Tacrolimus (TAC) is essential for prophylaxis of acute graft-versus-host disease (aGVHD) after allogeneic hematopoietic cell transplantation (allo-HSCT). We have sometimes observed large fluctuations in TAC concentration. However, links between the variability in the concentration or the concentration/dose (C/D) ratio of TAC and clinical complications remain ambiguous. To clarify relationships between various parameters of TAC and early complications such as pre-engraftment
immune reactions/engraftment syndrome, aGVHD, and transplant-associated thrombotic microangiopathy (TA-TMA), a total of 146 patients who underwent allo-HSCT were included. Intrapatient variabilities in the concentration and C/D ratio of TAC were estimated by intrapatient mean absolute deviation (iMAD). The mean concentration and C/D ratio of TAC were not significantly different between with and without complications. A strong association was observed between greater iMAD for TAC C/D ratio from days 15 to 21 and the development of TA-TMA. iMAD values for TAC C/D ratio of 11.4 or greater was a risk factor for TA-TMA and the cumulative incidence of nonrelapse mortality (NRM) was significantly higher in patients with iMAD values for TAC C/D ratio of 11.4 or greater. Intrapatient variability in the C/D ratio of TAC was associated with the incidence of TA-TMA and NRM and might be useful for predicting TA-TMA.