A 68-year-old woman with a history of hypertension and chronic kidney disease stage 3a (GFR 45 mL/min/1.73 m²) is admitted with acute dehydration secondary to gastroenteritis. On admission, her serum creatinine is 1.5 mg/dL (baseline 1.4 mg/dL), serum urea is 52 mg/dL (baseline 28 mg/dL), and urine osmolality is 650 mOsm/kg. Her urine sodium is 8 mEq/L. Over the next 48 hours, after fluid resuscitation with 2 liters of normal saline, her serum creatinine falls to 1.3 mg/dL and serum urea to 30 mg/dL. Which of the following best explains the disproportionate elevation of serum urea relative to serum creatinine during the acute dehydration phase?

Explanation

## Urea vs. Creatinine Clearance in Volume Depletion **Key Point:** During acute volume depletion, serum urea rises disproportionately relative to serum creatinine because urea is reabsorbed in the proximal tubule (and collecting duct), whereas creatinine is not reabsorbed. The BUN:Cr ratio (or urea:creatinine ratio) is a sensitive marker of prerenal azotemia. ### Mechanism of Disproportionate Urea Elevation 1. **Volume depletion** activates the renin-angiotensin-aldosterone system (RAAS) 2. **Decreased renal perfusion** reduces GFR 3. **Increased proximal tubular reabsorption** of urea occurs due to: - Increased peritubular capillary oncotic pressure (from volume loss) - Increased angiotensin II, which enhances proximal tubular urea reabsorption - Increased ADH, which enhances collecting duct urea reabsorption 4. **Creatinine**, being a non-reabsorbed solute, is filtered and excreted proportionally to GFR 5. Result: **Urea rises more than creatinine** ### Clinical Evidence in This Case | Parameter | Admission | After Resuscitation | Interpretation | |-----------|-----------|---------------------|----------------| | Serum creatinine | 1.5 mg/dL | 1.3 mg/dL | Mild rise from baseline (1.4) | | Serum urea | 52 mg/dL | 30 mg/dL | Marked elevation from baseline (28) | | BUN:Cr ratio | 34.7 | 23 | >20 indicates prerenal azotemia | | Urine osmolality | 650 mOsm/kg | — | Concentrated urine (volume depletion) | | Urine Na⁺ | 8 mEq/L | — | Avid sodium retention (RAAS activation) | **High-Yield:** A BUN:Cr ratio >20 (or urea:creatinine ratio >10 in SI units) strongly suggests **prerenal azotemia** or volume depletion. After fluid resuscitation, both fall, but the disproportionate rise in urea during depletion is pathognomonic for enhanced tubular reabsorption. **Mnemonic:** **RAAS = Reabsorb Amino-acid-like Solutes** — RAAS activation increases proximal tubular reabsorption of urea (and amino acids, glucose, etc.), but NOT creatinine. **Clinical Pearl:** The rapid improvement in both urea and creatinine after saline resuscitation confirms this was prerenal azotemia (functional, reversible) and not intrinsic renal disease. [cite:Guyton & Hall Textbook of Medical Physiology 14e Ch 31; Harrison 21e Ch 279]