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Increased use of immunosuppressive drugs has broadened the range and increased the incidence of invasive fungal diseases. Successful therapy relies on early diagnosis; however, fungal organisms show overlapping morphology on histologic sections. As commercially available fungal immunohistochemistry (IHC) has been validated for crossreactivity with few other fungi, we ran Aspergillus and Candida IHCs on formalin-fixed, paraffin-embedded tissue of fungal cases. The cases had also been cultured from fresh tissue submitted to the clinical microbiology laboratory. Commercially available polyclonal and monoclonal Aspergillus and Candida IHCs were run with antigen retrieval on 16 culture-confirmed fungal cases (9 Aspergillus, 2 Candida, 2 Bipolaris, 1 Fusarium, 1 Curvularia, and 1 Rhizopus). Three additional cases, which stained positive for Zygomycete IHC at the Centers for Disease Control and Prevention, which had not been confirmed by culture, were also tested against the antibodies. Preabsorption controls with formalin-fixed fungi were run on 2 cases. Fifteen of the 16 culture-confirmed cases stained with the polyclonal Aspergillus antibody, whereas only 7 of 14 cases stained with the monoclonal Aspergillus antibody. Sensitivities and specificities for the polyclonal and monoclonal Aspergillus antibodies were 100% and 29%, and 43% and 14%, respectively. Two Zygomycete IHC Centers for Disease Control and Prevention cases stained strongly positive for monoclonal Aspergillus. Although Aspergillus cultures were often correctly identified on IHC, false positive staining was seen with both polyclonal and monoclonal Aspergillus antibodies. The number of culture-confirmed Candida cases was too low to adequately evaluate sensitivity. More accurate and reliable fungal IHCs, including those for Zygomycete staining, are needed.

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