A 38-year-old woman presents with 2 days of pleuritic chest pain and dyspnoea. She recently had knee arthroscopy. Her heart rate is 110 beats per minute, blood pressure 118/74 mm Hg, oxygen saturation 95 percent on room air. Wells score is 3.0 (moderate probability). According to Thrombosis Canada guidance, what is the most appropriate next step in diagnosis?