OBJECTIVETo describe the pathologic features and diagnostic algorithm of pulmonary alveolar proteinosis (PAP).

METHODSThirty-nine biopsy and postmortem cases of PAP were studied by light microscopy and histochemical staining using periodic acid-Schiff (with digestion) (PAS-D), mucicarmine (with digestion) (mucicarmine-D) and alcian blue.

RESULTSHistologically, the affected lung tissue displayed the following characteristic features: (1) alveoli and some of the small bronchioles were filled with eosinophilic and fine granular proteinaceous material with needle-like clefts; (2) proteinaceous material was seen admixed with various numbers of degenerated and sometimes exfoliated pneumocytes; (3) pneumocytes were hyperplastic; (4) alveolar capillaries and alveolar septa had become hyperemic, but pulmonary interstitial inflammation was not obvious; (5) no significant inflammation was identified in the bronchial wall; (6) compensating emphysema was noted in the surrounding lung parenchyma. Fragments of eosinophilic, finely granular proteinaceous material with needle-like clefts were also found in the bronchoalveolar lavage fluid under light microscopy. The proteinaceous material was stained red by PAS-D. The staining for mucicarmine-D was negative, while alcian blue staining was either weakly positive (faint blue staining) or negative. Pathologic examination of lung biopsies and bronchoalveolar lavage fluid thus remaines the gold standard for diagnosis of PAP.

CONCLUSIONSIdentification of homogeneous, eosinophilic, finely granular and PAS-D-positive proteinaceous material with needle-like clefts in alveolar spaces or bronchoalveolar lavage fluid is of diagnostic importance in PAP. Bronchoalveolar lavage, being a relatively safe and non-invasive procedure, can be a useful adjunct in arriving at the final conclusion.