A 58-year-old man with chronic kidney disease (eGFR 35 mL/min/1.73m²) and hypertension is started on a loop diuretic for fluid overload. After 2 weeks of therapy, he develops severe muscle cramps, generalized weakness, and palpitations. Serum potassium is 2.8 mEq/L. Which investigation would be most appropriate to assess the mechanism of hypokalemia and guide further management?

## Investigation of Diuretic-Induced Hypokalemia ### Clinical Context Loop diuretics cause hypokalemia through increased urinary potassium wasting. In a patient with a clear clinical context (known loop diuretic use, symptomatic hypokalemia), the priority is to rapidly confirm renal potassium wasting and guide management. The **spot urine potassium-to-creatinine (K/Cr) ratio** is the most appropriate and practical first-line investigation in this setting. ### Why Spot Urine K/Cr Ratio is the Best Choice **Key Point:** A spot urine potassium-to-creatinine ratio >13 mEq/mmol (or >1.5 mEq/mmol in some references) indicates renal potassium wasting, confirming diuretic-induced losses as the mechanism — and results are available within hours. **High-Yield:** The spot urine K/Cr ratio has largely replaced the 24-hour urine potassium collection in modern clinical practice because: - It is **immediately available** (no timed collection required) - It **corrects for urine concentration** (unlike a random urine K+ alone) - It correlates well with 24-hour urinary potassium excretion - It is the recommended initial test in current nephrology guidelines (Harrison's Principles of Internal Medicine, 21st ed.) ### Why This Matters Clinically **Clinical Pearl:** Knowing the mechanism (renal vs. extrarenal wasting) guides therapy: - **Renal wasting** (spot urine K/Cr ratio >13 mEq/mmol) → potassium supplementation ± potassium-sparing agent (spironolactone, amiloride) - **Extrarenal loss** (spot urine K/Cr ratio <13 mEq/mmol) → investigate GI source ### Comparison of Investigations | Investigation | Utility | Limitation | |---|---|---| | **Spot urine K/Cr ratio** | Rapid, practical, corrects for concentration; recommended first-line | Slightly less precise than 24-hr in non-steady-state | | 24-hr urine K+ | Quantifies total renal potassium excretion; historical gold standard | Time-consuming; requires timed collection; impractical in acute setting | | ABG | Assesses acid-base status; hypokalemia → metabolic alkalosis | Does not quantify potassium losses; indirect marker only | | Serum Mg, Ca | Identifies concurrent deficiencies that impair K+ repletion | Does not explain mechanism of hypokalemia | **High-Yield:** While 24-hour urinary potassium was historically considered the gold standard, the **spot urine K/Cr ratio is the preferred initial investigation** in current clinical practice due to its speed and equivalent diagnostic accuracy in most clinical scenarios (Harrison's, 21st ed.; Kamel & Halperin, NEJM 2017). **Mnemonic:** **SPOT = Swift Potassium Output Test** — the spot K/Cr ratio rapidly identifies renal wasting without waiting 24 hours.