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Many cancer scans use a dye containing iodine. If your kidneys work normally the dye is very unlikely to harm them, and you do not need to stop eating or drinking beforehand. If your kidneys are known to be weak, the team checks a recent blood test, may give you fluid through a drip around the scan, and may ask you to pause metformin for two days.
Risk is judged from the estimated glomerular filtration rate (eGFR). For intravenous contrast the threshold for extra care is an eGFR below 30 ml/min/1.73 m², or acute kidney injury (ESUR 2026, ACR/NKF 2020, NICE NG148 2024); intra-arterial injection with first-pass renal exposure lowers the ESUR threshold to 45. ESUR measures eGFR within 7 days for inpatients or the acutely ill and within 3 months otherwise; NICE uses an eGFR from the past 6 months, or screening questions when there is none, and proceeds without a blood test if the answers are negative and the person is stable. Emergency imaging is not delayed for kidney function (NICE 1.1.7 [2024]; ACR/NKF).
Prevention in at-risk patients is intravenous volume expansion with 0.9% saline or isotonic bicarbonate, which ESUR finds equally effective (Level of evidence A) and recommends for at-risk patients (Level B). ESUR gives regimens (saline 1 ml/kg/h for 3 to 4 hours before and 4 to 6 hours after, or bicarbonate 3 ml/kg/h for 1 hour before; Level D) and does not accept oral fluids alone (Level D); NICE encourages oral hydration for those at increased risk and considers IV fluids for inpatients at particularly high risk. ACR/NKF prefer saline. N-acetylcysteine, statins, vitamin C and prophylactic dialysis are not recommended (ESUR Level A or B). Use low- or iso-osmolar contrast at the lowest diagnostic dose and recheck eGFR at 48 hours in at-risk patients.
The guidelines differ on ACE inhibitors and angiotensin receptor blockers: ESUR says they need not be stopped (Level B); NICE says consider stopping them temporarily when eGFR is below 30 [2013, amended 2024]. ESUR found insufficient evidence to withhold nephrotoxic chemotherapy before contrast (Level C). Metformin continues unless eGFR is below 30, there is acute kidney injury, or the injection is intra-arterial with first-pass renal exposure; then it stops at the time of contrast and restarts after an eGFR check at 48 hours (ESUR Level D; ACR/NKF agrees at eGFR below 30). People with myeloma and normal kidney function are not at higher risk if well hydrated; Bence Jones testing beforehand is not recommended (ESUR 2026). Fasting before low- or iso-osmolar contrast is not recommended (ESUR 2026).
Why: Weak kidneys are the main thing that raises the small risk of kidney injury after contrast.
“Before requesting a non-urgent iodine-based contrast media CT scan, assess whether the person has pre-existing kidney disease.”
NICE 2024 · [2024]
Why: Most people do not need a fresh blood test; the questions find those who do.
“If available, use an eGFR measurement from the past 6 months to support decision making about the use of iodine-based contrast media. If the person has been acutely unwell or clinically unstable since their last eGFR test, consider using a more recent eGFR.”
NICE 2024 · [2024]
Why: When a scan is urgent, missing a diagnosis is a bigger danger than the small risk to the kidneys.
“Do not delay the use of iodine-based contrast media in an emergency if the risk of delaying the contrast media is likely to be clinically significant.”
NICE 2024 · [2024]
Why: For someone with weak kidneys there may be another way to get the answer, and the choice is theirs to share.
“Discuss the benefits and risks of tests or treatments that use iodine-based contrast media with the person, and their family members and carers if appropriate.”
NICE 2024 · [2024]
Why: Fluid through a drip before and after the scan is the one measure shown to lower the risk.
“Preventive hydration should be used to reduce the incidence of PC-AKI in at-risk patients.”
ESUR Contrast Media Safety Committee (van der Molen et al., European Radiology) 2018 · Level of evidence B
Why: Both fluids work about equally well; too much fluid can be dangerous for a weak heart.
“Intravenous saline and bicarbonate protocols have similar efficacy for hydration.”
ESUR Contrast Media Safety Committee (van der Molen et al., European Radiology) 2018 · Level of evidence A
Why: Drinking is sensible but has not been shown to be enough on its own for people at real risk.
“Oral hydration as the sole means of prevention is not recommended.”
ESUR Contrast Media Safety Committee (van der Molen et al., European Radiology) 2018 · Level of evidence D
Why: None of them has been shown to protect the kidneys consistently.
“No pharmacological prophylaxis (with statins, renal vasodilators, receptor antagonists of endogenous vasoactive mediators or cytoprotective drugs, etc.) has been shown to offer consistent protection against CA-AKI.”
European Society of Urogenital Radiology (ESUR) Contrast Media Safety Committee 2026
Why: Less contrast, and contrast closer to the body's own concentration, means less strain on the kidneys.
“Use the lowest dose of contrast medium consistent with a diagnostic result.”
European Society of Urogenital Radiology (ESUR) Contrast Media Safety Committee 2026
Why: There is not enough evidence that stopping them for a scan helps, and stopping cancer treatment has its own cost.
“There is insufficient evidence to recommend withholding nephrotoxic drugs such as NSAIDs, antimicrobial agents or chemotherapeutic agents before CM administration.”
ESUR Contrast Media Safety Committee (van der Molen et al., European Radiology) 2018 · Level of evidence C
Why: Metformin does not harm the kidneys, but if the kidneys are injured it can build up and cause a rare dangerous acid build-up in the blood.
“Stop taking metformin from the time of contrast medium administration. Measure eGFR within 48 hours and restart metformin if renal function has not changed significantly.”
European Society of Urogenital Radiology (ESUR) Contrast Media Safety Committee 2026
Why: Myeloma often affects the kidneys and raises calcium, but with normal kidneys and good hydration the risk is not higher.
“Multiple myeloma patients with normal renal function are not at increased risk of CA-AKI if they are well hydrated and low- or iso-osmolar iodine-based contrast media are used.”
European Society of Urogenital Radiology (ESUR) Contrast Media Safety Committee 2026
Why: Their kidney care is specialised, but dialysis timing does not need to be changed for the dye.
“Discuss the person's care with a nephrology team before offering iodine-based contrast media to adults on renal replacement therapy, including people with a renal transplant, but do not delay emergency imaging for this.”
NICE 2024 · [2019]
Why: Fasting dates from older dyes that made people sick; it is not needed now and can leave people dehydrated.
“Fasting is not recommended before administration of low- or iso-osmolar non-ionic iodine-based contrast media, gadolinium-based agents or ultrasound contrast agents.”
European Society of Urogenital Radiology (ESUR) Contrast Media Safety Committee 2026
Why: Kidney injury from contrast shows up in blood tests over two to three days and needs following until it settles.
“Determine eGFR 48 hours after contrast medium administration.”
European Society of Urogenital Radiology (ESUR) Contrast Media Safety Committee 2026
How care is usually carried out, read from the guidelines and standards cited. Your own team's protocol and their judgement come first; this page is for understanding it, not for doing it yourself.
Judge risk from eGFR, with extra care for intravenous contrast only when eGFR is below 30 ml/min/1.73 m² or there is acute kidney injury (ESUR 2026, ACR/NKF 2020, NICE NG148 2024 all draw the line at 30); never delay emergency imaging for a kidney result (NICE [2024]; ACR/NKF: contrast for a life-threatening diagnosis should not be withheld on kidney function). For those at risk, intravenous 0.9% saline or isotonic bicarbonate around the scan (ESUR Level A for equivalence, Level B for use; ACR/NKF prefer saline; NICE considers it for inpatients at particularly high risk), no drug prophylaxis, the lowest diagnostic dose of low- or iso-osmolar contrast, and an eGFR at 48 hours. Metformin is stopped only below eGFR 30, in acute kidney injury or with first-pass intra-arterial contrast (ESUR Level D; ACR/NKF agree at eGFR below 30). The guidelines differ on when an eGFR must have been taken (ESUR: 7 days for inpatients or the acutely ill, 3 months otherwise; NICE: 6 months, or screening questions and no test if they are negative), on oral fluids (NICE encourages them; ESUR rejects them as the only measure) and on ACE inhibitors and ARBs (ESUR: need not be stopped, Level B; NICE: consider stopping when eGFR is below 30).
European Society of Urogenital Radiology (ESUR) Contrast Media Safety Committee 2026 · ESUR Contrast Media Safety Committee (van der Molen et al., European Radiology) 2018 · American College of Radiology and National Kidney Foundation (Davenport et al., Kidney Medicine; also published in Radiology, 10.1148/radiol.2019192094) 2020 · NICE 2024
Where it differs: Uses an eGFR from the past 6 months; if there is none, asks whether the person has kidney disease or a transplant, sees a kidney specialist or urologist, or has an acute illness, and proceeds without a blood test if all answers are no and the person is stable (NG148 1.1.9 to 1.1.12, 2024). ESUR offers measuring eGFR in all patients or only in those with a listed history, within 7 days for inpatients and the acutely ill and within 3 months for others.
Evidence: NICE's 2024 update rests on its evidence review of eGFR and risk tools for predicting contrast-associated kidney injury (not read here). ESUR's timing is a consensus recommendation.
NICE 2024 · European Society of Urogenital Radiology (ESUR) Contrast Media Safety Committee 2026
Where it differs: NICE recommends considering temporarily stopping ACE inhibitors and angiotensin receptor blockers in adults with chronic kidney disease and an eGFR below 30 having iodine-based contrast (1.2.9, [2013, amended 2024]). ESUR says they do not have to be stopped.
Evidence: ESUR: meta-analyses of trials found no lower risk; withholding them may be associated with a slightly lower risk, but not strongly enough to recommend it (Level of evidence B).
NICE 2024 · ESUR Contrast Media Safety Committee (van der Molen et al., European Radiology) 2018
Where it differs: NICE encourages oral hydration before and after intravenous contrast in adults at increased risk (1.2.7 [2019]) and reserves intravenous volume expansion for inpatients at particularly high risk (eGFR below 30, a kidney transplant, a large contrast volume, first-pass intra-arterial injection). ESUR supports unrestricted oral fluids only alongside intravenous volume expansion.
Evidence: ESUR graded oral hydration as the only preventive measure as not recommended (Level of evidence D).
NICE 2024 · ESUR Contrast Media Safety Committee (van der Molen et al., European Radiology) 2018
Where it differs: The US Food and Drug Administration label advice, as reported by ACR/NKF, withholds metformin before iodinated contrast at eGFR 30 to 59 as well as below 30. ACR/NKF say decisions at 30 to 59 should be individualised by the referring clinician; ESUR continues metformin normally above 30 and notes its advice may differ from EMA and FDA recommendations.
Evidence: ACR/NKF: metformin does not raise the risk of contrast-associated kidney injury, and the risk of contrast-induced injury at eGFR 30 to 59 is low. The FDA text was not read directly.
American College of Radiology and National Kidney Foundation (Davenport et al., Kidney Medicine; also published in Radiology, 10.1148/radiol.2019192094) 2020 · European Society of Urogenital Radiology (ESUR) Contrast Media Safety Committee 2026 · ESUR Contrast Media Safety Committee (van der Molen et al., European Radiology) 2018
Where it differs: The EANM FDG PET/CT procedure guideline (2015) stops metformin at the time of any intravenous contrast and withholds it for 48 hours, restarting without a kidney test when nephrotoxicity risk is low and only after a normal result when it is high. ESUR, ACR/NKF and the later ESUR handbook stop it only below eGFR 30, in acute kidney injury or with first-pass intra-arterial contrast.
Evidence: EANM states it as procedure advice without a grade; it predates the 2018 ESUR and 2020 ACR/NKF revisions, which rest on metformin not raising the risk of kidney injury.
European Association of Nuclear Medicine (Boellaard et al., European Journal of Nuclear Medicine and Molecular Imaging) 2015 · European Society of Urogenital Radiology (ESUR) Contrast Media Safety Committee 2026 · American College of Radiology and National Kidney Foundation (Davenport et al., Kidney Medicine; also published in Radiology, 10.1148/radiol.2019192094) 2020
Many hospitals still ask for a kidney blood test within three months before a CT scan with contrast dye, and cancel scans when it is missing. Since 2024 NICE has said a test from the past six months will do, and that when there is none, three questions about kidney disease and acute illness can decide whether a test is needed. How safely and how widely this works is not measured.
Being told not to eat for hours before a CT scan with contrast dye dates from older dyes that made people sick. Randomised trials, including one of more than 3,000 outpatients at a cancer centre, found no more sickness when people ate, and European radiologists recommend against fasting. Some hospitals still ask for it.
| Recommendation | Grade, as printed | Source |
|---|---|---|
| Preventive hydration in at-risk patients | Level of evidence B | ESUR Contrast Media Safety Committee (van der Molen et al., European Radiology) 2018 |
| Intravenous saline and bicarbonate are equally effective | Level of evidence A | ESUR Contrast Media Safety Committee (van der Molen et al., European Radiology) 2018 |
| Oral hydration alone is not recommended | Level of evidence D | ESUR Contrast Media Safety Committee (van der Molen et al., European Radiology) 2018 |
| ACE inhibitors and ARBs need not be stopped “ACE inhibitors and angiotensin receptor blockers do not have to be stopped before CM administration.” | Level of evidence B | ESUR Contrast Media Safety Committee (van der Molen et al., European Radiology) 2018 |
| Insufficient evidence to withhold nephrotoxic drugs, including chemotherapy, before contrast | Level of evidence C | ESUR Contrast Media Safety Committee (van der Molen et al., European Radiology) 2018 |
| Stop metformin at contrast when eGFR is below 30, in acute kidney injury or with first-pass intra-arterial contrast; restart after eGFR at 48 hours | Level of evidence D | ESUR Contrast Media Safety Committee (van der Molen et al., European Radiology) 2018 |
| Prophylaxis is indicated in acute kidney injury or eGFR below 30 (not on maintenance dialysis), may be considered at 30 to 44 in high-risk circumstances, and when given is isotonic normal saline “When prophylaxis is indicated, isotonic volume expansion with normal saline is the preferred method.” | Consensus statement (ungraded) | American College of Radiology and National Kidney Foundation (Davenport et al., Kidney Medicine; also published in Radiology, 10.1148/radiol.2019192094) 2020 |
| Contrast needed for a life-threatening diagnosis should not be withheld on kidney function “If contrast media administration is required for a life-threatening diagnosis, then it should not be withheld based on kidney function.” | Consensus statement (ungraded) | American College of Radiology and National Kidney Foundation (Davenport et al., Kidney Medicine; also published in Radiology, 10.1148/radiol.2019192094) 2020 |
| Metformin does not increase the risk of contrast-associated kidney injury “Metformin does not increase risk of CA-AKI.” | Consensus statement (ungraded) | American College of Radiology and National Kidney Foundation (Davenport et al., Kidney Medicine; also published in Radiology, 10.1148/radiol.2019192094) 2020 |
| Encourage oral hydration before and after intravenous contrast in adults at increased risk “Encourage oral hydration before and after procedures using intravenous iodine-based contrast media in adults at increased risk of contrast-associated acute kidney injury” | [2019] | NICE 2024 |
| For inpatients at particularly high risk, consider intravenous isotonic bicarbonate or 0.9% saline “For inpatients having iodine-based contrast media, consider intravenous volume expansion with either isotonic sodium bicarbonate or 0.9% sodium chloride if they are at particularly high risk, for example, if:” | [2019] | NICE 2024 |
| Consider temporarily stopping ACE inhibitors and ARBs when eGFR is below 30 (the unit after the figure breaks in the raw text and is left out of the quote) “Consider temporarily stopping ACE inhibitors and ARBs in adults having iodine-based contrast media if they have chronic kidney disease with an eGFR less than 30” | [2013, amended 2024] | NICE 2024 |
| Do not test for Bence Jones protein before contrast in myeloma “Assessment for Bence Jones proteinuria before contrast medium administration is not recommended.” | ungraded (handbook) | European Society of Urogenital Radiology (ESUR) Contrast Media Safety Committee 2026 |
Shares Iodinated and barium contrast for CT and X-ray, CT (computed tomography).
Shares Iodinated and barium contrast for CT and X-ray, CT (computed tomography).
Shares Iodinated and barium contrast for CT and X-ray, CT (computed tomography).
Shares Iodinated and barium contrast for CT and X-ray, CT (computed tomography).
Shares Iodinated and barium contrast for CT and X-ray, CT (computed tomography).
Shares Iodinated and barium contrast for CT and X-ray, CT (computed tomography).
Shares Iodinated and barium contrast for CT and X-ray, CT (computed tomography).
Shares Iodinated and barium contrast for CT and X-ray, CT (computed tomography).