Special stains: Diagnosis is based on morphology alone, as the organisms do not grow on ordinary culture media. New terms (in bold) have been proposed for the stages of Pneumocystis carinii, which is now classified as a fungus. The spore cases (cysts) of pneumocystis stained with the Gomori methenamine silver (GMS) stain are shown at low (A) and high (B) magnification. Spore cases are about 7 µm in diameter (size of RBC) and rounded or cup-shaped. Two dots, which are part of the spore case wall, can sometimes be seen (A and B (arrow)) and diagram (C). These thickenings may be the site of egress of the intracystic bodies (sporozoites), which are not seen with the silver stain [1]. The GMS stain is usually used to stain tissue samples.




Giemsa or Diff-Quik stains are usually used to stain sputum, lavage, or other fluids. A smear of induced sputum stained with the Diff-Quik method is shown below. It is from an AIDS patient.

Two macrophages indicate relative sizes of organisms and cells. The clumps of purple stain represent aggregates of trophic forms (trophozoites) with dot-like nuclei and ill-defined purple cytoplasm (right arrow). Among the trophic forms are outlines of unstained spore cases, some of which contain several (up to 8) purple dots--intracystic bodies (left arrow). Other spore cases are empty.

Summary of stains for Pneumocystis carinii:

Identification of organisms: Smears of induced sputum or sedimented cells from lavage fluid are usually stained with the Giemsa or Diff-Quik stains. The GMS stain is used for tissue sections. An immunohistochemical stain is sometimes used for diagnosis of fluids or tissues.

The foamy pink exudate stains with the PAS stain after diastase digestion. It is composed of trophic forms, surfactant phospholipids, cell debris, and host-derived proteins [2]. The faint dots in the exudate correspond to nuclei of the trophic forms.

Differential diagnosis of pink alveolar exudate: The differential diagnosis includes pulmonary edema--homogeneous pink exudate (PAS-D negative); alveolar proteinosis--granular exudate with dark clumps and unstained clefts (PAS-D positive); and mucus of bronchioloalveolar carcinoma--homogeneous pink exudate that stains with alcian blue in addition to PAS-D.

Pathogenesis of Cavity Formation

Rarely, the organisms spread to the interstitium of the lung, widen alveolar walls, and invade blood vessel walls (arrow). Some authors believe that alveolar walls then undergo lysis to form cavities [3]. Other authors have attributed the lysis to ischemic necrosis caused by a necrotizing angiitis [4]. Note that there is little inflammatory reaction.

An adjacent section was stained with the GMS stain to show the presence of spore cases both in the alveolar space (A) and alveolar wall (W).

In our patient with CLL, the edge of the cavity was necrotic and without fibrosis or inflammation. Active foamy exudate was present in the rest of both lungs. No vascular occlusions or angiitis were noted, and it is not clear whether septal lysis, ischemia, or both were responsible for the cavitation.


1. Cushion M. Taxonomy, genetic organization, and life cycle of Pneumocystis carinii. Sem Respir Infect 1998; 13:302-312.

2. Pottratz S. Pneumocystis carinii interactions with respiratory epithelium. Sem Respir Infect 1998; 13:323-329.

3. Murry C, Schmidt R. Tissue invasion by Pneumocystis carinii: A possible cause of cavitary pneumonia and pneumothorax. Hum Pathol 1992; 23:1380-1387.

4. Liu Y, Tomashefski Jr J, Tomford J, Green H. Necrotizing Pneumocystis carinii vasculitis associated with lung necrosis and cavitation in a patient with acquired immunodeficiency syndrome. Arch Pathol Lab Med 1989; 113:494-497.

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