Isavuconazole is an antifungal medicine used to prevent or treat serious fungal infections in people who have received a solid organ transplant (mainly lung, and also kidney or liver) or a hematopoietic stem cell (bone marrow) transplant. Because these patients take medicines that suppress the immune system, they are at high risk of fungal infections. The amount of isavuconazole in the blood can vary widely from person to person, and it is not fully understood what drives these differences or whether higher blood levels are linked to side effects such as liver problems.This study reviews the medical records of transplant recipients who were treated with isavuconazole at a single hospital in China between January 2024 and April 2026. Using results from routine therapeutic drug monitoring (blood tests that measure the drug level), the researchers describe how isavuconazole blood levels are distributed, how much they vary within and between patients, and which clinical and genetic factors are associated with higher or lower levels. The study also examines whether isavuconazole blood levels are related to liver function abnormalities and to survival. Because this is an observational study, no treatment was assigned for research purposes; the study only analyzes data collected during routine clinical care. The findings are intended to help guide individualized dosing and monitoring of isavuconazole in transplant recipients.