A 45-year-old woman comes to the clinic for follow-up of recently diagnosed hypertension. … — USMLE Step 3

A $45$ -year-old woman comes to the clinic for follow-up of recently diagnosed hypertension. Her blood pressure was $162/98$ mm Hg at a health fair two months ago and $158/96$ mm Hg during a clinic visit two weeks ago. Today, she is entirely asymptomatic and reports no headaches, palpitations, diaphoresis, or muscle weakness. She takes no medications and has no significant past medical history. Her family history is unremarkable. Vital signs today show a blood pressure of $160/94$ mm Hg and a pulse of $76$ /min. BMI is $24$ kg/m $^2$ . Physical examination is normal, with no abdominal bruits, striae, or edema. Routine laboratory testing shows:

Sodium: $144$ mEq/L
Potassium: $2.9$ mEq/L
Chloride: $102$ mEq/L
Bicarbonate: $31$ mEq/L
Blood urea nitrogen: $12$ mg/dL
Creatinine: $0.8$ mg/dL

Which of the following is the most appropriate next step in laboratory evaluation?

Plasma aldosterone concentration to plasma renin activity ratioAnswer
B
Computed tomography angiography of the renal arteries
C
Adrenal venous sampling
D
24-hour urine measurement of fractionated metanephrines
E
Overnight 1-mg dexamethasone suppression test

Answer

The most appropriate next step is to obtain a plasma aldosterone concentration to plasma renin activity ratio.

This patient's presentation of new-onset hypertension, unprovoked hypokalemia, and metabolic alkalosis is highly suspicious for primary hyperaldosteronism. The appropriate, evidence-based initial screening test is the ratio of plasma aldosterone concentration to plasma renin activity (PAC/PRA ratio). In primary hyperaldosteronism, autonomous aldosterone production leads to sodium retention and potassium/hydrogen excretion, which in turn causes volume expansion and suppression of renin. This results in a characteristically elevated PAC and a low or undetectable PRA (generating a high PAC/PRA ratio).

Step-by-Step Solution

Identify the patient's classic clinical triad.

The patient presents with unprovoked hypertension, significant hypokalemia (

2.9

mEq/L), and metabolic alkalosis (bicarbonate

31

mEq/L) without any symptoms of other endocrine disorders.

Recognizing this triad narrows the differential diagnosis down to causes of mineralocorticoid excess.

Formulate a diagnostic algorithm for suspected primary hyperaldosteronism.

The standard diagnostic approach begins with a simple biochemical screening test.

Biochemical screening is non-invasive, cost-effective, and highly sensitive before proceeding to confirmatory or localizing tests.

Select the correct initial laboratory test.

The plasma aldosterone concentration (PAC) to plasma renin activity (PRA) ratio is chosen.

An elevated ratio (due to high autonomous aldosterone secretion suppressing renin production) effectively screens for primary hyperaldosteronism.

Key Concept

Patients with unprovoked hypokalemia and hypertension should undergo initial screening for primary hyperaldosteronism by measuring the plasma aldosterone concentration to plasma renin activity ratio.

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