## Distinguishing Hyperkalemia: Renal Excretion Defect vs. Transcellular Shift ### Pathophysiology **Renal excretion defect** (decreased GFR, hypoaldosteronism, medications): - Kidneys cannot excrete potassium adequately - Urine potassium remains low despite elevated serum K⁺ **Transcellular shift** (acidosis, rhabdomyolysis, tumor lysis, exercise): - Potassium moves from intracellular to extracellular space - Kidneys still function normally and excrete excess K⁺ in urine ### Key Discriminator: Urine Potassium Excretion **High-Yield:** The **transtubular potassium gradient (TTKG)** and **24-hour urine potassium** are the gold-standard discriminators: | Feature | Renal Cause | Transcellular Shift | |---------|-------------|--------------------| | **Urine K⁺ (24-hr)** | <20 mEq/day | >40 mEq/day | | **TTKG** | <2 (inappropriate low) | >6 (appropriately high) | | **Mechanism** | Kidney failure to excrete | Normal renal response to ↑ serum K⁺ | | **Serum K⁺ trend** | Persistent elevation | Resolves with treatment of underlying cause | ### Clinical Pearl **Key Point:** In a hyperkalemic patient, measure **24-hour urine potassium or spot urine K⁺/creatinine ratio**. A low urine K⁺ despite high serum K⁺ indicates renal dysfunction; high urine K⁺ indicates the kidney is responding appropriately and the problem is transcellular shift or ongoing intake. ### Why ECG Timing Differs ECG changes (peaked T waves → prolonged PR → widened QRS) depend on **absolute serum K⁺ level and rate of rise**, not the mechanism. Both renal and transcellular causes produce identical ECG patterns at the same K⁺ concentration. [cite:Harrison 21e Ch 280]
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