Preparing for a PET scan requires switching to a no-sugar diet the day before the scan. People with diabetes may have special instructions regarding medications.

Doctors typically schedule PET scans days to weeks in advance. While preparation for a PET scan isn’t extensive, you still need to follow some essential steps in the lead-up to the procedure.

Your medical team will give you precise instructions, but this guide can give you a general idea of what to expect.

As soon as a doctor schedules a PET scan, let them know if:

  • You are or might be pregnant: Radioactivity can affect the fetus. If the PET scan is medically necessary, the radiology team may need to make special considerations.
  • You are breastfeeding or chestfeeding: This may influence which type of radioactive tracer they use in the scan. Otherwise, they may reschedule the scan.
  • You have diabetes: Your doctor may need to adjust your diabetes medication dosage because blood sugar levels can affect the scan.

Experts advise adjusting to a low carb, no-sugar diet 24 hours before your PET scan. Consuming sugar can interfere with the test results because the scan uses radioactive sugar as a tracer.

“Sugar” doesn’t just mean foods and beverages that taste sweet or have added sugar. Lots of foods contain sugar and starches, which break down into sugar. Still, there are a lot of foods you may eat.

It may help to know which foods to eat and which to avoid:

Foods you can eat

  • meat
  • cheese
  • eggs
  • nonstarchy vegetables (e.g., broccoli, lettuce, peppers)

Foods you should avoid

  • cereal
  • pasta
  • bread
  • milk
  • starchy vegetables (e.g., potatoes, corn, peas)

In addition to adjusting your diet, you’ll need to avoid:

  • alcohol
  • tobacco
  • caffeine

You’ll also need to refrain from vigorous exercise for 24 hours before your scan. Strenuous activity beforehand can cause your body to absorb the tracer more quickly.

Experts recommend that you stop eating and drinking for 6 hours before a PET scan. The exact number of hours may vary depending on your medical team.

However, you can drink water during this period. It will be important to stay hydrated.

Diabetes medication and PET scans

If you have diabetes, the American College of Radiology advises medical teams to schedule your PET scan for early in the morning, when your glucose level is lowest.

You should take your insulin the day before your scan. But do not take an insulin injection the morning of the scan.

You can take oral medications as usual. If you take metformin, though, your doctor may ask you to stop taking it 48 hours before your scan.

If you use an insulin pump, turn it off 4 hours before the scan.

Most places request that you check in 15 to 30 minutes early for your appointment. Arriving on time is important, as the radioactive tracer can only work briefly.

You’ll want to wear loose, comfortable clothes without any metal parts, such as zippers, buttons, or buckles. Some places may have you change out of your clothes into a hospital gown.

If you wear glasses, you’ll need to remove them for the scan. You’ll also need to remove jewelry and other metal objects.

Anxiety can affect PET scan results, so let your medical team know if you’re feeling anxious about the test or if you have claustrophobia. Though sedatives can also affect PET scan results, the doctor may be able to administer a mild one to help you relax.

The scan will take 45 minutes to 2 hours, but the results could take weeks. If your results come back quickly or take a long time, it’s helpful not to make assumptions about what that time frame means. It’s best to wait for instructions from your doctor, who can walk you through the results.