The BAL cytology smears show many pigmented alveolar macrophages. The question: is this melanin, carbon or hemosiderin? The answer lies in the staining for hemosiderin (Prussian Blue). In this case the presence of many (more than 20%) positive for hemosiderin macrophages, fulfills one of CDC's criteria for pulmonary alveolar hemorrhage. They indicate that the bleeding has been present for at least 2 days.
Did you know that the name of the pigment came from its use as a dye in the German Army uniforms? And was made from ( Good Lord !) cattle blood?...To know more about this fascinating dye, click here