Start with the diagnostic question: will contrast-enhanced CT change urgent treatment? In suspected dissection, bowel ischemia or major bleeding, compare the harm of diagnostic delay with kidney risk. If the scan is necessary and no adequate alternative exists, proceed with the treating and radiology teams while resuscitation continues. These decisions concern adult IV iodinated contrast; arterial procedures need separate assessment. CAR emergency-imaging guidance.

Definitions & Terminology

Contrast-associated AKI (CA-AKI) describes AKI occurring within 48 hours of contrast exposure, without proving causation. Contrast-induced AKI (CI-AKI) is injury actually caused by contrast. A creatinine rise after CT cannot distinguish them. Shock, sepsis, congestion, obstruction and medications remain competing explanations. Consensus definitions.

Renal vasoconstriction and tubular toxicity are proposed mechanisms; their clinical contribution is uncertain. At the bedside, assess perfusion, congestion and urine output rather than treating the contrast exposure as the diagnosis. ACR Manual 2026.

AKI Diagnostic Criteria (KDIGO)

• Creatinine increase ≥0.3 mg/dL (26.5 µmol/L) within 48 hours.
• Creatinine ≥1.5 times baseline, with the increase known or presumed within the preceding seven days.
• Urine output <0.5 mL/kg/hour for at least six hours.

Any one criterion establishes AKI. Review previous results and the trajectory; creatinine-based eGFR is unreliable during evolving AKI. Investigate and treat reversible causes. KDIGO AKI guideline.

Clinical Evidence: How Risky Is Contrast, Really?

Controlled observational studies support low attributable risk in stable kidney function, but disagree in severe CKD: Davenport 2013 found an association below eGFR 30, whereas McDonald 2014 did not. Propensity matching reduces measured confounding; it cannot establish zero risk. The 2013 systematic review is useful background, not proof that contrast never causes injury.

• Stable eGFR ≥45: estimated CI-AKI risk is near zero.
• Stable eGFR 30–44: estimated risk is approximately 0–2%.
• eGFR <30 or AKI: greater uncertainty; discuss benefit, alternatives and precautions.

These ACR–NKF estimates describe attributable risk, not all AKI after scanning. Do not apply stable-CKD estimates to evolving AKI. All eGFR values here use mL/min/1.73 m².

When to Check Renal Function Before Contrast

Review kidney disease, recent AKI, renal surgery, albuminuria and metformin use. Check current creatinine when relevant and feasible. A reassuring isolated number does not exclude early AKI. ACR screening guidance.

The screening extract above is background for routine studies. It does not set an emergency waiting period. Metformin screening concerns drug accumulation if kidney function deteriorates. ACR screening and metformin chapters.

 Emergency Imaging: When Contrast Should Not Be Delayed

Obtain eGFR before emergency imaging only when waiting will not harm the patient. If unavailable, use feasible high-risk precautions. Avoid a subdiagnostic contrast dose: agree the protocol that answers the question. ESUR 2025 guidance.

Arterial contrast is different: first-pass renal exposure delivers relatively undiluted contrast to the kidneys, and catheterization adds embolic risk. ESUR uses eGFR <45 for first-pass arterial exposure or ICU patients, versus <30 for IV or second-pass arterial exposure. These thresholds prompt precautions, not automatic refusal. ESUR route-specific guidance.

Prevention Strategies for High-Risk Patients

Under ACR–NKF guidance, give IV saline prophylaxis for AKI or eGFR <30 when the patient is not on maintenance dialysis and fluids are appropriate. Consider it individually at stable eGFR 30–44 with additional high-risk circumstances. Routine prophylaxis is unnecessary for stable eGFR ≥30. Review nonessential nephrotoxins, particularly NSAIDs, in high-risk patients. ACR–NKF recommendations.

Hydration needs a prescription and reassessment. ESUR lists 0.9% saline 1 mL/kg/hour for 3–4 hours before and 4–6 hours after IV contrast. Individualize for severe heart failure or advanced renal failure. Emergency circumstances may preclude this regimen. Do not force fluids into a congested patient or delay a necessary scan to complete it. ESUR hydration guidance.

AMACING found no prophylactic hydration noninferior in elective patients with eGFR 30–59; 5.5% of hydrated participants had hydration-related complications. It excluded eGFR <30 and does not settle prophylaxis in AKI or unstable emergencies. AMACING trial.

PRESERVE found no advantage of bicarbonate over saline, or acetylcysteine over placebo, in high-risk angiography patients. Bicarbonate is an alternative fluid, not a superior antidote. PRESERVE trial. Do not add mannitol or furosemide solely for kidney prophylaxis. ACR preventive measures.

Metformin Management

ACR: continue metformin for routine IV contrast when there is no AKI and eGFR ≥30. With AKI, eGFR <30, or arterial catheter studies that might embolize renal arteries, withhold at/before exposure and for 48 hours; reassess before restarting. ACR metformin chapter.

FDA labeling is stricter: withhold at/before iodinated contrast for eGFR 30–60, a history of liver disease, alcoholism or heart failure, or any intra-arterial administration. Recheck eGFR after 48 hours; restart only if stable and otherwise appropriate. Metformin remains contraindicated below eGFR 30. The concern is accumulation and lactic acidosis if renal clearance falls. Follow the governing product label and local pharmacy/radiology policy. FDA GLUMETZA label, August 2026, sections 2.2–2.3.

Key ED Takeaways

• Before: establish urgency, kidney trajectory and volume status.
• During: obtain a diagnostic study and continue appropriate resuscitation.
• After: for at-risk patients, arrange renal reassessment around 48 hours, with earlier clinical review for oliguria or deterioration. ESUR follow-up guidance.

Do not start dialysis or change its schedule solely to remove iodinated contrast. Patients retaining urine output need individualized discussion about residual function. ACR dialysis guidance.

Final Word for ED Teams

Make the imaging decision and the kidney-care plan together. Document why the scan is needed, what precautions fit this patient, and who will review kidney function and withheld medication.