Can p40 (Polyclonal) Replace p63 (Clone 4A4) in the Cytologic Diagnosis of Pulmonary Non–Small Cell Carcinoma?

Objectives: Differentiating squamous cell carcinoma from adenocarcinoma (ACA) in cytology specimens can be challenging. Recent literature showed p40 had higher specificity than p63 for this purpose. Methods: We identified 190 cytology cases with p40 (polyclonal) and p63 (monoclonal clone 4A4) immunohistochemistry, including specimens from fine-needle aspirations (FNAs) and effusions. Results: ACAs of lung origin stained for p40 and p63 in 21% and 20% of cases, respectively, regardless of specimen site. Among lung FNAs of primary pulmonary ACAs (n = 42), 14% were positive for p40 and 24% were positive for p63. Of the 20 pulmonary ACAs in effusions, more cases showed p40 positivity (40%) compared with FNAs, whereas p63 were positive in 15%. Among metastatic ACAs from other sites (n = 14), more cases were positive for p40 than p63. Conclusions: Polyclonal p40 yields a level of false positivity in ACAs similar to p63, which is highest in effusions and is not limited to lung origin.