caution is required since AIN 1 frequently exhibits block staining; the prognostic value of p16 warrants further investigation.
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