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Difficulty: MediumOrdering Laboratory Tests

A 48-year-old male is evaluated during a routine follow-up visit for poorly controlled blood pressure. He was diagnosed with essential hypertension two years ago and is currently taking maximally tolerated doses of amlodipine and lisinopril. Despite compliance with his medication regimen, his home blood pressure readings consistently range from 150/90 to 160/95 mmHg. He reports occasional muscle cramps and generalized weakness but is otherwise healthy. His body mass index is 26 kg/m². In the clinic today, his blood pressure is 156/94 mmHg and his heart rate is 72/min. Physical examination is unremarkable with no abdominal bruits or striae. Laboratory studies obtained earlier this week reveal a serum sodium level of 143 mEq/L and a serum potassium level of 3.1 mEq/L. Which of the following is the most appropriate next step in laboratory evaluation?

  1. A
    Adrenal venous sampling
  2. B
    Computed tomography scan of the abdomen
  3. Plasma aldosterone concentration to plasma renin activity ratioAnswer
  4. D
    24-hour urine free cortisol
  5. E
    Repeat basic metabolic panel in 1 week

Answer

The most appropriate next step is to order a plasma aldosterone concentration to plasma renin activity ratio.
The patient presents with resistant hypertension and spontaneous hypokalemia, a classic presentation for primary hyperaldosteronism. The appropriate diagnostic sequence for suspected primary hyperaldosteronism begins with biochemical screening using the plasma aldosterone concentration to plasma renin activity ratio (ARR). An elevated ratio indicates autonomous aldosterone production independent of renin stimulation.

Step-by-Step Solution

1
Identify the primary clinical problem from the vignette.
The patient has resistant hypertension (blood pressure above target despite being on two antihypertensive agents) and unprovoked hypokalemia (potassium of 3.1 mEq/L while taking an ACE inhibitor, which typically raises potassium).
Recognizing the pattern of resistant hypertension accompanied by hypokalemia points strongly toward a secondary endocrine cause.
2
Determine the most likely suspected diagnosis based on the clinical pattern.
Primary hyperaldosteronism (Conn syndrome) is the most likely underlying condition.
Excess aldosterone promotes sodium retention (causing hypertension) and potassium excretion (causing hypokalemia and muscle cramps).
3
Select the appropriate initial laboratory test to screen for the suspected diagnosis.
The plasma aldosterone concentration to plasma renin activity ratio (ARR) is the recommended initial diagnostic step.
This biochemical screening test is highly sensitive for primary hyperaldosteronism and must be performed prior to any advanced imaging or invasive confirmatory procedures.

Key Concept

The initial laboratory evaluation for patients presenting with resistant hypertension and unprovoked hypokalemia is the plasma aldosterone concentration to plasma renin activity ratio to screen for primary hyperaldosteronism.
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Ordering Laboratory Tests

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