Achievement of target blood concentrations of cyclosporine (CsA) early after transplantation is known to be highly effective for reducing the incidence of acute graft versus host disease (aGVHD). However, no research has been conducted for predicting aGVHD occurrence with low CsA concentrations at different time periods. The objective of this study was to investigate the risk of aGVHD according to low CsA concentrations at lag days in children with allogenic hematopoietic stem cell

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... on (HSCT). The records of 61 consecutive children who underwent allogeneic HSCT and received CsA as prophylaxis against aGVHD between May 2012 and March 2015 were retrospectively evaluated. The main outcome was any association between low CsA concentrations at lag days and aGVHD occurrence, which was examined for the first month after transplantation. Mean CsA concentrations at three lag periods were calculated: lag days 0-6, 7-13, and 14-20 before aGVHD occurrence. Patients whose mean CsA concentrations at lag days 0-6 did not reach the initial target concentration had 11.0-fold (95% confidence interval [CI]: 2.3-51.9) greater incidence of aGVHD. In addition, the AORs of low CsA concentrations at lag days 7-13 and 14-20 for developing aGVHD were 108.2 (95% CI: 7.7-1515.5) and 12.1 (95% CI: 1.1-138.1), respectively. After low CsA concentrations are detected, careful attention needs to be paid to prevent aGVHD.