An established 51-year-old male patient presents to the office for ongoing management of r… — USMLE Step 3

An established $51$ -year-old male patient presents to the office for ongoing management of resistant hypertension. He was initially diagnosed with elevated blood pressure four years ago. His current home and in-office readings average $162/98$ mmHg despite documented compliance with a maximum-dose three-drug regimen comprising amlodipine, olmesartan, and chlorthalidone. The patient reports feeling generally well without muscle weakness, palpitations, or episodic sweating.

Vital signs today show a blood pressure of $164/96$ mmHg and a heart rate of $74$ beats/minute. Physical examination is unremarkable, with no abdominal bruits.

Recent serum laboratory studies show a sodium level of $142$ mEq/L and a potassium level of $3.6$ mEq/L.

The physician suspects a secondary endocrine cause and correctly transitions the patient's antihypertensive regimen to verapamil, hydralazine, and doxazosin to prevent assay interference. After a $4$ -week washout period, biochemical screening reveals:
- Plasma aldosterone concentration (PAC): $21$ ng/dL (Reference: $3$ - $15$ ng/dL)
- Plasma renin activity (PRA): $0.4$ ng/mL/hr (Reference: $0.6$ - $4.3$ ng/mL/hr)
- PAC/PRA ratio: $52.5$

Which of the following is the most appropriate next step in the diagnostic workup?

Intravenous isotonic saline infusionAnswer
B
Computed tomography scan of the adrenal glands
C
Adrenal venous sampling
D
Overnight low-dose dexamethasone suppression test
E
Addition of spironolactone to the medical regimen

Answer

The most appropriate next step is an intravenous isotonic saline infusion test.

The correct answer is an intravenous isotonic saline infusion. This patient presents with resistant hypertension and a positive screening test for primary hyperaldosteronism (aldosterone-to-renin ratio >

20

with PAC >

15

ng/dL). According to endocrine guidelines, the diagnostic workup for primary hyperaldosteronism follows a strict three-step sequence: 1) Screening, 2) Confirmation, and 3) Subtype Localization. A positive screening test must be definitively confirmed by demonstrating autonomous aldosterone production. This is achieved through a suppression test, most commonly the intravenous isotonic saline infusion test or oral sodium loading test. In a normal physiologic state, a saline load suppresses renin and consequently aldosterone; in primary hyperaldosteronism, aldosterone remains inappropriately elevated. Only after this biochemical confirmation is achieved should the physician proceed to cross-sectional imaging (CT scan).

Step-by-Step Solution

Interpret the clinical presentation and initial labs.

The patient has resistant hypertension with a normal potassium level, prompting a screen for secondary causes like primary hyperaldosteronism.

Resistant hypertension (uncontrolled on 3+ drugs including a diuretic) is a classic indication for endocrine hypertension screening.

Analyze the screening test results.

The PAC is elevated (

21

ng/dL) and the PRA is suppressed (

0.4

ng/mL/hr), resulting in a ratio >

20

, which constitutes a positive screening test.

An elevated aldosterone-to-renin ratio indicates autonomous aldosterone production independent of the renin-angiotensin system.

Determine the next sequential diagnostic step according to guidelines.

A confirmatory biochemical test (e.g., IV saline infusion or oral sodium loading) must be performed.

Screening tests have a high false-positive rate. Autonomous secretion must be confirmed by demonstrating a failure to suppress aldosterone under salt-loading conditions BEFORE ordering any imaging.

Key Concept

In the diagnostic algorithm for primary hyperaldosteronism, a positive initial screening test (elevated aldosterone-to-renin ratio) must be followed by a confirmatory biochemical suppression test prior to pursuing cross-sectional imaging.

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