A 62-year-old man with chronic kidney disease (eGFR 28 mL/min/1.73 m²) requires imaging of the renal arteries to evaluate for stenosis. Which investigation is most appropriate to assess renal artery patency while minimizing the risk of contrast-induced nephropathy?

Explanation

## Investigation of Choice in CKD with Renal Artery Evaluation ### Clinical Context In patients with advanced CKD (eGFR <30 mL/min/1.73 m²), contrast-induced nephropathy (CIN) is a significant risk. The investigation must balance diagnostic accuracy with safety. ### Why Doppler Ultrasound is Optimal **Key Point:** Doppler ultrasound is a non-invasive, non-nephrotoxic modality that provides real-time assessment of renal artery blood flow and can detect hemodynamically significant stenosis with sensitivity 85–90% and specificity 92–95%. **High-Yield:** In CKD patients, Doppler ultrasound is the first-line non-invasive imaging for renal artery stenosis screening because it: - Requires no contrast (eliminates CIN risk) - Is operator-dependent but highly accurate in experienced hands - Allows functional assessment via resistive index - Can be repeated without cumulative toxicity ### Comparison of Modalities in CKD | Modality | Contrast Type | CIN Risk | Sensitivity | Specificity | Utility in eGFR <30 | |----------|---------------|----------|-------------|-------------|---------------------| | **Doppler US** | None | Nil | 85–90% | 92–95% | **Preferred** | | CT angiography | Iodinated (osmotic load) | High | 94% | 90–95% | Avoid unless essential | | MR angiography | Gadolinium | Moderate–High* | 90% | 95% | Relative contraindication | | Captopril renography | Radiotracer ± contrast | Low | 60–70% | 80–85% | Alternative if US inconclusive | *Gadolinium carries risk of nephrogenic systemic fibrosis (NSF) in eGFR <30. **Clinical Pearl:** When Doppler is inconclusive or technically limited (obesity, bowel gas), captopril renography is a safer alternative than contrast-enhanced CT or MRI in severe CKD. ### Prevention of CIN If iodinated contrast is unavoidable: - Hydration with isotonic saline (1 mL/kg/hr × 12 hrs pre- and post-procedure) - Metformin: hold for 48 hrs post-contrast if CIN occurs - Consider N-acetylcysteine (evidence mixed but low harm) - Use lowest osmolarity contrast (iso-osmolar or low-osmolar) [cite:Harrison 21e Ch 281]