Napsin A (TA02) is a useful alternative to thyroid transcription factor-1 (TTF-1) for the identification of pulmonary adenocarcinoma cells in pleural effusions.

The purpose of this study was to test napsin A as a diagnostic marker of metastatic lung adenocarcinoma in pleural effusions, and to compare its performance with TTF-1. Napsin A and TTF-1 reactivities were determined immunohistochemically on formalin-fixed paraffin embedded cell blocks from 50 pleural effusion (5 mesotheliomas, 10 mesothelial proliferations, 12 pulmonary, and 23 nonpulmonary metastases). The results were evaluated separately, and correlated to the final diagnoses. Concordant results were obtained in 48/50 cases. TTF-1 and Napsin A were positive in 8/12 and 10/12 pulmonary adenocarcinomas, respectively. Both markers were negative in 42 cases, including two lung carcinomas. Napsin reactivity was found in more than 75% of the tumor cells in 9/10 positive cases, whereas TTF-1 reactivity was seen in more than 75% of the tumor cells in 2/8 positive cases only (P < 0.05). This makes napsin A an alternative to TTF-1 in cytological diagnosis of effusions in which tumor cells may be scanty.