Contrast Media Cautions

Expansion

Iodinated (CT) contrast

• Contrast associated acute kidney injury: risk rises with eGFR under 30, and with diabetes, heart failure, dehydration, myeloma, nephrotoxic drugs and repeated doses. Recent evidence suggests the risk has been substantially overestimated for intravenous contrast
• Prevention is essentially hydration, usually intravenous sodium chloride before and after. N-acetylcysteine and bicarbonate are not supported
• Metformin: withhold for 48 hours after contrast if eGFR is below 30 or there is acute kidney injury, because of the lactic acidosis risk if renal function deteriorates. No action is needed at higher eGFR
• Allergy: true anaphylaxis is rare. A previous severe reaction is the main contraindication. Shellfish allergy is irrelevant, a persistent myth
• Thyroid: the iodine load can precipitate thyrotoxicosis in nodular goitre, and it interferes with radioiodine treatment and scanning for weeks

Gadolinium (MRI) contrast

• Nephrogenic systemic fibrosis, a rare but severe scleroderma-like condition, in severe renal impairment. Group I agents are avoided below an eGFR of 30, and in dialysis patients
• Gadolinium retention in brain and bone after repeated doses, of uncertain significance
• Generally safer than iodinated contrast, with far fewer reactions

Pregnancy and breastfeeding: iodinated contrast is given when necessary, with neonatal thyroid checking; gadolinium is avoided unless essential. Breastfeeding need not be interrupted for either.

Practical checklist before contrast: recent renal function, previous reaction, metformin, thyroid disease, pregnancy, and adequate intravenous access, since extravasation of a pressure injection can cause significant soft tissue injury.