Abstract: Objective: To explore the role of serum magnesium and platelet-to-lymphocyte ratio (PLR) in the auxiliary diagnosis of limb post-traumatic osteomyelitis (PTOM).Methods: A total of 78 patients with limb PTOM treated in our hospital from 2013 to 2018 were included in the PTOM group, 89 patients with aseptic nonunion (ABN) were included in the ABN group, and 100 patients with fracture healing in the same period were included in the control group.The serum magnesium level and whole blood cell counts were detected by Hitachi AU5832 automatic biochemical analyzer and Sysmex XN-2800 automatic blood cell analyzer, and PLR as well as neutrophil-to-lymphocyte ratio (NLR) were further calculated.Results: Serum magnesium in the PTOM group was lower than that in the control group and ABN group, PLR was higher than that in the control group and ABN group, and NLR was higher than that in the control group (P< 0.05).Serum magnesium level and PLR in the PTOM group were not related to age and gender stratification (P> 0.05).Logistic regression analysis showed that serum magnesium and PLR were independent influencing factors of limb PTOM (P< 0.05).There was no significant correlation between serum magnesium level and PLR in patients with PTOM (r=-0.086, P> 0.05).The area under ROC curve (AUC) showed that the acceptable accuracy of serum magnesium level (AUC=0.816, 95% CI:0.753-0.879) and PLR (AUC=0.728, 95% CI:0.649-0.808) was high (AUC> 0.7).The combined prediction of AUC was 0.886 (95% CI:0.835-0.936), and the sensitivity and specificity were 85.9% and 78.0%, respectively.Serum magnesium levels of 2.25mg/dL and PLR of 181.35 were determined as the best cut-off values.Conclusion: The decrease of serum magnesium and the increase of PLR are closely related to PTOM.Serum magnesium level combined with PLR detection may be an effective method to assist the diagnosis of limb PTOM.