Accuracy of Genus-Level Diagnosis of Fungal Organisms in Pathology Specimens from the Lung Based on Morphology Alone: Analysis of 52 Specimens with Microbiology/PCR as Gold Standard

Objectives To assess the accuracy of a genus-level pathologic diagnosis of fungal organisms in lung specimens in which the diagnosis was made without knowledge of microbiologic culture or polymerase chain reaction (PCR) results. Methods Retrospective review (1999-2025) of 1002 specimens (cytology or histopathology) in which fungi were identified in the lung. Specimens were included in the study if culture or PCR results were not known prior to pathologic diagnosis, a specific fungus was diagnosed by the pathologist based on morphology alone, and the identity of the fungus was subsequently confirmed by cultures or PCR. In 52 specimens (39 histology, 13 cytology) in which pathologists committed to a single organism by morphology alone, the pathologic diagnosis was compared to the gold standard (subsequent microbiologic cultures or PCR). Results The morphologic diagnosis was concordant with cultures or PCR in 50/52 specimens (96%), including 23/24 (96%) fungal yeasts. Of the 2 discrepancies, one was diagnosed on cytology as consistent with Aspergillus ; cultures grew Pseudallescheria boydii . The other was diagnosed on transbronchial biopsy as Histoplasma ; cultures grew Cryptococcus . No adverse clinical consequences occurred in either instance. Conclusions For fungal hyphae, we do not recommend genus-level diagnosis based on morphology alone. However, for fungal yeasts in lung specimens with classic morphologic features, a genus-level diagnosis based on histopathology or cytopathology alone is feasible and accurate, with a low error rate.