Conditions which can mimic PAP:
(1) Pneumocystis carinii pneumonia (PCP)
(2) pulmonary amyloidosis
(3) resolving pneumonia
(4) drug-induced pulmonary toxicity
Features of pneumocystosis:
(1) foamy, amorphous material that is PAS positive and diastase resistance
(2) presence of organisms on GMS or other special stains
Features of pulmonary amyloidosis:
(1) unusual in bronchial lavage but may be encountered in bronchial biopsies
(2) orange staining on Congo red with apple-green birefingence under polarized light
Features of resolving pneumonia:
(1) presence of macrophages and inflammatory cells
(2) micro-organisms may be present in small numbers but are typically absent
(3) amorphous material is fibrin and stains blue on Mallory's PTAH (phosphotungstic acid hematoxylin)
Features of drug-induced pulmonary toxicity:
(1) history of recent therapy with amiodorone, chlorphenteramine or other implicated medication
(2) presence of increased cellularity (neutrophils, type II pneumocytes or vacuolated histiocytes)
(3) lamellar bodies may be found within cells but are not the dominant feature of extracellular material
(4) does not recur if implicated drugs are discontinued
Some cases may be difficult to diagnose because PAP can occur in patients who are immunosuppressed and who are at risk for opportunistic infections such as PCP or bacterial pneumonia. Thus, the presence of PCP or bacterial pneumonia does not exclude PAP. However, PAP would be favored by recurrence while on prophylactic antibiotics and demonstration of lamellar bodies on electron microscopy.
