A 50-year-old construction worker continues to have elevated BP of 160/100 mmHg after a third agent (thiazide) is added to his anti-hypeensive regimen. Physical examination is normal, electrolytes are normal and he is not taking any over the counter medications. Which is the next helpful step in the diagnosis of this patient?

The patient is suffering from resistant hypeension and the leading cause of resistant hypeension is non- compliance on the pa of patient. Resistant hypeension is defined as blood pressure that remains above goal in spite of the concurrent use of 3 antihypeensive agents of different classes. Ideally, one of the 3 agents should be a diuretic and all agents should be prescribed at optimal dose amounts.