Evaluation of the use of Congo red staining in the differential diagnosis of Candida vs. various other yeast-form fungal organisms.

The Congo red staining properties of Candida organisms in clinical tissue specimens have not, to the best of our knowledge, previously been reported. The objective of this study was to determine if the Congo red staining characteristics of Candida vs. Histoplasma, Pityrosporum and Blastomyces could provide useful diagnostic information. Archival tissue specimens that contained Histoplasma, Pityrosporum, Candida and Blastomyces were stained with Congo red. The results of the Congo red staining were compared with the diagnoses which were originally rendered on the tissue. Nine out of nine cases (100%) of Blastomyces were Gomori methenamine silver (GMS) positive and Congo red positive, seven out of seven cases (100%) of Histoplasma were GMS positive and Congo red negative, and eight out of eight cases (100%) that had Pityrosporum were GMS positive and Congo red positive; these results corroborate with previously described staining patterns for each respective organism. Nine out of nine cases (100%) that had Candida were GMS positive and Congo red negative. Differential Congo red staining of Candida organisms can provide a rapid and accurate method of diagnosis in tissue specimens vs. Blastomyces and Pityrosporum, but not vs. Histoplasma.