A 72-year-old man with chronic kidney disease stage 3b (baseline creatinine 2.0 mg/dL) undergoes elective coronary angiography for stable angina. He receives 150 mL of iodinated contrast medium. On postoperative day 2, his serum creatinine rises to 3.8 mg/dL, and urine output is 1.2 mL/kg/hr. Urine dipstick shows no proteinuria, and microscopy reveals muddy brown casts. What is the most likely diagnosis?

Explanation

## Clinical Analysis ### Key Features - **Contrast exposure** (150 mL iodinated contrast during angiography) - **Timing:** AKI develops within 24–72 hours post-contrast (peak at 3–5 days) - **Creatinine rise:** 2.0 → 3.8 mg/dL (90% increase) - **Preserved urine output** (1.2 mL/kg/hr — non-oliguric AKI) - **Muddy brown casts** (hallmark of acute tubular necrosis) - **No proteinuria** (rules out glomerulonephritis) - **Risk factors present:** CKD stage 3b, advanced age ### Contrast-Induced Acute Kidney Injury (CI-AKI) **Definition:** Acute rise in serum creatinine (≥0.5 mg/dL or ≥25% from baseline) within 48–72 hours of intravascular contrast administration, in the absence of other causes. ### Pathophysiology ```mermaid flowchart TD A[Iodinated Contrast Injection]:::action --> B[Osmotic diuresis + renal vasoconstriction]:::outcome B --> C[Renal medullary hypoxia]:::outcome C --> D[Mitochondrial dysfunction + ROS generation]:::outcome D --> E[Tubular epithelial cell apoptosis]:::outcome E --> F[Acute Tubular Necrosis]:::outcome F --> G[Muddy brown casts in urine]:::outcome ``` **Key Point:** CI-AKI is a form of **intrinsic renal disease (ATN)**, not prerenal azotemia. The contrast causes direct tubular injury through: 1. **Osmotic load** → diuresis and tubular obstruction 2. **Renal vasoconstriction** → medullary hypoperfusion 3. **Oxidative stress** → ROS and tubular apoptosis ### Risk Factors for CI-AKI | High Risk | Moderate Risk | |-----------|---------------| | CKD (eGFR <30) | CKD stage 3 (eGFR 30–60) | | Diabetes mellitus | Age >70 years | | Congestive heart failure | Contrast volume >100 mL | | Dehydration | Nephrotoxic drugs (NSAIDs, ACEi) | | Multiple myeloma | Repeated contrast within 72 hrs | This patient has **CKD stage 3b + age 72** = moderate-to-high risk. ### Clinical Features of CI-AKI - **Timing:** 24–72 hours post-contrast (peak at 3–5 days) - **Urinalysis:** Muddy brown casts (ATN), no RBC casts, minimal proteinuria - **Urine electrolytes:** FE~Na~ >2% (intrinsic renal disease) - **Course:** Usually reversible within 7–14 days if uncomplicated - **Non-oliguric in most cases** (as in this patient) **High-Yield:** Muddy brown casts are pathognomonic for acute tubular necrosis and strongly support CI-AKI in the context of recent contrast exposure. **Clinical Pearl:** Modern low-osmolar contrast media (LOCM) and iso-osmolar contrast media (IOCM) have lower nephrotoxicity than high-osmolar contrast media (HOCM), but CI-AKI can still occur in high-risk patients. ### Prevention of CI-AKI 1. **Hydration:** Normal saline 1 mL/kg/hr × 12 hours pre- and post-procedure 2. **Minimize contrast volume:** Use <3 mL/kg body weight 3. **Avoid nephrotoxic drugs:** Hold NSAIDs, ACEi/ARB, metformin for 48 hrs post-contrast 4. **N-acetylcysteine (NAC):** 600 mg PO BID × 2 days (controversial; may offer marginal benefit) 5. **Use low-osmolar or iso-osmolar contrast** **Mnemonic: CI-AKI Prevention = HMN** — **H**ydration, **M**inimize contrast, **N**ephrotoxic drug avoidance.