Purpureocillium lilacinum Fungemia Masquerading as an Unidentified Yeast in a Heavily Immunosuppressed Patient With Relapsed Multiple Myeloma and Therapy-Related Mixed-Phenotype Acute Leukemia: A Case Report.

Purpureocillium lilacinum (formerly Paecilomyces lilacinus) is an environmental saprophytic mold and an increasingly recognized opportunistic pathogen in immunocompromised hosts, most often causing cutaneous, ocular, or sinopulmonary disease; bloodstream infection (fungemia) is rare. The initial microscopic morphology on Gram stain of a positive blood culture can simulate yeast, delaying correct identification and appropriate antifungal therapy. We describe a 67-year-old man with relapsed multiple myeloma and therapy-related mixed-phenotype acute leukemia, further complicated by a prior Mycobacterium abscessus vertebral epidural abscess, who was admitted with neutropenic fever, hypoxic Burkholderia cenocepacia pneumonia, and multiorgan dysfunction. Blood culture demonstrated a yeast-like organism that could not be identified as any common Candida species by rapid multiplex polymerase chain reaction (PCR). The patient was later found to have port-associated Paecilomyces/Purpureocillium fungemia, with only one day of documented fungemia. The port was removed, and repeat blood cultures were negative. The isolate was submitted to a reference mycology laboratory and was identified as P. lilacinum approximately two weeks after the index blood culture. Susceptibility testing reported resistance to amphotericin B and a high minimum inhibitory concentration (MIC) to itraconazole, with lower MICs to posaconazole and voriconazole. He received posaconazole and liposomal amphotericin B briefly, followed by voriconazole, with clinical improvement and no recurrent fungemia. This case illustrates a clinically important diagnostic pitfall: rare filamentous molds such as P. lilacinum can mimic yeast morphologically and evade rapid yeast-identification panels, leading to delayed diagnosis in critically ill, heavily immunosuppressed patients. Early involvement of a reference mycology laboratory with molecular sequencing capabilities, when phenotypic identification is inconclusive, is warranted. The possibility of an amphotericin B-resistant mold simulating "yeast" fungemia should be considered in high-risk hematology-oncology patients.