Question

A 72-year-old woman comes to the clinic due to a 4-week history of new-onset headaches and generalized fatigue. She has no vision changes, jaw claudication, or scalp tenderness. She has a history of osteoarthritis and hypertension. Her temperature is 37.2°C (99.0°F), blood pressure is 140/85 mm Hg, and pulse is 78/min. Physical examination shows normal, non-tender temporal arteries. Laboratory studies show an erythrocyte sedimentation rate of 45 mm/h (reference range: 0-30 mm/h). Given the atypical presentation but elevated inflammatory markers in an older adult, the physician suspects giant cell arteritis (GCA) but estimates the pre-test probability to be only 20%. To aid in the diagnosis before deciding on an invasive temporal artery biopsy, the physician orders a high-resolution color Doppler ultrasound of the temporal arteries. Evidence shows that a 'halo sign' (hypoechoic wall thickening) on this ultrasound has a positive likelihood ratio of 12.0 and a negative likelihood ratio of 0.2 for GCA. The patient's ultrasound reveals a clear halo sign. Which of the following is the best estimate of this patient's post-test probability of having giant cell arteritis?