Preparing for a CT urogram involves a few straightforward steps: fasting for a couple of hours, drinking water to fill your bladder, and possibly getting a blood test to check your kidney function beforehand. The exact instructions can vary slightly between imaging centers, but the core prep is consistent. Here’s what to expect.
Fasting and Clear Liquids
Most facilities ask you to stop eating solid food about 2.5 hours before your scan. You can typically drink clear liquids up to two hours before the appointment. Clear liquids include water, black coffee or tea, apple juice, clear soda, and clear broth. The short fast helps reduce the chance of nausea from the contrast dye that gets injected during the scan.
Drinking Water and Holding Your Bladder
A CT urogram is specifically designed to image your kidneys, ureters, and bladder. To get clear images of your bladder, you’ll likely be asked to drink water before the scan and avoid urinating until the test is finished. This fills and expands the bladder so your radiologist can see its walls and interior clearly. Your imaging center will give you specific timing, but plan on arriving with a comfortably full bladder.
Kidney Function Blood Work
Because a CT urogram uses iodine-based contrast dye injected into a vein, your care team needs to confirm your kidneys can safely filter it out. You’ll need a blood test measuring your kidney filtration rate (called eGFR) taken within 30 days of the scan under current U.S. guidelines, or within 3 months under European guidelines. Patients with a filtration rate below 30 are at higher risk for contrast-related kidney injury and may need extra precautions, like IV fluids before and after the scan.
Many facilities will order this blood test at a routine appointment before your scan date, so ask when you schedule whether you need lab work done in advance.
Metformin and Other Medications
If you take metformin for diabetes, you’ll need to stop it at the time of or before the scan and keep it paused for 48 hours afterward. The combination of metformin and iodine-based contrast can, in rare cases, cause a dangerous buildup of lactic acid. Your doctor will tell you when it’s safe to restart the medication, usually after confirming your kidneys are functioning normally post-scan.
For most other medications, you can take them as usual. But always tell the scheduling team about everything you’re taking, including supplements, so they can flag anything that needs to be adjusted.
If You Have a Contrast Allergy
If you’ve had a reaction to iodine-based contrast dye in the past (hives, swelling, difficulty breathing), let your doctor know well before scan day. You’ll need a premedication regimen that starts the day before the procedure. The standard outpatient protocol involves taking a steroid at three separate times: 13 hours, 7 hours, and 1 hour before the contrast injection. You’ll also take an antihistamine within an hour of the injection.
This schedule means you need advance planning. If your scan is at 10 a.m., your first steroid dose would be at 9 p.m. the night before. Missing a dose can mean rescheduling the entire procedure, so set alarms or reminders. For hospital inpatients or emergency situations, a shorter IV-based steroid protocol starting 4 hours before the scan is used instead.
Clothing and Metal
You’ll change into a hospital gown for the scan. Remove all jewelry, piercings, and any metal accessories in the area being scanned (abdomen and pelvis). Metal creates bright streaks on CT images that can obscure the exact structures your radiologist needs to see. Leaving valuables at home simplifies this step.
What Happens During the Scan
The scan itself is quick and painless. You’ll lie on a narrow table that slides into a large, doughnut-shaped scanner. A technologist will insert a small IV line, usually in your arm, and inject the contrast dye. You may feel a warm, flushing sensation or a brief metallic taste in your mouth as the dye enters your bloodstream. This is normal and passes within seconds.
The scanner takes images in multiple phases. First, it captures your urinary tract without contrast, then again as the dye flows through your kidneys, and finally a delayed set of images after the dye has traveled down through your ureters into your bladder. This last phase is why a full bladder matters. Between passes, you’ll wait on the table for several minutes while the contrast makes its way through your system.
After the Scan
Once the scan is done, you can eat and drink normally right away. In fact, drinking plenty of water afterward helps your kidneys flush out the contrast dye more quickly. Most people feel completely fine and can drive themselves home and return to their usual activities immediately.
If you have reduced kidney function, your care team may keep you for a short period to give you IV fluids before and after the contrast injection. This extra hydration protects the kidneys. For patients with moderate kidney impairment, a common approach involves receiving about 250 mL of saline before imaging and another 250 mL afterward, each infused over about 30 minutes.