[Type A influenza pneumonia with diffuse alveolar damage diagnosed by increased antibody titers and immunohistochemical staining].

A 58-year-old man presented to a local physician with cough, fever, myalgia and dyspnea. His chest X-ray film showed abnormal shadows and therefore he was admitted to our hospital. Chest computed tomography showed bilateral ground-glass opacities and bilateral consolidation. We suspected influenza pneumonia, but the results of both an influenza rapid antigen test and reverse transcriptase-polymerase chain reaction test for novel influenza (H1N1 2009) were negative. Transbronchial lung biopsy showed diffuse alveolar damage patterns. We diagnosed acute interstitial pneumonia and initiated corticosteroid therapy. Moreover, because influenza pneumonia could not be excluded according to his clinical picture, oseltamivir was administered. His condition improved and he was discharged. After discharge, the levels of antibody titers for influenza A virus significantly increased. We therefore re-evaluated his transbronchial lung biopsy specimen and found that immunohistochemical staining was positive for influenza A antigen in his bronchial and bronchiolar cells. We re-diagnosed his condition as influenza pneumonia. The possibility that influenza pneumonia may present in cases originally diagnosed as acute interstitial pneumonia must be considered.