A 58-year-old woman presents with a 24-hour history of a painful, swollen right calf after a long car journey.

 She is a smoker, has recently become less mobile because of sciatica, and reports marked tenderness over the deep venous system.

Findings on examination:

Entire right leg appears swollen

Calf circumference is 3.5 cm larger than the other side

Localised pitting oedema confined to the symptomatic leg

No clear alternative diagnosis

Her 2-level DVT Wells score is 3  

A proximal leg vein ultrasound is available immediately and is performed within 4 hours, reported as negative for DVT.

 According to NICE NG158, what is the most appropriate next step?