Contrast Safety for Referrers: Renal Function, Metformin and Allergy

Contrast agents - iodinated contrast for CT and gadolinium-based agents for MRI - are safe for the great majority of patients. The small groups at genuine risk are identifiable in advance, and the referring GP is usually the person best placed to identify them.

A referral that documents renal function, allergy history, metformin use and pregnancy status allows the radiology practice to plan the correct protocol from the outset. A referral that omits them triggers callbacks, day-of-scan blood tests, rescheduled appointments, or - in the worst case - a preventable adverse event.

The radiologist always retains discretion over whether contrast is used; your role is not to make that decision but to supply the information it depends on. This guide summarises the main considerations at a general level - clinics follow RANZCR contrast guidelines, which are the authoritative reference and are updated periodically.

The principal renal concern with iodinated contrast is contrast-associated acute kidney injury. Current evidence suggests the risk from modern intravenous contrast is lower than historically believed, but it remains a genuine consideration in patients with significantly impaired renal function.

Practical points for referrers:

• Include a recent eGFR (with the date) on any referral where contrast is likely - contrast CT of the chest, abdomen or pelvis, and CT angiography in particular • Patients with a stable, normal eGFR and no risk factors generally need no special preparation • In patients with moderate to severe renal impairment, the radiology practice may use a reduced dose, arrange pre-hydration, choose a non-contrast protocol, or discuss an alternative modality • Acute kidney injury, dehydration and haemodynamic instability raise the risk more than stable chronic impairment

How recent the eGFR needs to be depends on the patient's stability - clinics will advise, but a result within the past few months is a reasonable default for stable outpatients.

Metformin does not damage kidneys and does not interact with contrast directly. The concern is indirect: if contrast were to cause significant renal impairment, continued metformin in that setting could contribute to lactic acidosis - a rare but serious event.

Contemporary practice, reflected in RANZCR guidance, stratifies by renal function rather than applying a blanket rule. In patients with normal or mildly reduced renal function receiving standard intravenous contrast, metformin can generally be continued. In patients with more significant renal impairment, or where large contrast volumes are anticipated, the usual advice is to withhold metformin at the time of the scan and for around 48 hours afterwards, restarting once renal function is confirmed stable.

What matters on your referral is simply documenting that the patient takes metformin and providing the eGFR - the clinic will then apply the current protocol and give the patient specific instructions. Avoid giving patients firm withholding instructions that may contradict the clinic's advice.

A history of prior contrast reaction is the strongest predictor of a future one, so this history belongs prominently on the referral. Useful documentation includes:

• What agent was involved (iodinated contrast or gadolinium - a reaction to one does not imply risk with the other) • The nature of the reaction - urticaria and itch versus bronchospasm, hypotension or anaphylaxis • When it occurred and how it was managed

For patients with a previous mild reaction, options include using a different contrast agent, corticosteroid and antihistamine premedication protocols, or choosing a non-contrast or alternative study. For previous severe reactions, contrast is generally avoided unless the indication is compelling and appropriate precautions are in place.

Two common misconceptions worth correcting with patients: shellfish allergy is not a specific contraindication to iodinated contrast, and topical iodine (antiseptic) sensitivity is unrelated. Genuine atopy and asthma modestly raise reaction risk but rarely change the plan on their own.

Gadolinium-based contrast agents used in MRI carry a different renal consideration: nephrogenic systemic fibrosis (NSF), a rare but serious fibrosing condition described almost exclusively in patients with severe renal impairment - particularly those on dialysis or with an eGFR below about 30 - who received certain older, less stable gadolinium agents.

Since the shift to more stable (macrocyclic) agents and routine renal screening, new NSF cases have become exceedingly rare. Nonetheless, clinics continue to screen, and in severe renal impairment the radiologist will weigh the necessity of gadolinium, use the most stable agent at the lowest effective dose, or proceed without contrast.

For the referrer, the actions are the same as for CT: include a recent eGFR on MRI referrals where contrast is plausible, and flag dialysis or known chronic kidney disease explicitly. There is also ongoing research into gadolinium retention in tissues in patients with normal renal function; no clinical harm has been established, but it reinforces the general principle of using contrast only when it adds diagnostic value.

Before sending any referral where contrast may be used, a quick mental checklist:

• Recent eGFR documented, with date • Metformin (and other relevant medicines) listed • Prior contrast reactions described - agent, severity, management • Asthma and significant atopy noted • Pregnancy status or possibility documented for patients who could be pregnant • Dialysis or known chronic kidney disease flagged • The clinical question stated clearly enough for the radiologist to judge whether contrast is needed at all

Even a line reading "no known contrast allergy, eGFR normal (date), not on metformin" saves the clinic a phone call and the patient a delay.

This article is general professional information rather than clinical advice - the RANZCR contrast guidelines and your local radiology practice's protocols are the current authoritative references, and clinics are generally happy to discuss individual patients before you refer.