A 42-year-old woman presents with muscle weakness, polyuria, and polydipsia. Lab results: serum K+ 2.8 mEq/L, serum bicarbonate 32 mEq/L, arterial pH 7.48, serum sodium 142 mEq/L, urine K+ 48 mEq/L on spot urine (high). Blood pressure is 160/100 mmHg. Serum magnesium is normal. What is the TRANSTUBULAR POTASSIUM GRADIENT (TTKG) in this scenario, and what does it indicate?
- A TTKG < 2 indicates gastrointestinal potassium loss (appropriate renal conservation)
- B TTKG > 4 indicates renal potassium wasting (inappropriate urinary K+ loss), consistent with mineralocorticoid excess ✓
- C TTKG cannot be calculated without urine osmolality and serum osmolality
- D TTKG = 1–3 is normal; this patient's TTKG indicates dietary potassium deficiency
Explanation
The TTKG = (Urine K+ / Serum K+) ÷ (Urine osmolality / Serum osmolality), and represents the K+ concentration ratio across the cortical collecting duct. A TTKG > 4 in the setting of hypokalaemia indicates inappropriate renal K+ wasting due to increased aldosterone activity at the collecting duct. This patient has hypertension, metabolic alkalosis, hypokalaemia with high urine K+ — the classic triad of primary hyperaldosteronism. TTKG < 2 would indicate appropriate renal K+ conservation (pointing to GI loss, redistribution, or inadequate intake). Note: TTKG requires urine osmolality to calculate accurately — the question tests knowledge of TTKG interpretation rather than mathematical calculation.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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Written and medically reviewed by the StethoPrep medical team.