A 72-year-old man with chronic kidney disease stage 3 (baseline creatinine 2.0 mg/dL) undergoes elective coronary angiography for stable angina. He receives 150 mL of iodinated contrast medium. On post-procedure day 2, his serum creatinine rises to 2.8 mg/dL and urine output is 1.2 mL/kg/hr. Urine dipstick shows no protein or blood. Urine osmolality is 320 mOsm/kg and urine sodium is 45 mEq/L. What is the most likely diagnosis?

Explanation

## Diagnosis: Contrast-Associated Acute Kidney Injury (CA-AKI) ### Clinical Context and Timing **High-Yield:** Contrast-associated AKI is the third leading cause of in-hospital AKI. The temporal relationship is key: - **Exposure:** Iodinated contrast during angiography - **Onset:** Post-procedure day 2 (typical 24–72 hours) - **Risk factor:** Pre-existing CKD (baseline Cr 2.0) - **Magnitude:** Acute rise of 0.8 mg/dL (>25% increase) ### Laboratory Pattern: Intrinsic Renal (Tubular) Injury | Parameter | Finding | Interpretation | |-----------|---------|----------------| | **Urine osmolality** | 320 mOsm/kg (LOW-NORMAL) | Loss of concentrating ability → tubular dysfunction | | **Urine sodium** | 45 mEq/L (HIGH) | Inability to reabsorb sodium → tubular damage | | **Urine dipstick** | Negative for protein/blood | No glomerular or vascular injury | | **Urine output** | 1.2 mL/kg/hr (normal) | Non-oliguric AKI (better prognosis) | | **BUN:Cr ratio** | ~(BUN not given, but expected ~20:1) | Suggests intrinsic renal process | **Key Point:** The combination of **high urine sodium (45 mEq/L) + low-normal urine osmolality + absence of proteinuria** is pathognomonic for **intrinsic renal tubular injury**, not prerenal azotemia or glomerular disease. ### Pathophysiology of Contrast-Induced Tubular Injury ```mermaid flowchart TD A[Iodinated contrast injection]:::action --> B[Osmotic load to proximal tubule]:::outcome B --> C[Increased intratubular pressure]:::outcome C --> D[Renal vasoconstriction + hypoxia]:::outcome D --> E[Direct tubular epithelial toxicity]:::outcome E --> F[Loss of tight junctions]:::outcome F --> G[Increased urine sodium, low osmolality]:::outcome G --> H[Acute tubular dysfunction]:::outcome ``` **Mnemonic: CONTRAST** — **C**ontamination of tubule, **O**smotic load, **N**ephrotoxic metabolite, **T**ubular pressure ↑, **R**enal hypoxia, **A**cute tubular injury, **S**odium wasting, **T**ransient azotemia ### Why This Is NOT Atheroembolic Disease **Clinical Pearl:** Atheroembolic AKI typically occurs **3–7 days post-procedure** (this patient is day 2), presents with **eosinophiluria** (not tested here), may have **systemic features** (rash, livedo reticularis, eosinophilia), and often shows a **more indolent rise in creatinine**. The acute rise on day 2 with preserved urine output is more consistent with contrast toxicity. ### Risk Factors for CA-AKI - **Pre-existing CKD** (this patient: stage 3) — strongest risk factor - Advanced age (>70 years) - Diabetes mellitus - Dehydration - High contrast volume - Repeated procedures within 72 hours ### Prevention and Management 1. **Hydration:** IV isotonic saline 1–1.5 mL/kg/hr for 3–4 hours pre- and post-procedure 2. **Avoid nephrotoxins:** Hold ACE inhibitors, NSAIDs, metformin temporarily 3. **Use lowest contrast volume:** <3 mL/kg body weight 4. **Monitor creatinine:** Day 2–3 post-procedure 5. **N-acetylcysteine:** Controversial; some benefit in high-risk patients **High-Yield:** Most cases of CA-AKI are non-oliguric and recover within 5–7 days with supportive care. Oliguric CA-AKI carries higher mortality and may require dialysis. [cite:Harrison 21e Ch 279; Robbins 10e Ch 20]