Antifungal Susceptibility Testing Experience in the Management of Culture-Positive Mucormycosis: Observation from a Large Healthcare System

Background: Mucormycosis, an invasive fungal infection with high morbidity and mortality rates, requires prompt surgical and antifungal therapies; however, the role of antifungal susceptibility testing (AFST) in clinical management of mucormycosis remains underexplored. We aimed to describe the experience of using AFST in the clinical management of mucormycosis. Methods: We conducted a retrospective study from 1 October 2017 to 8 February 2023. We included non-pregnant patients aged ≥ 18 years old with a positive culture for Mucorales and with proven or probable mucormycosis. We collected clinical and microbiological data using a chart review. Results: Over the study period, a total of 119 patients were included, with 36 (30%) undergoing AFST. Of all patients, the median age was 54 years, with 80 (67%) being White and not Hispanic and 73 (61%) being male. Fifty-three (45%) patients had DM, 27 (23%) had hematological malignancy, 15 (13%) had SOT, and 23 (19%) had COVID-19. Half of the cases met the criteria of proven invasive mucormycosis, with pulmonary involvement being the most common presentation (46, 39%), followed by rhino-cerebral-orbital involvement (35, 29%). The majority of Mucorales isolates were Rhizopus species (79, 66%). Among the 36 who underwent AFST, posaconazole minimal inhibitory concentrations (MICs) were lower than isavuconazole (range 0.03 to 2 µg/mL versus 0.1 to 16 µg/mL, respectively). AFST resulted in a change in antifungal therapy from isavuconazole to posaconazole in 3/36 (8%) cases. There was no statistically significant difference in the mortality between the patients whose isolates received AFST versus those who did not have AFST performed. Conclusions: AFST led to a change in antifungal therapy in a minority of mucormycosis cases. Further studies to understand the epidemiological range of antifungal MICs and the effect of AFST-informed antifungal therapy are needed.