Contrast-Associated AKI & Acute Tubular Necrosis vs Pre-Renal AKI — SAQ

Stem

A 68-year-old man with type 2 diabetes mellitus, hypertension and known CKD (baseline creatinine 1.8 mg/dL, eGFR 38 mL/min/1.73 m2) is admitted with an acute coronary syndrome and undergoes primary percutaneous coronary intervention with intra-arterial iodinated contrast (220 mL of iopamidol). Forty-eight hours later his creatinine has risen to 2.9 mg/dL. He has been oliguric (240 mL over the last 12 hours). Serum potassium is 5.9 mmol/L. Urine sodium is 44 mmol/L, urine osmolality is 310 mOsm/kg, BUN/creatinine ratio is 12, and the urine sediment shows muddy brown granular casts and renal tubular epithelial cells. He takes metformin 1 g twice daily and aspirin.

Questions

a) What is the diagnosis and what two criteria does he meet? (2 marks)

Contrast-associated acute kidney injury (CA-AKI), a form of acute tubular necrosis. He meets both the absolute rise criterion (rise of 1.1 mg/dL, well above the 0.3 mg/dL threshold) and the timing criterion (within 48-72 hours of iodinated contrast exposure), per the ESUR/ACR definition.

b) How do the urine indices distinguish this from pre-renal AKI? (3 marks)

The picture is intrinsic (ATN), not pre-renal. The urine sodium is over 40 mmol/L (the injured tubule cannot reabsorb sodium), the FENa would be over 2 percent, the urine osmolality is isosthenuric (under 350 mOsm/kg, reflecting loss of concentrating ability), the BUN/creatinine ratio is under 20, and the sediment shows muddy brown granular casts - the pathognomonic finding of ATN. A pre-renal picture would show urine sodium under 20, FENa under 1 percent, osmolality over 500, BUN/Cr over 20, and a bland sediment.

c) Outline the four pillars of prevention of contrast-associated AKI and the single best-proven measure. (3 marks)

The four pillars are: (1) risk-stratify by eGFR and risk factors (this patient was high-risk: CKD stage 3, diabetes, intra-arterial contrast); (2) isotonic saline hydration before and after the procedure (1 mL/kg/h for 6-12 h pre and post, or 3 mL/kg/h for 1 h pre in urgent settings); (3) minimise the contrast dose using the lowest volume and an iso- or low-osmolar agent; (4) hold nephrotoxins (NSAIDs, metformin if AKI develops, aminoglycosides). The single best-proven measure is isotonic saline hydration. The PRESERVE trial (Weisbord, NEJM 2018) showed that neither N-acetylcysteine nor bicarbonate is effective.

d) What is the correct management of his metformin? Justify. (2 marks)

Withhold metformin now that AKI has developed, and do not restart until renal function returns to baseline. The rationale is that, if AKI persists, metformin accumulates and risks severe lactic acidosis (it inhibits mitochondrial complex I). Contrast does not directly interact with metformin - the link is the change in renal function. Restart only when eGFR has returned to baseline and the AKI has resolved.