CT With Contrast: Your Preparation Checklist

Here's a number that surprises almost everyone who hears it: between 2001 and 2019, the rate of CT scanning in Australia climbed from roughly 4,662 scans per 100,000 Medicare-eligible people to 14,505 per 100,000 — more than a tripling in under two decades, according to a whole-of-population analysis of Medicare data published in BMJ Open. Put another way, the CT scanner has quietly gone from a specialised hospital tool to something a huge slice of the Australian population will lie inside at some point — often at short notice, often while feeling unwell, and often with very little idea of what's about to happen.

And a large proportion of those scans involve contrast — that clear liquid injected into a vein through a cannula that lights up your blood vessels, organs, and any trouble spots on the images. If you've just been handed a referral that says "CT abdomen/pelvis with IV contrast," you may have questions that the 90-second conversation in the consult room didn't quite cover. Do I need to fast? What if I'm allergic to shellfish? I'm on metformin — do I stop it? I'm breastfeeding. My kidneys aren't great. Will it hurt?

We're NextClinic, an Australian telehealth service, and a big part of what we do every day is help people navigate exactly this kind of moment — the gap between "your doctor ordered a test" and "you actually feel ready for it." We issue online medical certificates, prescriptions, and specialist referrals for patients right across Australia, and we talk to people about imaging preparation constantly.

So this article is your practical, plain-English preparation checklist for a CT with contrast in Australia. We'll walk through what contrast actually is, exactly what your imaging clinic needs to know about your allergies, kidney health, pregnancy status and medicines, what the injection feels like in the moment (it's weirder than most people expect), and what to watch for afterwards. By the end, you'll know what questions to ask, what to bring, and when to pick up the phone.

Let's get you prepared.

First, what is "contrast" and why does your scan need it?

A CT (computed tomography) scanner is essentially a very clever X-ray machine. As InsideRadiology — a patient information service run by the Royal Australian and New Zealand College of Radiologists (RANZCR) explains, CT takes images in fine slices through a part of your body, a bit like slicing through a loaf of bread.

The problem is that on a plain CT, many soft tissues look frustratingly similar — blood, muscle, tumour, inflammation and normal organ tissue can all read as roughly the same shade of grey. Contrast medium fixes that. The type used in CT is iodinated contrast (often abbreviated ICCM, for iodine-containing contrast medium). Iodine is dense, so it blocks X-rays. When it's injected into a vein, it floods your arteries, veins and well-perfused tissues, and suddenly the radiologist can see a blood clot in a lung artery, a bleeding vessel, an abscess wall, an inflamed appendix, or a suspicious lesion that would otherwise be invisible.

You may also be given oral contrast to drink (to outline the bowel) or, less commonly, rectal contrast. Whether you need any of these depends entirely on the clinical question your doctor is trying to answer.

The short version: contrast isn't an optional extra your clinic tacked on. It's usually the difference between a scan that answers the question and a scan that doesn't.

Before you book: the Australian referral and Medicare piece

In Australia, you can't simply walk into a radiology practice and request a CT. You need a valid request (referral) from a registered medical practitioner — usually your GP, sometimes a specialist — and Medicare will only contribute towards the cost when your scan matches an item on the Medicare Benefits Schedule (MBS) and the referral is valid.

A few practical points Australians consistently get caught out by:

• Bulk billing is a clinic-by-clinic decision. Two practices can look at the identical referral and quote you two very different out-of-pocket amounts. Always ring ahead and ask, "Do you bulk bill this item, and if not, what's the gap?"
• Referrals have an expiry. Standard GP imaging requests generally remain valid for 12 months, but don't leave it to chance — if you've had a referral sitting in the glovebox for a year, check.
• Take the referral with you. No referral on the day can mean no Medicare rebate, or a rebooked appointment.
• Some items require a specialist request. Most notably, PET scans and many adult MRI items — but standard CT requests from a GP are typically fine.

If your referral has expired, gone missing in the washing machine, or you've moved interstate and your old practice is no longer practical to reach, that's the sort of thing our team can help sort out through a telehealth consultation without you needing to take half a day off work. You can browse more of our guides on navigating the Australian health system over on the NextClinic blog.

The preparation checklist: what your imaging clinic needs to know

When you book, most radiology practices will run through a safety questionnaire — either over the phone, on a form, or both. Treat this as one of the most important conversations of the whole process. Here's everything worth volunteering, even if they don't ask.

1. Allergies — and the truth about the shellfish myth

Let's clear this up first, because it's the single most persistent piece of misinformation in radiology.

A shellfish or seafood allergy does not mean you're allergic to contrast. Seafood allergy is a reaction to proteins (like tropomyosin) in the shellfish — not to iodine. Iodine is an essential element found throughout your own body, including in your thyroid hormones; you cannot be "allergic to iodine" in any meaningful sense. Major radiology bodies have been trying to bust this myth for years — the Radiological Society of North America has publicly addressed it, and reviews such as this one in the Journal of Emergency Medicine have documented how widely the belief persists among both patients and clinicians.

That said, do still mention it. Mention every allergy you have. A history of significant allergy in general — severe hay fever, food anaphylaxis, multiple drug allergies — and especially asthma may modestly increase your risk of a reaction, and your radiology team wants the full picture. InsideRadiology's guidance on CT coronary angiography specifically advises telling staff if you've had a previous reaction to a contrast agent or have a strong history of allergy to things like foods, pollens or dust.

The genuinely critical question is: have you ever reacted to contrast before? If you have, say so loudly and early, and try to bring details — which hospital, roughly when, what happened, how it was treated. Depending on the severity, the radiologist may choose a different contrast agent, arrange premedication with steroids and antihistamines, switch you to a non-contrast study or an ultrasound/MRI alternative, or scan you in a setting with more immediate support.

How likely is a reaction, honestly? The published data is reassuring. A large study in the American Journal of Roentgenology looking at more than 300,000 injections found an overall acute allergic-like reaction rate to iodinated contrast of about 0.48% — roughly one in 200, and the overwhelming majority of those are mild (itching, hives, a few sneezes). Research summarised in a six-year cohort of over 26,000 procedures puts severe reactions including anaphylaxis at approximately 0.04–0.28% of cases, with fatal reactions extremely rare at an estimated one in 170,000. Reactions also tend to happen fast — one analysis found over 90% occurred within 20 minutes of injection, which is precisely why clinics ask you to wait around afterwards.

2. Kidney health — the blood test you may need first

Your kidneys clear the contrast out of your body. If they're already struggling, that matters.

Historically, the medical world was extremely nervous about "contrast-induced nephropathy," and plenty of patients were denied useful scans as a result. The evidence has shifted considerably. RANZCR's position, referenced in NICE's surveillance documentation, is that intravascular iodinated contrast should be given to any patient regardless of renal function status if the radiologist and referrer judge the diagnostic benefit justifies it.

Still, thresholds exist for a reason. A 2024 systematic appraisal in *Insights into Imaging* found that nine of ten major international guidelines use an eGFR below 30 mL/min/1.73m² as the point at which the risk–benefit conversation should happen. The joint American College of Radiology and National Kidney Foundation consensus statements similarly identify patients with CKD stages 4 or 5 who aren't on maintenance haemodialysis as the group at potential risk.

What this means for you:

• Tell the clinic if you have chronic kidney disease, a single kidney, a kidney transplant, are on dialysis, or have had kidney problems in the past.
• Also mention diabetes, heart failure, high blood pressure, or a recent episode of severe dehydration (vomiting, diarrhoea, gastro), as these all feed into kidney risk.
• You may be asked for a recent blood test (eGFR/creatinine) before your appointment — commonly within the last three months, though timeframes vary by clinic and by how unwell you are. Don't leave this to the day before. If you need a pathology request, a quick telehealth consult can usually arrange one.
• If you're on dialysis, bring your dialysis schedule. The timing conversation is a real one.

3. Your medicines — especially metformin

Bring a complete, current list of everything you take, including over-the-counter anti-inflammatories, herbal supplements and anything you've started in the last month.

Metformin is the big one. Metformin itself doesn't damage your kidneys, but if contrast were to cause a temporary drop in kidney function while metformin keeps accumulating, there's a rare risk of lactic acidosis. The classic position, described in this review in *Radiology*, is that metformin must be withheld for 48 hours after contrast administration, during which any contrast-related decline in kidney function would become clinically apparent.

Modern practice is more nuanced and depends heavily on your kidney function. Many patients with normal eGFR are now told to simply continue as normal; those with reduced function are more likely to be asked to pause. The rule is simple: do not guess, and do not stop a prescribed diabetes medicine on the basis of a blog post — including ours. Ask the radiology clinic what their protocol is, and confirm with your GP.

Other medicines worth flagging:

• NSAIDs (ibuprofen, naproxen, diclofenac) and diuretics — sometimes paused around the scan in people with reduced kidney function.
• Beta-blockers — if you're having a CT coronary angiogram, you may be given these to slow your heart rate. InsideRadiology notes the whole CTCA process, including preparation and recovery, can take up to three or four hours, so clear your afternoon.
• Blood-thinners — usually fine for a straightforward contrast CT, but relevant if any biopsy or intervention is planned.
• Beta-blockers and adrenaline interactions — another reason the full list matters, in the unlikely event a reaction needs treating.

If you're due for a repeat script before your scan and you're running low, sorting that out in advance is one less thing to think about. Our online prescription service exists for exactly these small-but-annoying logistical moments.

4. Pregnancy, trying to conceive, and breastfeeding

Always tell them if you are, or might be, pregnant. This is never an awkward question — it's a routine safety one, and staff ask it dozens of times a day.

CT uses ionising radiation, so the conversation is genuinely about weighing benefit against risk. InsideRadiology's resource on radiation risk in pregnancy, developed with RANZCR and referencing ARPANSA safety guidance, is an excellent Australian starting point. In practical terms, when the fetus isn't in the scanned area — a head CT, a chest CT — the dose reaching it is negligible. When the abdomen and pelvis are being scanned, doses are higher, though a review in Applied Radiology notes a single-pass abdomen/pelvis CT typically delivers 25 mGy or less to the fetus, well below the threshold associated with malformations. Where appropriate, your doctor may substitute ultrasound or MRI. But if you have a suspected pulmonary embolism or major trauma, the risk of not scanning is often far greater than the risk of scanning.

Breastfeeding is more straightforward than most people assume. Only a tiny fraction of iodinated contrast passes into breast milk, and less again is absorbed by the baby's gut. The Academy of Breastfeeding Medicine's clinical protocol on radiology studies in lactating women notes that the American College of Radiology considers it safe to continue breastfeeding after intravenous iodinated contrast, and European guidance takes the same position. If you'd still prefer to pump and discard for 24 hours for peace of mind, that's your call to make — but it is not medically required by current evidence. Bring a pump if you'll be away from bub for a few hours.

5. Other conditions worth mentioning

• Thyroid disease, particularly hyperthyroidism, or a planned radioactive iodine treatment or thyroid scan — iodinated contrast can interfere with these for weeks.
• Multiple myeloma or paraproteinaemia.
• Asthma, especially if poorly controlled.
• Heart failure — relevant to fluid loading.
• Claustrophobia or anxiety. CT is a ring, not a tunnel, and you're in and out in minutes, so it's far less confronting than MRI. But if you know you'll struggle, say so — mild sedation or simply having a support person nearby can be arranged.
• Difficult veins or a history of failed cannulas. Flag it, and drink water beforehand — well-hydrated veins are easier to find.
• Recent contrast exposure. If you've had contrast for another scan in the last day or two, mention it.

6. Fasting, food and fluids

This one varies enormously by scan type, so follow your clinic's instructions, not the internet's. InsideRadiology points out that for contrast-enhanced CT, most departments will ask you to fast before the scan, and that these instructions matter — getting them wrong can affect the accuracy of the test or force a rebooking.

Typically you might be asked to avoid solid food for four to six hours beforehand. Water is usually still encouraged, and being well hydrated is generally helpful for both your kidneys and your veins. If you're diabetic and fasting, ask specifically how to manage your insulin or tablets that morning — this is not a one-size-fits-all instruction.

7. The practical day-of list

Pack and plan:

• Your referral/request form and any previous imaging (films, discs, reports) — comparison studies are genuinely valuable to the radiologist.
• Your Medicare card, and private health or DVA card if relevant.
• Your medicines list and recent blood test results.
• Comfortable clothing with no metal — no zips, studs, underwires or belts over the area being scanned. You may be asked to change into a gown regardless.
• Leave jewellery at home.
• Arrange a support person if you're anxious, need an interpreter, or have been told you may receive sedation (in which case you must not drive).
• Allow more time than you think. The scan itself may take minutes; the paperwork, cannula, oral contrast drinking time and post-injection observation period do not.

What actually happens on the day

A radiographer will insert a cannula, usually into a vein in your arm or the back of your hand. The contrast is then given through a pump injector, often partway through the scan while you're on the table.

Here's the part nobody warns you about properly. As healthdirect, the Australian government-funded health information service, describes, the contrast produces some genuinely strange sensations: a spreading warm flush through your chest, pelvis and sometimes your whole body, a metallic taste in your mouth, and — the famous one — a convincing feeling that you have just wet yourself. You haven't. Everyone has this thought. healthdirect notes these effects generally last only about 20 seconds.

You'll be asked to hold still and hold your breath for a few seconds at a time. The whole scanning portion is often under five minutes.

Afterwards, expect to be asked to wait around 15 to 30 minutes with the cannula still in place before it's removed. This isn't bureaucracy — it's because, as the research above shows, the large majority of reactions happen within the first 20 minutes, when staff and equipment are right there.

After the scan: what's normal and what isn't

Drink plenty of water for the rest of the day unless you've been told to restrict fluids. It helps flush the contrast out.

Normal and expected: a mild warm feeling that fades quickly, a slightly odd taste, a small bruise or tenderness at the cannula site, and passing more urine than usual.

Contact the clinic or your doctor if you notice:

• A rash, hives or itching — these can appear anywhere from an hour to several days after the injection. Delayed reactions are usually mild and settle with antihistamines, but get them checked.
• Significant pain, swelling, redness or blistering at the injection site, which can indicate extravasation (contrast leaking into the tissue rather than the vein). Most cases resolve with elevation and cold packs, but it needs assessing.
• A marked drop in how much you're weeing, swelling of the ankles, or unusual fatigue in the days afterwards, particularly if you have known kidney disease.
• Any breathing difficulty, facial or throat swelling, severe dizziness or widespread hives — this is an emergency. Call 000 immediately.

If you were told to pause metformin, confirm with your doctor before restarting it — often after a repeat kidney function check.

Getting your results

Your report goes to the doctor who referred you, usually within a day or two for routine studies and much faster if urgent. Book a follow-up appointment to actually discuss it, rather than trying to decode the radiologist's language from the patient portal at 11pm. Radiology reports are written doctor-to-doctor, and phrases that sound alarming are frequently routine — and occasionally the reverse.

If your regular GP is booked out for a fortnight, this is a very common reason people come to us. A short telehealth consult to go through results, arrange the next investigation, or generate a specialist referral can save a lot of waiting and worrying. And if the scan or its aftermath means you need to be off work, we can issue a medical certificate without you having to drag yourself into a waiting room to prove you're unwell.

A quick word on radiation, since everyone wonders

Yes, CT uses ionising radiation, and yes, that's precisely why your doctor orders a specific study rather than scanning everything. ARPANSA, Australia's radiation protection authority, provides solid plain-language information on what those doses actually mean, and healthdirect also directs parents to ARPANSA for information about CT in children.

The key idea is justification: a scan should be done when the information it provides outweighs the small risk. For a single clinically indicated CT in an adult, that balance is almost always comfortably in favour of scanning. The problem the Australian data highlights isn't any one scan — it's cumulative, unnecessary scanning across a population. Which is a great reason to ask your doctor a simple question: "What will this scan change about my treatment?" A good doctor will welcome it.

Recapping the essentials

If you take nothing else from this article, take these:

1. Contrast makes the scan useful. It's iodinated dye that lights up blood vessels and soft tissue so the radiologist can actually answer your doctor's question.
2. Shellfish allergy is not contrast allergy — but tell them about every allergy anyway, and especially about any previous contrast reaction.
3. Kidney function matters, and you may need a recent eGFR blood test before your appointment. Sort it out early, not the night before.
4. Bring your full medicines list, and ask specifically about metformin rather than deciding on your own.
5. Always disclose possible pregnancy. Breastfeeding, by current evidence, does not need to be interrupted.
6. Follow your clinic's fasting instructions exactly — they're the only ones that count.
7. The warm flush and the "did I just wee?" feeling are normal and last about 20 seconds.
8. Wait the full observation period, hydrate afterwards, and know the red flags for delayed reactions.
9. Check the referral and the billing before you go. Valid request, Medicare item, gap fee.

Here's your challenge for this week. Pick just one item from that list and action it. If you've got a CT booked, ring the imaging clinic today and ask the three questions that trip people up most: Do I need to fast? Do you need a recent kidney blood test? What do I do about my regular medicines? If you haven't got a scan booked, do the easier version — sit down and write out a current, accurate list of every medicine, supplement and allergy you have, and save it as a note on your phone. It will be useful at the imaging clinic, at the pharmacy, in the emergency department, and in every telehealth consult you ever have.

Then tell us how you went. Which one did you choose, and what did you learn? Drop a comment below — if the clinic told you something that surprised you, or you discovered your referral had quietly expired, someone else reading this will benefit from hearing it. And if you've had a contrast CT before, we'd love to know: did anyone warn you about the warm flush, or did you find out the hard way like the rest of us?

This article is general information only and is not a substitute for personalised medical advice. Always follow the specific instructions given to you by your referring doctor and your imaging clinic. If you experience difficulty breathing, facial or throat swelling, or severe symptoms after a contrast injection, call 000 immediately. For 24-hour health advice, you can call healthdirect on 1800 022 222 (NURSE-ON-CALL in Victoria). To speak with an Australian-registered practitioner about a referral, prescription or medical certificate, you can book a consultation with us at [NextClinic](https://nextclinic.com.au/), or browse more health explainers on our [blog](https://nextclinic.com.au/blog).

FAQs