Cryptococcus neoformans/gattii and Histoplasma capsulatum var. capsulatum infections on tissue sections: diagnostic pitfalls and relevance of an integrated histomolecular diagnosis

Cryptococcus neoformans/gattii and Histoplasma capsulatum var. capsulatum may present atypical histopathological features inducing diagnostic errors. We aimed to estimate the frequency of these atypical features on formalin-fixed tissue samples (FT) and to assess the relevance of an integrated histomolecular diagnosis using specific Histoplasma capsulatum PCR and panfungal PCR followed by Sanger sequencing and/or targeted-massive parallel sequencing (MPS). Twenty-seven FT from 23 patients with a histopathological diagnosis of cryptococcosis (n=16 FT from 13 patients) or histoplasmosis (n=11 FT from 10 patients) were retrospectively included. All FT were consultation cases. Mycological identifications on equivalent fresh tissue were available for 11/23 (47.8%) patients. The expert pathologist review modified the diagnosis suggested by the initial pathologist in 7/27 (25.9%) FT. Fungal morphology and tissue inflammation were compared between both mycoses. The most discriminant atypical criterion was the presence of dented-looking yeasts, observed in 68.75% (11/16) of C. neoformans/gattii and none (0/11) of H. capsulatum var. capsulatum (p=0.002). Although a trend towards more frequent pseudohyphae in C. neoformans/gattii (p=0.2663) and more "protozoa-like" features in H. capsulatum var. capsulatum (p=0.1304) was seen, it did not reach significance. For the 12/23 (52.2%) patients without mycological identification on fresh tissue, an integrated histomolecular diagnosis on FT using specific PCR or panfungal PCR followed by Sanger sequencing and/or MPS led to fungal identification in 9/12 (75%) cases; for cryptococcosis, the targeted-MPS sensitivity was higher than that of Sanger sequencing (p=0.041). Thus, because atypical histopathological features may be tricky, integrated histomolecular diagnosis is essential for optimal patient care.