Objective:To study the clinical significance of plasma D-dimer for extended anticoagulation in the elderly patients with hip fracture after discharge.Methods:The data were retrospectively analyzed of the 355 elderly patients with hip fracture who had been admitted to Emergence Center of Trauma and Department of Orthopedic Surgery, The Third Hospital of Hebei Medical University from January 2016 to November 2020. There were 107 males and 248 females with a mean age of 79.0 years (from 65 to 102 years). There were 179 femoral neck fractures, 159 intertrochanteric fractures, and 17 subtrochanteric fractures. Plasma D-dimer concentrations were detected at admission, before surgery, and 35 days after surgery. Color Doppler ultrasonography of deep veins of lower extremities was performed at admission, before surgery, before discharge, and 35 days after surgery. The patients with deep vein thrombosis (DVT) at the time of discharge should were reexamined 15 days after discharge. Strategies for anticoagulation prevention and treatment were formulated according to the patients' plasma D-dimer, color Doppler ultrasonography results, symptoms and signs. Plasma D-dimer concentrations were compared between patients with thrombosis and without thrombosis at different time points.Results:The plasma D-dimer concentrations for the 355 patients at admission, before surgery and 35 days after surgery were 2.73 (1.72, 5.13) mg/L, 1.31 (0.72, 2.58) mg/L, and 0.49 (0.25, 0.80) mg/L, respectively. The level of D-dimer at 35 days after surgery was significantly lower than that at admission and before surgery ( P<0.05). There were, respectively, 105, 126,191 and 148 patients with DVT at admission, before surgery, before discharge and 35 days after surgery, and the DVT was distal in most cases. The D-dimer concentrations for patients with thrombosis and without thrombosis at 35 days after surgery was 0.64 (0.32, 1.00) mg/L and 0.37 (0.22, 0.68) mg/L respectively, showing a significant difference ( P<0.05). The area under the receiver operating characteristic curve of plasma D-dimer for the diagnosis of DVT at 35 days after surgery was 0.665. Conclusions:Extended anticoagulation and continuous monitoring of plasma D-dimer are very important for elderly patients with hip fracture after discharge. For patients with DVT at discharge, negative plasma D-dimer is a sign for termination of anticoagulation; for patients without DVT at discharge, it is reasonable to extend anticoagulation 35 days after surgery.