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p16/Ki-67 Immunocytochemistry in Gynecological Cytology: Limitations in Practice

Objective: Immunochemical detection of the protein p16INK4a in cervical cytology is used in combination with Ki-67. Cells positive for both proteins are certain to have been transformed by high-risk HPV. p16/Ki-67 immunocytochemistry provides a significant improvement in specificity over cytology and HPV DNA testing. However, p16/Ki-67 immunocytochemistry also has its limitations. Study Design: The research is based on the follow-up of 1,131 patients for whom p16/Ki-67 immunocytochemistry was performed with cytology. Dependencies on the age of patients with LSIL, number of p16/Ki-67-positive cells, and different results during repeated examinations were analyzed. Results: In LSIL, positive p16/Ki-67 is less specific for ≥CIN2/HSIL for patients younger than 30 years compared to patients aged 30 years or older (61.1 vs. 75.7%, p < 0.013). Using a score of 10 p16/Ki-67-marked cells as a positive result instead of 1 led to significantly higher specificity (89.0 vs. 70.2%, p < 0.001). This modified threshold offers better risk assessment in LSIL. In repeated immunocytochemical investigations, 28.4% of the results deviated from the first examination. Conclusion: The abovementioned discrepancies can be interpreted as hints about the molecular biological causes of suboptimal performance of p16/Ki-67. An efficient and reliable application of p16/Ki-67 immunocytochemistry requires knowledge of its methodological limitations.