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Difficulty: MediumOrdering Laboratory Tests

A 41-year-old man presents to the primary care clinic for evaluation of elevated blood pressure. He has a 3-year history of hypertension that has been difficult to control despite strict adherence to a regimen of maximum tolerated doses of lisinopril, amlodipine, and hydrochlorothiazide. The patient notes occasional episodes of muscle cramping and generalized weakness but denies headaches, palpitations, or diaphoresis. His blood pressure is 168/102 mm Hg, and his pulse is 76/min. The physical examination is unremarkable, with no abdominal bruits or peripheral edema.

Initial laboratory studies reveal the following:

Laboratory ParameterResultReference Range
Sodium144 mEq/L136-145 mEq/L
Potassium3.1 mEq/L3.5-5.0 mEq/L
Chloride102 mEq/L95-105 mEq/L
Bicarbonate29 mEq/L22-28 mEq/L
Blood urea nitrogen15 mg/dL7-18 mg/dL
Creatinine0.8 mg/dL0.7-1.3 mg/dL

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

  1. A
    Computed tomography of the abdomen
  2. Measurement of plasma aldosterone concentration to plasma renin activity ratioAnswer
  3. C
    Oral sodium loading test
  4. D
    24-hour urine collection for fractionated metanephrines
  5. E
    Adrenal venous sampling

Answer

Measurement of plasma aldosterone concentration to plasma renin activity ratio
The correct answer identifies the primary screening test for primary hyperaldosteronism. This patient presents with classic features: resistant hypertension, hypokalemia, and metabolic alkalosis. According to established endocrine guidelines, the most appropriate initial laboratory evaluation is measuring the plasma aldosterone concentration to plasma renin activity ratio. A high ratio (typically > 20) with a concurrent plasma aldosterone concentration > 15 ng/dL indicates a positive screen, prompting subsequent confirmatory testing.

Step-by-Step Solution

1
Analyze the patient's clinical presentation.
The patient has resistant hypertension (uncontrolled on three medications including a diuretic) combined with hypokalemia and metabolic alkalosis.
Identifying the clinical syndrome points toward a secondary cause of hypertension, specifically an endocrine etiology.
2
Formulate the most likely differential diagnosis.
The triad of resistant hypertension, spontaneous or diuretic-induced hypokalemia, and muscle weakness strongly suggests primary hyperaldosteronism (Conn's syndrome).
Excess aldosterone promotes sodium retention (hypertension) and potassium and hydrogen ion excretion (hypokalemia and metabolic alkalosis).
3
Determine the correct algorithm for diagnostic testing.
The diagnostic sequence for primary hyperaldosteronism requires screening first, followed by biochemical confirmation, and finally radiographic localization.
Adhering to strict test sequencing avoids false positives and limits unnecessary invasive procedures or imaging.
4
Select the appropriate initial screening test.
The initial screening test is the early morning plasma aldosterone concentration to plasma renin activity (PAC/PRA) ratio.
An elevated PAC/PRA ratio is highly sensitive for screening before subjecting the patient to confirmatory salt-loading tests.

Key Concept

The initial diagnostic step for suspected primary hyperaldosteronism is screening with the plasma aldosterone concentration to plasma renin activity ratio.
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Ordering Laboratory Tests

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