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A PET-CT scan uses a sugar-like tracer, so food, sugar, exercise and cold can all blur the pictures. Do not eat for at least 4 hours before (plain water is fine and helps), take your usual medicines, avoid hard exercise the day before, and keep warm. If you have diabetes, the unit will give you a plan for your tablets or insulin and will check your blood sugar when you arrive.
EANM 2015 (version 2.0, ungraded): nothing but plain water for at least 4 hours before the FDG injection; medication as prescribed; about 1 litre of water in the 2 hours before; no coffee, caffeinated or 'sugar-free' drinks; glucose-containing intravenous fluids and parenteral nutrition stopped at least 4 hours before; no strenuous exercise for at least 6 hours and preferably 24; warmth from 30 to 60 minutes before injection to limit brown fat uptake; seated or lying and silent during the uptake phase, especially in head and neck cancer; void before imaging. Blood glucose must be measured before FDG; in clinical studies scanning proceeds below 11 mmol/L (about 200 mg/dL) and is rescheduled at or above it, though hyperglycaemia in brittle diabetes is not an absolute contraindication. Insulin to lower glucose is given only if more than 4 hours will pass before FDG (rapid-acting, subcutaneous). Type 2 diabetes on tablets: late morning slot, tablets continued; insulin-treated diabetes: late morning after normal breakfast and insulin with FDG no sooner than 4 hours after rapid-acting insulin, or early morning; insulin pumps off for at least 4 hours. Uptake time 60 minutes (55 to 75), kept within 10 minutes on repeat scans for response assessment. Pregnancy needs a clinical decision; breastfeeding need not stop, with close contact limited for 12 hours. An Australian joint diabetes and nuclear medicine guideline (2026) gives diabetes-specific protocols; it was read as a summary abstract only.
Why: Each changes how the scan is prepared or timed, and finding out early avoids a wasted visit.
“A specific inquiry about claustrophobia at the time the patient is scheduled for the study may decrease the number of nondiagnostic studies and cancellations, and allow premedication planning.”
Why: Food raises blood sugar and insulin, which pull the tracer into muscle and away from the cancer.
“Nondiabetic patients should not consume any food, simple carbohydrates or liquids other than plain (unflavoured) water for at least 4 h prior to the start of the FDG PET/CT study (i.e. with respect to the time of injection of FDG).”
Why: Water dilutes the tracer in the urine, giving clearer pictures and a lower radiation dose to the bladder.
“Adequate prehydration is important to ensure a sufficiently low concentration of FDG in the urine (fewer artefacts) and for radiation safety reasons.”
Why: Glucose in a drip raises blood sugar just as food does.
“Parenteral nutrition and intravenous fluids containing glucose should be discontinued at least 4 h before the time of FDG injection.”
Why: Working muscles take up the tracer and can hide or mimic disease.
“Patients must avoid strenuous exercise for at least 6 h before the FDG PET/CT study, and preferably for 24 h.”
Why: High sugar competes with the tracer and lowers the quality of the scan; low sugar is unsafe.
“Blood glucose level must be measured prior to administering FDG.”
Why: Very high sugar makes the scan less accurate, but an important scan should not be refused just for that.
“If the plasma glucose level is higher than or equal to 11 mmol/L (about 200 mg/dL), the FDG PET/CT study should be rescheduled or the patient excluded depending on the patient’s circumstances and the trial being conducted.”
Why: Insulin pushes the tracer into muscle, which spoils the scan if given too close to it.
“Insulin should not be given to reduce glucose levels (this leads to greater muscle uptake of FDG) unless the interval between administration of insulin and administration of FDG is more than 4 h.”
Why: Timing the scan around insulin keeps sugar steady without leaving insulin active at the injection.
“FDG should be injected no sooner than 4 h after subcutaneous injection of rapid-acting insulin or 6 h after subcutaneous injection of short-acting insulin.”
Why: Tablets do not interfere with the scan in the way insulin does.
“Patients continue to take oral medication to control their blood sugar.”
Why: Cold switches on brown fat and talking or chewing works muscles, both of which take up the tracer.
“The patient should be kept warm starting 30 – 60 min before the injection of FDG and continuing throughout the subsequent uptake period and examination to minimise FDG accumulation in brown fat (especially relevant in winter or if the room is air-conditioned).”
Why: Tracer uptake keeps rising with time, so comparing scans fairly needs the same wait each time.
“When repeating an FDG PET/CT study in the same patient, especially in the context of therapy response assessment, it is essential to apply the same uptake interval to within 10 min”
Why: Little tracer passes into milk, but the mother's body gives off radiation for some hours.
“The ICRP does not recommend interruption of breastfeeding after FDG administration since little FDG is excreted in the milk”
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.
EANM procedure guideline version 2.0 (2015), the current European adult oncology FDG guideline: at least 4 hours' fast with plain water encouraged, no strenuous exercise for at least 6 hours, warmth and quiet rest during uptake, blood glucose measured before injection with scanning below 11 mmol/L, no insulin within 4 hours of FDG, diabetes-specific timing of tablets, insulin and pumps, and a standard 60-minute uptake kept consistent across response scans. The guideline is ungraded consensus; no newer adult oncology FDG guideline from EANM or SNMMI was found.
Where it differs: A joint guideline of the Australian Diabetes Society and the Australian and New Zealand Society of Nuclear Medicine sets preparation by diabetes type, covering fasting, oral glucose-lowering medicines, insulin regimens and insulin pumps.
Evidence: Described as developed from a literature review and refined by a multidisciplinary expert panel. Only the summary abstract was read, so its specific instructions (for example on SGLT2 inhibitors or GLP-1 receptor agonists) are not stated here.
PET-CT scans for cancer measure how much sugar tissues take up, and many people now take newer diabetes and weight-loss medicines that change sugar handling. The main European guideline predates their wide use and says nothing about them, while reports suggest they can blur parts of the scan. There is no agreed advice on whether to pause them.
| Recommendation | Grade, as printed | Source |
|---|---|---|
| Scan can proceed when plasma glucose is below 11 mmol/L (clinical studies) “If the plasma glucose level is lower than 11 mmol/L (about 200 mg/dL), the FDG PET/CT study can be performed.” | ungraded | European Association of Nuclear Medicine (Boellaard et al., European Journal of Nuclear Medicine and Molecular Imaging) 2015 |
| Hyperglycaemia in brittle or poorly controlled diabetes is not an absolute contraindication “Therefore we recommend the same advice and suggest recording the blood glucose level and any other information that could be relevant for interpretation of the examination.” | ungraded | European Association of Nuclear Medicine (Boellaard et al., European Journal of Nuclear Medicine and Molecular Imaging) 2015 |
| Standard uptake time of 60 minutes “Note that consistency of SUV measurements (in-house and when compared to the literature) depends on strict observance of the uptake time, and therefore a 60 min interval is recommended with an acceptable range of 55 – 75 min” | ungraded | European Association of Nuclear Medicine (Boellaard et al., European Journal of Nuclear Medicine and Molecular Imaging) 2015 |
Shares A written plan for SGLT2 inhibitors and GLP-1 medicines before an FDG PET-CT scan, PET/CT.