A 72-year-old woman with chronic kidney disease (baseline creatinine 2.2 mg/dL) develops acute kidney injury after cardiac catheterization with contrast injection. Urine microscopy is bland (no casts or cells). FENa is 0.8%. Which finding best distinguishes contrast-induced nephropathy (CIN) from acute tubular necrosis in this clinical context?

Explanation

## Distinguishing Contrast-Induced Nephropathy from ATN **Key Point:** The temporal relationship—acute creatinine rise within 24–48 hours of contrast exposure—is the best discriminator for CIN. This narrow temporal window, combined with the procedural history, is pathognomonic for CIN and distinguishes it from ATN, which typically develops over hours to days from a different mechanism (ischemia, sepsis, rhabdomyolysis). ### Temporal and Clinical Characteristics | Feature | Contrast-Induced Nephropathy (CIN) | Acute Tubular Necrosis (ATN) | |---------|-------------------------------------|------------------------------| | **Onset** | 24–48 hours after contrast | Hours to days (variable) | | **Peak creatinine** | 3–5 days post-contrast | 5–7 days | | **Urine microscopy** | Bland or minimal findings | Muddy brown casts, granular casts | | **FENa** | <1% (tubules intact) | >2% (tubular injury) | | **Urine output** | Often preserved | Oliguria common | | **Recovery** | Days to weeks | Weeks to months | | **Mechanism** | Renal vasoconstriction + osmotic injury | Tubular epithelial necrosis | **High-Yield:** CIN is a **non-oliguric** AKI with **bland urine** and **preserved FENa <1%**. This paradox (low FENa despite AKI) is the classic teaching point. The mechanism is vasoconstriction and osmotic toxicity, NOT tubular necrosis. ### Why This Case Points to CIN 1. **Procedural trigger:** Cardiac catheterization with contrast (clear inciting event) 2. **Timing:** Creatinine rise within 24–48 hours (classic CIN window) 3. **Urine:** Bland microscopy (no muddy brown casts → no ATN) 4. **FENa:** 0.8% <1% (tubules still reabsorbing sodium → no tubular injury) 5. **Baseline CKD:** Risk factor for CIN (reduced renal reserve) **Clinical Pearl:** In a patient with CKD presenting with AKI after contrast, the combination of **procedural timing + bland urine + FENa <1%** = CIN until proven otherwise. ATN would show muddy brown casts and FENa >2%. ### Mnemonic: CIN vs ATN **"CIN is Clean, ATN is Muddy"** - **C**IN = **C**lean urine (bland microscopy) - **ATN** = **A**bnormal casts (**A**lways muddy brown) **Warning:** FENa <1% in the setting of AKI is unusual and should raise suspicion for: - Contrast-induced nephropathy - Hepatorenal syndrome - Rhabdomyolysis (early phase) - Glomerulonephritis Do NOT assume low FENa always means prerenal azotemia in AKI. [cite:Harrison 21e Ch 297; KD Tripathi 8e Ch 12]