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Difficulty: MediumDiagnostic Workup and Test Sequencing

A 34-year-old man presents to the clinic for a routine medical evaluation. He has no significant past medical history and takes no medications. He reports experiencing occasional muscle weakness and cramps in his legs over the past three months. His blood pressure is 168/104 mmHg168/104 \text{ mmHg} on two separate readings during the visit. His heart rate is 76/min76/\text{min}, and his body mass index is 24 kg/m224 \text{ kg/m}^2. Physical examination reveals no abdominal bruits, and peripheral pulses are normal and symmetric. Initial laboratory studies show the following:

Laboratory TestResultReference Range
Sodium144 mEq/L144 \text{ mEq/L}136−145 mEq/L136-145 \text{ mEq/L}
Potassium3.1 mEq/L3.1 \text{ mEq/L}3.5−5.0 mEq/L3.5-5.0 \text{ mEq/L}
Chloride104 mEq/L104 \text{ mEq/L}95−105 mEq/L95-105 \text{ mEq/L}
Bicarbonate29 mEq/L29 \text{ mEq/L}22−28 mEq/L22-28 \text{ mEq/L}
Creatinine0.9 mg/dL0.9 \text{ mg/dL}0.7−1.3 mg/dL0.7-1.3 \text{ mg/dL}

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

  1. Measurement of plasma aldosterone concentration and plasma renin activityAnswer
  2. B
    Computed tomography (CT) of the abdomen
  3. C
    Intravenous saline infusion test
  4. D
    Renal artery duplex ultrasonography
  5. E
    24-hour urine collection for fractionated metanephrines

Answer

Measurement of plasma aldosterone concentration and plasma renin activity is the correct first step to screen for primary hyperaldosteronism.
The correct sequence for diagnosing primary hyperaldosteronism begins with demonstrating autonomous hormone production. The first, least invasive, and most sensitive step is measuring the morning plasma aldosterone concentration (PAC) and plasma renin activity (PRA). An elevated ratio indicates primary hyperaldosteronism (where aldosterone suppresses renin). Only after this screening is positive should a physician proceed to confirmatory testing (e.g., oral sodium loading, IV saline infusion) and subsequently to imaging (CT of the abdomen).

Step-by-Step Solution

1
Analyze the patient's clinical and laboratory presentation.
The patient is a young male with severe hypertension, spontaneous hypokalemia, and mild metabolic alkalosis without any medication use.
Identifying the classic triad of hypertension, unexplained hypokalemia, and metabolic alkalosis narrows the differential diagnosis toward states of mineralocorticoid excess.
2
Determine the optimal diagnostic sequencing algorithm for suspected mineralocorticoid excess.
The initial step must distinguish between primary and secondary hyperaldosteronism using a biochemical screening test.
Screening must precede confirmatory testing (like saline infusion) or localization imaging (like CT scans) to avoid the high false-positive rates associated with adrenal incidentalomas.
3
Select the appropriate non-invasive screening modality.
The measurement of plasma aldosterone concentration (PAC) and plasma renin activity (PRA) is selected.
An elevated PAC/PRA ratio (usually > 20) with an elevated absolute aldosterone level points to a primary adrenal source (Conn's syndrome), whereas a low ratio with high renin points to a secondary source (like renal artery stenosis).

Key Concept

The diagnostic sequencing for primary hyperaldosteronism requires initial biochemical screening (PAC/PRA ratio), followed by biochemical confirmation (e.g., saline suppression test), and finally localization with cross-sectional imaging (CT abdomen).
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Diagnostic Workup and Test Sequencing

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