Detection of Melanin by the Fontana-Masson Silver Stain Is Not Specific for Cryptococcus in Tissue Sections.
Justin A Bishop, Anne Marie Nelson, William G Merz, Frederic B Askin, Stefan Riedel. The Johns Hopkins Medical Institutions, Baltimore, MD; Armed Forces Institute of Pathology, Washington, DC
Background: With the steadily rising number of immunocompromised patients, it is not uncommon for surgical pathologists to encounter yeast and yeast-like organisms in tissue sections. Correct identification of these organisms is imperative for guiding appropriate therapy. Although fungal culture is considered the gold standard for organism identification, very often tissue samples for cultures either are not submitted or are negative. Although most yeast, yeast-like, and dimorphic organisms have characteristic morphologic features, findings often overlap among these organisms, especially in tissue samples of limited volume. Fontana-Masson (FM), a form of a silver stain for detecting melanin in tissue, has been used and accepted as a relatively specific stain for Cryptococcus neoformans in tissue based on few studies with limited numbers and types of organisms. This study was designed to test the value and specificity of the FM by investigating a large collection of tissues with organisms that may mimic Cryptococccus neoformans.
Design: Cases of Cryptococcus and other organisms that can morphologically mimic Cryptococcus were identified in the surgical pathology archives of The Johns Hopkins Hospital and The Armed Forces Institute of Pathology. Cases were included if organism identification was culture-proven and/or if cases were highly diagnostic at the morphologic level.
Results: Overall, FM was positive in 26/46 (57%) cases, with 9/9 Cryptococcus neoformans, 1/1 Cryptococcus gattii, 7/7 Coccidioides immitis, 4/10 Blastomyces dermatitidis, 2/2 Paracoccidioides brasiliensis, 1/1 Loboa loboi, 1/1 Rhinosporidium seeberi, and 1/1 Chrysosporium parvum staining. FM was negative in 10 Histoplasma capsulatum, 1 Histoplasma duboisii, 1 Sporothrix schenckii, and 2 cases of the alga genus Prototheca.
Conclusions: FM was 100% sensitive, staining all Cryptococcus neoformans and gattii tested. Low specificity, however, limits the value of the FM stain; only 4 of 10 non-Cryptococcal species tested were negative in all cases. These results need to be confirmed and extended to other isolates and species, but it is clear that many organisms in the morphologic differential diagnosis can be FM-positive. Accordingly, results of the FM stain, especially a positive, should be interpreted cautiously and only in the context of the organism's morphologic and host features.
Wednesday, March 2, 2011 1:00 PM
Poster Session VI # 224, Wednesday Afternoon