Utility of transplant conditioning intensity score in allogeneic haematopoietic stem cell transplantation for genetic disorders

Summary In allogeneic haematopoietic stem cell transplantation (allo‐HSCT) for genetic disorders (GDs), predicting outcomes using the currently used myeloablative/reduced‐intensity conditioning classification is challenging. We examined the utility of the transplant conditioning intensity (TCI) score in allo‐HSCT for GDs. We retrospectively analysed 1077 patients who underwent first allo‐HSCT for GDs between 1987 and 2018. While no significant difference was observed in 3‐year non‐relapse mortality (NRM) between the myeloablative and reduced‐intensity conditioning group, 3‐year NRM in the TCI score ≥4.0 (TCI‐high) group (17.3%) was higher than that in the TCI score 0.5–3.5 (TCI‐low/intermediate) group (11.6%, p = 0.013). Multivariate analysis identified a high TCI score as a significant factor for NRM (HR 1.56, p = 0.011). Infection‐related mortalities were more prevalent in the TCI‐high group (7.9%) compared with the TCI‐low/intermediate group (3.2%). In contrast, patients in the TCI‐high group achieved a significantly higher rate of complete donor chimerism (71.7%) than those in the TCI‐low/intermediate group (62.2%, p = 0.022). TCI score is a valuable tool for predicting outcomes after allo‐HSCT for GDs as well as for guiding the selection of conditioning regimens tailored to the unique characteristics of GDs.