c Estimate based on Simes et al72 (INSPIRE) of synthesis of Brighton et al70 (ASPIRE) and Becattini et al71 (WARFASA). d Estimate taken from Douketis et al.228 Bibliography: Simes et al72 (INSPIRE) 2 weeks, or sooner if there is concern; there is no widely accepted protocol for surveillance US testing).75 Because about 15% of untreated isolated distal DVT are expected to subsequently extend into the popliteal vein and may cause PE, it is not acceptable to neither anticoagulate nor do surveillance to detect thrombus extension.1,76-79 In AT9, we judged that there was high-quality evidence that anticoagulant therapy was effective for the treatment of proximal DVT and PE, but uncertainty that the bene ts of anticoagulation outweigh its risks in patients with isolated distal DVT because of their lower risk of progressive or recurrent VTE. We suggest the following as risk factors for extension of distal DVT that would favor anticoagulation over surveillance: (1) D-dimer is positive (particularly when markedly so without an alternative reason); (2) thrombosis is extensive (eg, >5 cm in length, involves multiple veins, >7 mm in maximum diameter); (3) thrombosis is close to the proximal veins; (4) there is no reversible provoking factor for DVT; (5) active cancer; (6) history of VTE; and (7) inpatient status.1,75-77,80-84 We consider thrombosis that is con ned to the muscular veins of the calf (ie,, soleus, gastrocnemius) to have a lower risk of extension than thrombosis that involves the axial (ie, true deep; peroneal, tibial) veins.76,81,85 Severe symptoms favor anticoagulation, a high risk for bleeding (Table 11) favors surveillance, and the decision to use anticoagulation or surveillance is expected to be sensitive to patient preferences. 