From the Guidelines
Patients should abstain from consuming glucose or foods containing sugar for at least 4 hours before the PET scan. This fasting period is necessary because PET scans often use a radioactive form of glucose (FDG or fluorodeoxyglucose) as a tracer to identify areas of high metabolic activity in the body. If you have recently consumed glucose, your normal cells will take up the sugar, which can interfere with the scan's ability to detect abnormal uptake in areas of concern, such as tumors. The fasting ensures that your blood glucose levels are stable and that the radioactive glucose will be preferentially taken up by metabolically active tissues of interest.
According to the most recent and highest quality study, patients without diabetes are advised to abstain from eating any food, simple sugars, or beverages, except for plain water, for a minimum of 4 hours prior to the commencement of the PET/CT scan 1. This recommendation is supported by other studies, which suggest that a fasting period of at least 4 hours is necessary to ensure low blood glucose and low insulinaemia, allowing for optimal uptake of the radioactive glucose by metabolically active tissues 1.
It is also important to note that patients with diabetes should consult with their healthcare provider about managing their medication during this fasting period, as they may need to adjust their insulin or oral diabetes medications to prevent hypoglycemia while maintaining appropriate glucose levels for the scan 1.
Some studies suggest that a longer fasting period, such as 6 hours, may be beneficial in certain cases, such as in patients with cardiac device-related infections or endocarditis 1. However, the most recent and highest quality study recommends a minimum fasting period of 4 hours 1.
In summary, a minimum fasting period of 4 hours is recommended before a PET scan, and patients with diabetes should consult with their healthcare provider to manage their medication during this period.
From the Research
Glucose Intake Before PET Scan
- The European Association of Nuclear Medicine (EANM) guidelines suggest a cutoff level of serum glucose (SG) ≥11.1 mmol/l for PET-CT scanning 2.
- A study found that a significant number of patients were diagnosed with diabetes mellitus, impaired glucose tolerance, or impaired fasting glucose as a result of being referred for the PET-CT scan 2.
- Another study suggested that blood glucose levels should be less than 200 mg/dl before F-FDG PET/CT scanning, with higher levels associated with an increased proportion of abnormal biodistribution 3.
- However, the same study found that most patients with blood glucose levels less than or equal to 200 mg/dl had normal or mildly abnormal biodistribution 3.
Fasting Requirements
- A study found that the mean provided fasting duration was 5 hours, with a range of 4-8 hours 4.
- Another study suggested that a 24-hour metformin discontinuation was associated with lower accumulation of fluorodeoxyglucose in the bowel, compared to no discontinuation or 24-hour discontinuation of metformin 5.
- The same study found that metformin discontinuation for 48 hours prior to PET/CT was associated with lower accumulation of fluorodeoxyglucose in the bowel, compared to 24-hour discontinuation or no discontinuation 5.
Timing of Glucose Intake
- A study found that an approximately 90-min time window from 18F-FDG injection to PET imaging would significantly improve target-to-background ratio and, thus, quantitation and visual interpretation 6.
- However, the optimal timing of glucose intake before a PET scan is not explicitly stated in the provided studies.
- It is recommended to follow the guidelines and instructions provided by the specific PET scan facility or healthcare provider regarding glucose intake and fasting requirements before the scan.