Answer
A: Plasma aldosterone concentration/plasma renin activity ratio
Educational Objective
Diagnose primary hyperaldosteronism as a secondary cause of hypertension.
Critique
The most appropriate diagnostic test to perform next is the plasma aldosterone concentration/plasma renin activity ratio (Option A). Primary hyperaldosteronism, in which aldosterone production cannot be suppressed with sodium loading, is the most common cause of secondary hypertension in middle-aged adults and an important cause of resistant hypertension. A triad of resistant hypertension, metabolic alkalosis, and hypokalemia (including in patients treated with low-dose thiazide diuretics) should raise suspicion. Screening is recommended if any of the following are present: resistant hypertension; hypokalemia (spontaneous or substantial, if diuretic induced); incidentally discovered adrenal mass; family history of early-onset hypertension; moderately severe hypertension (>160/100 mm Hg); or stroke at age <40 years. It is common for patients with primary hyperaldosteronism to have normal serum potassium levels; a high index of suspicion is therefore required. This patient has substantial hypokalemia (serum potassium level of 2.9 mEq/L [2.9 mmol/L]) with initiation of low-dose hydrochlorothiazide and persistent, moderately severe hypertension. The patient also has a metabolic alkalosis.
Plasma fractionated metanephrines (Option B) are obtained to screen for a pheochromocytoma. The absence of episodic palpitations, headaches, and tachycardia and the presence of metabolic alkalosis and hypokalemia make pheochromocytoma a less likely diagnosis for this patient.
Renal artery CT angiography (Option C) is an appropriate screening test for renovascular disease, including fibromuscular dysplasia. Older patients with renovascular hypertension often have other manifestations of atherosclerosis, including the presence of coronary artery, cerebrovascular, or peripheral vascular disease. Lateralizing abdominal bruits may be auscultated. Abrupt onset of hypertension in patients <35 years of age suggests fibromuscular dysplasia. None of these clinical findings is present to suggest the diagnosis of renovascular disease.
The 24-hour urine free cortisol (Option D) is a frequently used screening test for hypercortisolism. In adults, hypercortisolism may manifest as hypertension and hypokalemia, but it is relatively rare compared with primary hyperaldosteronism. In the absence of Cushingoid facies, central obesity, proximal muscle weakness, and ecchymosis, Cushing syndrome is a very unlikely cause of this patient's hypertension.
Key Point
Primary hyperaldosteronism is the most common cause of secondary hypertension and should be suspected in patients with resistant hypertension, hypokalemia, family history of early-onset hypertension, blood pressure >160/100 mm Hg, or stroke at age <40 years. The plasma aldosterone concentration/plasma renin activity ratio is the recommended screening test for primary hyperaldosteronism.
Bibliogrpahy
Lee FT, Elaraj D. Evaluation and management of primary hyperaldosteronism. Surg Clin North Am. 2019;99:731-745. doi:10.1016/j.suc.2019.04.010
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