<i>Cryptococcus neoformans/gattii</i> and <i>Histoplasma capsulatum</i> var. <i>capsulatum</i> 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 in formalin-fixed tissue (FT) samples and to assess the relevance of an integrated histomolecular diagnosis using specific H. capsulatum PCR and panfungal PCR followed by Sanger sequencing and/or targeted massive parallel sequencing (MPS). A total of 27 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 = .002). 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 = .041). Thus, because atypical histopathological features may be tricky, integrated histomolecular diagnosis is essential for optimal patient care.