Table 1.

Characteristic Features	Pulmonary Alveolar Proteinosis	Pneumocystis Carinii
Appearance of the alveolar cast-like structures	Hyaline	Foamy
Crushed "ping-pong" ball-like cyst-structures	Absent	Present
Dark central dots in the cyst-like structures by fungal stains (GMS, PAS)	Absent	Present
Back-ground debris	Could be present	Absent