Napsin A is a pepsin-like aspartic proteinase. It is expressed in type II pneumocytes and in adenocarcinomas of the lung and kidney.2
Studies have shown that Napsin A is both more sensitive and specific than TTF-1. When compared to TTF-1, Napsin A showed a higher specificity (94.3%) for adenocarcinoma in non-small cell lung carcinoma as compared to TTF-1 (76.1%).4
Unlike TTF-1, Napsin A is positive in some renal cell carcinomas (RCC). Several studies have shown that Napsin A was positive in 83%-90.7% of primary lung adenocarcinomas. 1-3 Other neoplastic tissues such as ovarian cancers show low expression with different staining patterns from that of primary lung cancer which shows granular cytoplasmic staining in tumor cells. In studies comparing TTF-1 and SP-A, Napsin A stained more tumor cells and a higher percentage of lung adenocarcinomas than either of these antibodies.