An ambulatory 42-year-old man is evaluated in the hypertension specialty clinic due to per… — USMLE Step 3

An ambulatory $42$ -year-old man is evaluated in the hypertension specialty clinic due to persistently elevated blood pressure. He was diagnosed with essential hypertension $5$ years ago, but his readings remain consistently above $150/90$ mm Hg despite strict adherence to a daily regimen of amlodipine, olmesartan, and indapamide. The patient reports occasional episodes of proximal muscle weakness and nocturnal polyuria, but is otherwise asymptomatic. Vital signs today show a blood pressure of $158/96$ mm Hg and a heart rate of $76$ /min. Physical examination is unremarkable, with no abdominal bruits, cushingoid features, or radiofemoral delay.

Basic metabolic panel reveals the following:

Laboratory Parameter	Value
Sodium	$146$ mEq/L
Potassium	$2.8$ mEq/L
Chloride	$102$ mEq/L
Bicarbonate	$29$ mEq/L
Creatinine	$0.9$ mg/dL

Which of the following is the most appropriate next step in this patient's diagnostic workup?

A
Dedicated computed tomography scan of the adrenal glands
B
Intravenous saline infusion test with measurement of serum aldosterone
C
Computed tomography angiography of the renal arteries
Measurement of early morning plasma aldosterone concentration and plasma renin activityAnswer
E
Bilateral adrenal venous sampling

Answer

Measurement of early morning plasma aldosterone concentration and plasma renin activity

This patient presents with resistant hypertension, unprovoked hypokalemia, and mild metabolic alkalosis, generating a high pre-test probability for primary hyperaldosteronism. The diagnostic evaluation for primary hyperaldosteronism strictly requires sequential progression through three distinct phases to avoid diagnostic error. The first phase is case detection (screening), which is accomplished by measuring the early morning plasma aldosterone concentration (PAC) and plasma renin activity (PRA). In primary hyperaldosteronism, autonomous aldosterone secretion causes sodium retention and volume expansion, which suppresses renin, resulting in a significantly elevated PAC/PRA ratio. Only if this screening test is positive should the clinician proceed to confirmatory testing (e.g., oral sodium loading or intravenous saline infusion), followed finally by subtype evaluation (adrenal CT scan and potentially adrenal venous sampling).

Step-by-Step Solution

Identify the clinical syndrome based on patient presentation and laboratory results.

The patient has resistant hypertension (elevated blood pressure despite three antihypertensive medications from different classes, including a diuretic) accompanied by unprovoked hypokalemia (

2.8

mEq/L) and metabolic alkalosis.

These findings are the classic hallmark of mineralocorticoid excess, raising a high clinical suspicion for primary hyperaldosteronism (Conn syndrome).

Determine the proper diagnostic testing sequence for the suspected condition.

The workup for primary hyperaldosteronism mandates a strict three-tier sequence: 1) Case detection (biochemical screening), 2) Case confirmation (biochemical loading/suppression), and 3) Subtype evaluation (cross-sectional imaging followed by invasive sampling if surgery is considered).

Strict adherence to this sequence prevents misdiagnosis from common adrenal incidentalomas and avoids unnecessary invasive procedures.

Select the appropriate modality for the first tier (case detection).

The initial step is to measure the early morning plasma aldosterone concentration (PAC) and plasma renin activity (PRA) to calculate the PAC/PRA ratio.

An elevated ratio (typically

>

20

, with an absolute aldosterone

>

15

ng/dL) provides the biochemical evidence needed to proceed to confirmatory testing.

Key Concept

The diagnostic workup for primary hyperaldosteronism must strictly follow a sequential algorithm starting with biochemical screening (PAC/PRA ratio), followed by biochemical confirmation (e.g., saline loading), and ending with anatomical localization (adrenal CT) to prevent misdiagnosis due to high rates of benign adrenal incidentalomas.

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