The t test and ROC curve analyses showed differences in PA, SA, and the SA/PA ratio between the two groups of patients in the discovery set (A, B, and C; n = 28; P < 0.05, AUC > 0.7). The SA/PA ratio was more effective in the diagnosis of II–IV aGVHD compared with each of the two metabolite biomarkers alone. These data are confirmed by the results from the validation data set (D; n = 86; P < 0.01, AUC = 0.719).