Differentiating Pneumocystis cysts from Candida sp. yeasts in pulmonary specimens using methenamine silver.
Pneumocystis carinii (PC) pneumonia in the immunocopromised patient requires prompt therapeutic intervention; therefore, rapid identification of PC organisms in cytopathologic specimen is essential. Conclusive diagnoses of PC are achievable using Grocott's methenamine silver (GMS), the "gold-standard" stain for PC cyst visualization. However, non-budding Candida sp. yeasts can simulate PC cysts with GMS and thus pose significant diagnostic challenges. After qualitative and semi-quantitative analysis of 49 cytopulmonary cases, this study aimed to establish morphologic criteria that differentiate these organisms using GMS. The results showed that spherical/demilune PC cysts (4 to 7 microns in diameter) are monomorphic and mainly transparent, with intracyst densities being commonly evident. Demilune cysts typically display wall wrinkling with longitudinal clefts. Relative to cysts, Candida sp. yeasts reveal increased argyrophilia, range 4 to 10 microns in diameter, are mainly oval and budding, polymorphic, and exhibit wall deformation with variable internal structure. Differentiating criteria are (a) budding; (b) cyst transparency, demilune shape; (c) longitudinal cyst clefts; (d) paired comma-like intracyst densities; (e) cyst monomorphism ; (f) alveolar cast formations; (g) overall cytomorphologic presentation; and (h) relative argyrophilia.
Chantziantoniou
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Department of Pathology and Laboratory Medicine, Cytology Section, King Faisal Specialist Hospital and Research Centre, Riyadh, Saudi Arabia.