Pneumocystis carinii morphology

The cysts of P. carinii are spherical in shape and measure approximately 4-7 mm

They can be stained by a number of tissue stains, like the Gomori's methenamine silver stain shown in this figure, and the typical black cup-shaped appearance makes them stand out against a paler background.

Diagnostic problems occur when organism is few in numbers or found in clumps with many areas of the slide devoid of organisms.

Pneumocystis organisms must be differentiated from yeasts and fungi.