Quick answer: Contrast dye leaves your body through your kidneys, so before some CT or MRI scans with contrast, your imaging center may ask for a recent blood test of kidney function (creatinine and eGFR). It’s mainly needed if you have a history of kidney disease, and some centers also check people with diabetes, high blood pressure or older age. The test helps the team pick the safest approach. For most people, the result is normal and the scan goes ahead as planned; even if it’s low, a needed scan can usually still be done safely.
Why kidneys matter for contrast
After a CT or MRI with contrast, your kidneys filter the contrast out of your blood, and you pass it in your urine, usually within a day if your kidneys are healthy.
- CT (iodine) contrast: In people whose kidneys already work poorly, contrast can occasionally cause a short-term drop in kidney function. The risk is lower than doctors once thought, and it matters mainly for people with severe kidney disease or a recent kidney injury.
- MRI (gadolinium) contrast: In people with severe kidney disease, some older agents were linked to a rare condition called nephrogenic systemic fibrosis. With the newer agents most centers use, the risk is extremely low.
What the blood test measures
- Creatinine: a waste product your kidneys clear from the blood. Higher levels can mean the kidneys aren’t filtering as well.
- eGFR (estimated glomerular filtration rate): a calculation from your creatinine, age and sex that estimates how well your kidneys filter. A lower eGFR means lower kidney function.
Your imaging team looks at the eGFR. Guidance from the American College of Radiology and the National Kidney Foundation says the people at real risk are mainly those with a very low eGFR or a sudden kidney injury.
Who needs a test before contrast?
According to the 2020 ACR–NKF guidance, testing is mainly recommended for people with a history of kidney disease, such as:
- Chronic kidney disease or dialysis
- A kidney transplant
- Kidney surgery or kidney cancer
- A recent kidney injury (for example, during a hospital stay)
Screening people with diabetes is considered optional. In practice, policies vary: many centers also test people over a certain age, people with high blood pressure or diabetes, or anyone with an unclear history. Your center will tell you if you need a test.
Most people without these risk factors don’t need a blood test before contrast.
How recent does the test need to be?
It depends on your health and the center’s policy. For stable outpatients, many centers accept a result from the past few weeks. If you’re in the hospital, recently sick or your kidney function has been changing, they’ll want a more recent result. Some centers can do a quick finger-stick test on the day.
Tip: If you’ve had blood work recently, bring the results or make sure the imaging center can see them. It can save you an extra test and a delay.
What happens if my kidney function is low?
A low result doesn’t automatically cancel your scan. Your care team may:
- Go ahead if the scan is needed and there’s no good alternative. The ACR–NKF guidance says a needed contrast CT shouldn’t be withheld only because of kidney risk.
- Give IV fluids before and after the scan to help protect your kidneys, if you’re at high risk.
- Use the lowest amount of contrast that gives a good result.
- Choose another test, such as a scan without contrast, ultrasound or MRI without contrast.
- Review your medicines with your doctor.
Diabetes medicines and contrast
If you take medicine for diabetes, especially metformin, your doctor or imaging center may give you specific instructions, usually only if your kidney function is low. Don’t stop or change any medicine on your own. Follow the instructions you’re given.
If you’re on dialysis
People on dialysis can usually have contrast scans. Your care team will coordinate the timing with your dialysis schedule if needed. Tell the imaging center that you’re on dialysis when you book.
From the scanner room
[TU EXPERIENCIA: cómo funciona en tu servicio: quién necesita creatinina antes del contraste, cuánto tiempo vale el análisis, si hacéis pruebas rápidas, qué pasa con los pacientes en diálisis; errores frecuentes (pacientes que llegan sin análisis).]
FAQ
Why do I need a blood test before a CT scan with contrast?
To check your kidney function, because your kidneys clear the contrast. It’s mainly needed if you have a history of kidney disease, but some centers test more people.
Do I need a blood test before an MRI with contrast?
Sometimes, especially if you have kidney disease. Your MRI center will tell you.
What eGFR is too low for contrast?
The risk is mainly for people with severe kidney disease. Your radiologist decides based on your result, your health and how much you need the scan.
Can I drink water to protect my kidneys?
Staying normally hydrated helps, unless your doctor has told you to limit fluids. High-risk patients may be given IV fluids by the care team.
Can contrast damage healthy kidneys?
In people with normal kidney function, the risk is very low.
This article is for general information only and is not medical advice. Follow your doctor’s and imaging center’s instructions about blood tests and medicines before a contrast scan.
Sources
- ACR–NKF Consensus Statement on Use of Intravenous Iodinated Contrast Media in Patients With Kidney Disease (Radiology, 2020).
- ACR–NKF Consensus Statement on Use of Intravenous Gadolinium-Based Contrast Media in Patients With Kidney Disease (Radiology, 2021).
- ACR Manual on Contrast Media.
- Radiology Business. Patient-reported risk factors increase unnecessary testing before contrast-enhanced CT (AJR study).
- National Kidney Foundation. eGFR explained.