Reactions to iodinated contrast dye used in CT scans

A plain CT scan shows bone and dense tissues well but is limited at distinguishing blood vessels, tumours, inflammation and bowel from surrounding tissue. Iodine absorbs X-rays strongly, so an iodine-based contrast medium outlines the structures it reaches. The NHS states that it can be given in a drink, injected into a vein or put in via the back passage, depending on what part of the body is being examined.

Not every scan needs it, and the radiology team decides whether the added detail justifies the small risk. If a scan is being done for a specific question, such as looking for a blood clot in the lungs or assessing a liver lesion, the information from contrast is often essential.

When the dye is injected, most people feel a wave of warmth that spreads through the body, a metallic taste in the mouth and a sudden feeling of needing to pass urine, though the bladder is not actually full. The NHS says these feelings usually pass quickly. Telling the radiographer that you feel hot is unlikely to cause alarm; they hear it many times a day.

It is worth knowing about these sensations beforehand, because anxiety about them can be mistaken for a reaction, and a calm, prepared patient is easier to scan.

The NHS MRI page describes serious allergic responses as uncommon and says most reactions occur soon after injection or within the first hour. That timing is the reason for the observation period at the end of the scan.

Radiology departments ask a standard set of questions before injecting contrast. A previous reaction to the same type of dye is the strongest predictor of another. Asthma, other allergies and some medicines such as beta blockers may be asked about because they can make reactions harder to treat. Anxiety and a history of fainting do not indicate allergy but staff like to know.

The NHS advises telling staff if you have kidney or thyroid problems and if you are pregnant, might be pregnant or are breastfeeding. Reduced kidney function does not usually stop contrast being used when it is needed, but the team may check recent blood results, arrange extra fluids and review medicines that affect the kidneys. Our guides to contrast dye nephropathy and to metformin with contrast explain that issue further.

The NHS CT page says people who receive contrast wait 30 minutes in the department so that staff can respond if a reaction begins, and asks you to tell staff about any discomfort during or after the scan. Departments have emergency equipment and trained staff and treat serious reactions with adrenaline and other emergency measures.

Once home, drinking normally helps the kidneys clear the dye. Mild skin itching or a rash that appears later usually settles by itself and can be discussed with a pharmacist or GP, although a hives-type rash with swelling or breathing trouble needs emergency care.

• Stay for the full observation period, even if you feel well.

• Report a new rash, itching, cough, wheeze or swelling at once.

• Keep drinking fluids unless you have been told to restrict them.

• Tell your GP if you notice reduced urine output or swelling in the days after.

Bring any record of the reaction, including the name of the contrast agent and what happened. The radiologist may suggest a different contrast agent, a non-contrast or alternative scan such as ultrasound or MRI, or, in selected cases, medicines taken in advance, although premedication is no longer used routinely. If a reaction was severe, an allergy referral can help to characterise it and to decide what is safe in future. Having a clear record in your notes is useful for every future scan.

During the scan, speak up if you feel a tight throat, wheeze, faintness or widespread itching; staff are beside you. After you have gone home, call 999 for swelling of the lips, mouth, throat or tongue, difficulty breathing or swallowing, collapse or severe confusion, which are the NHS signs of a serious allergic reaction.