Lipid-laden alveolar macrophage

Lipid-laden alveolar macrophages, also known as pulmonary foam cells,[2] are cells found in bronchoalveolar lavage (BAL) specimens that consist of macrophages containing deposits of lipids (fats).[3] The lipid content of the macrophages can be demonstrated using a lipid targeting stain like Oil Red O or Nile red.[2][4][5][6] Increased levels of lipid-laden alveolar macrophages are associated with various respiratory conditions, including chronic smoking,[3] gastroesophageal reflux, lipoid pneumonia, fat embolism,[7] pulmonary alveolar proteinosis[3][8] and pulmonary aspiration.[2][9] Lipid-laden alveolar macrophages have been reported in cases of vaping-associated pulmonary injury.[10][1][11]

Lipid-laden alveolar macrophages in a case of vaping-associated pulmonary injury. Left: Papanicolaou stain; right: Oil Red O stain.[1]

The lipid-laden macrophage index (LLMI) can be calculated by counting 100 macrophages in a BAL specimen treated with a lipid stain and scoring each macrophage from 0 to 4 based on the amount of lipids present in the cell.[7][12] A LLMI score greater than 100 is considered positive for pulmonary aspiration. However, the test is limited by poor reproducibility[7] and low specificity for pulmonary aspiration, as lipid-laden macrophages occur in many respiratory conditions.[9]