Switching to a different contrast agent, chosen by allergy testing, instead of steroid premedication after a reaction

The evidence. McDonald and colleagues reviewed 1,973 people with a previous allergic-like reaction who had 4,360 later contrast CT scans at one United States centre between 2009 and 2017. Repeat reactions occurred in 80 of 423 examinations (19 per cent) with the same contrast agent and steroid premedication, and in 10 of 322 (3 per cent) with a different agent and no steroids (odds ratio 0.14, 95 per cent confidence interval 0.06 to 0.33). At the first repeat scan, people given the same agent reacted at the same rate with or without steroids (26 per cent against 25 per cent). It is a retrospective single-centre cohort, not a trial.

The guideline. The ESUR Contrast Media Safety Committee (European Radiology 2025) recommends referring at least everyone with a moderate or severe reaction to a drug allergy specialist and choosing an alternative agent from the allergy evaluation, which it calls safer and more effective than premedication. It cites an American allergology guideline finding evidence lacking for corticosteroid premedication, and notes that steroids can raise blood glucose. The ACR Manual on Contrast Media, the main source of North American premedication regimens, could not be read for this record, so the North American position is named but not quoted.

The open questions. There is no randomised comparison of switching against premedication, allergy services able to test contrast agents are scarce in many places, and how often departments switch rather than premedicate has not been measured.