Immunocytochemical Detection of Napsin-A in Pulmonary Alveolar Proteinosis: A Report of Two Cases

INTRODUCTION: Cytological evaluation of bronchoalveolar lavage (BAL) fluid is useful for diagnosing pulmonary alveolar proteinosis (PAP), with periodic acid-Schiff staining being the most used ancillary test. There are more specific diagnostic tests (ultrastructure, surfactant antibodies), but unfortunately, they are not available to most laboratories. Following the observation of case of PAP with intense expression of Napsin-A and a recently published study supporting this finding, we decided to test this antibody in BAL samples of patients with PAP. CASES PRESENTATION: We found two out of six cases of PAP that had adequate cytological samples for immunocytochemical evaluation. The patients were a 61-year-old man, and a 55-year-old woman diagnosed with secondary and primary PAP, respectively. Both had complete clinical and imaging study, as well as histopathologic analysis. Cytological and histological samples were evaluated for Napsin-A expression, with positive results in both cases. To serve as controls we selected 15 BAL samples from patients without PAP, five showing Pneumocystis jirovecii infection. CONCLUSION: We have observed intense Napsin-A expression in the globular bodies seen in BAL samples from patients with PAP. We believe that it is a useful marker for confirming this diagnosis, especially in the absence of electron microscopy or surfactant-specific antibodies. Its application to cytological samples increases diagnostic accuracy, allowing, in an appropriate clinical and imaging context, a more precise diagnosis.