p16 is not used routinely for all biopsies - the LAST project by Dr. Darragh et al describes 4 scenarios when p16 should be used: (1) to differentiate between HSIL and mimics (2) to classify AIN2 into either LSIL or HSIL (3) to unify disagreement among pathologists (4) special circumstance "a priori" in high-risk situations (eg prior cytology with HSIL, ASC-H or HPV16+ ASCUS). It should not be used in obvious AIN1 or AIN3 since this will lead to overdiagnosis/false positives - it is estimated that, when used appropriately, institutions should be using p16 staining in 20-25% of biopsies.
See this quote from Darragh, T M, ISSN: 0956-5507, 1365-2303; DOI: 10.1111/cyt. 12299; PMID: 26767600 Cytopathology., 2015, Vol.26(6), p.343-345:
““The LAST Project's recommendations for biomarkers in HPV-associated lower anogenital squamous intraepithelial lesions
1. p16 immunohistochemistry is recommended when the H&E morphological differential diagnosis is between pre-cancer (-IN2 or -IN3) and a mimic of pre-cancer (e.g. processes known to be not related to neoplastic risk such as immature squamous metaplasia, atrophy, reparative epithelial changes, tangential cutting, etc.). Strong and diffuse block positive p16 results support a categorisation of pre-cancerous disease.
2. If the pathologist is entertaining an H&E morphological interpretation of -IN 2 [under the old terminology; that is a biologically equivocal lesion falling between the morphological changes of HPV infection (low-grade lesion) and precancer], p16 IHC is recommended to help clarify the situation. Strong and diffuse block positive p16 results support a categorisation of pre-cancer. Negative or non-block positive staining strongly favours an interpretation of low-grade disease or a non-HPV-associated pathology.
3. p16 is recommended for use as an adjudication tool for cases in which there is a professional disagreement in histological specimen interpretation, with the caveat that the differential diagnosis includes a pre-cancerous lesion (-IN2 or -IN3).
4. LAST recommends against the use of p16 IHC as a routine adjunct to the histological assessment of biopsy specimens with morphological interpretations of negative, -IN1, and -IN3.
o Special circumstance: p16 IHC is recommended as an adjunct to morphological assessment of biopsy specimens interpreted as ≤-IN1 that are at high risk for missed high-grade disease, which is defined as a prior cytological interpretation of HSIL, ASC-H, ASC-US/HPV16+, or AGC(NOS). Any identified p16 positive area must meet H&E morphological criteria for a high-grade lesion to be re-interpreted as such.
To date, the biomarker, p16, has the most robust published literature on its utility to help make H&E morphological diagnoses of HPV-associated squamous lesions more objective and reproducible. A recent meta-analysis indicates improved interobserver agreement for the diagnosis of high-grade cervical lesions with the adjunctive use of p16 compared with H&E morphology alone.7 Additionally, individual studies published since the LAST Project's recommendations have shown that p16 immunostaining is useful for the diagnosis of high-grade lesions in a variety of anogenital sites including the cervix, anus and genital skin.8-11 However, p16 is not a panacea; LSIL and benign squamous mucosa can be p16 block positive.12 Thus, the judicious use of p16 is recommended by the LAST Project for diagnostically challenging situations, as noted above. In the future, other biomarkers may provide more of a magic bullet and help discriminate those lesions destined to become invasive if left untreated.”
Cristina Brickman and Jessica Korman
