Planning ahead before a test

Different tests and scans are used to help diagnose cancer. Some of these tests are also used during treatment and as part of your follow-up care. Many of these tests will not affect your diabetes. But you should contact the hospital before each test and tell them that you have diabetes. They can give you advice. If you are not following your usual diet, speak to your diabetes team to check if any changes are needed to your diabetes medication.

Some tests will take a few hours. When you contact the hospital, ask how long you will need to be there. You can then plan ahead, so that you can bring everything you need. This could include your diabetes equipment and some food.

Some tests need a bit more planning. You may need to follow a careful diet so that you have an empty bowel. Or you may need to stop eating for a few hours before. The hospital staff will tell you how to prepare for the tests. You may need to go into hospital the day before.

Not eating before a test

You may be asked not to eat for a few hours before some tests. This could be before an MRI scan or a CT scan.

Not eating may be a problem for some people with diabetes. Because of this, you may have a shorter time when you cannot eat than people who do not have diabetes.

The hospital staff will tell you when you can eat and when to take your diabetes medicines. It is important to follow this advice carefully. They may also be able to give you an early morning appointment. This will help shorten the amount of time when you cannot eat.

Most people manage to keep their blood sugar level in a safe range with advice and support. But if you notice symptoms of low blood sugars, the fastest way to treat this is to eat or drink 15 to 20g of fast-acting carbohydrate straight away. Take hypo treatment with you when you go to the hospital. Depending on the test, you may then need to rearrange the appointment for a different day.

If you use an insulin pump or glucose monitors

CT and MRI scanners and some x-rays can affect the way your insulin pump works. If you use an insulin pump, it may need to be removed before the scan or x-ray starts. A few days before the scan, you should tell the scanning or x-ray staff (radiographers) that you have an insulin pump. If you need to have radiotherapy treatment, you should also tell the therapeutic radiographers that you have an insulin pump.

Some scans and x-rays take more than 1 hour. If your pump is removed, you may need to have injections of a short-acting insulin during this time. Ask the staff how long your scan or x-ray will last. This will help you plan how to manage your diabetes. Your diabetes team can also give you advice.

If you use a continuous glucose monitor (CGM) or flash glucose monitor, whether you have to remove it depends on different things. This includes which type you are using, the scan or x-ray you are having, and where on the body you are being scanned. Your diabetes team and cancer team can give you advice about this and how to monitor and manage your blood sugar level.

Having a colonoscopy or a barium x-ray

A colonoscopy and a barium x-ray look at the lining of the bowel from the inside. The bowel needs to be completely empty, so you will need to follow a careful diet before the test. You will not be able to eat for a few hours before the test. You will also need to take a laxative.

The staff doing the test can give you information on controlling your blood sugar before, during and after the test. It is important to follow this advice. Contact your diabetes team if you are not sure how to manage your blood sugar during the procedure.

You should also be given a morning appointment for these tests. This helps to shorten the amount of time you cannot eat and drink. Contact the hospital if you have been given an afternoon appointment. It might be possible to change the time.

Having a PET scan

A PET scan uses low-dose radioactive sugar to measure the activity of cells in different parts of the body. A PET scan is often combined with a CT scan (PET-CT).

About 2 hours before the scan, you have an injection of a sugary fluid that has a small amount of radiation. The amount of radioactive sugar you are given is small. It should not affect your diabetes.

Cancer cells are usually more active than normal cells, so they take up more of the radioactive sugar. The sugar helps the cancer cells show up on the scan.

For the PET scan to work well, your blood sugars need to be within a certain range. Aim for a stable blood glucose within the range of 4 to 11mmol/L. But your diabetes team may set you a different range that is suitable for you. If the blood sugar level is too high, you may have to have the scan on another day. You will have your blood sugar level checked before your PET scan.

  • If the blood sugar level is outside the range, it can affect the test:
  • If your blood sugar level is high, the cancer cells may not take up (absorb) much of the radioactive sugar. This is because they already have enough sugar. This means they will not be seen on the scan.

If the amount of insulin in your blood is high, your normal cells will absorb more of the radioactive sugar. This means the cancer cells absorb less sugar and do not show up on the scan as clearly.
The staff in the scanning department will give you information about how to prepare before the scan. They will tell you:

  • what and when to eat
  • which diabetes medicines to take in the days before the scan
  • what food and diabetes medicines you should take with you to the hospital.

Scans using a dye (contrast)

Some types of scan include having an injection of a dye. This dye makes some areas of the body show up more clearly. The dye is called a contrast. It is used in different types of scan, including:

  • a CT scan
  • an MRI scan
  • an IVU (intravenous urogram) – a test to look at the urinary system
  • a venogram or an angiogram – tests to look at the blood vessels.

Contrast can temporarily affect how your kidneys work. This can be a problem if you take the diabetes medicine metformin. If you do, you will be asked not to take it on the day of your appointment and for about 2 days afterwards. You will be given information about when to take it and how to manage your blood sugars during this time.

Booklets and resources

About our information

This information has been written, revised and edited by Macmillan Cancer Support’s Cancer Information Development team. It has been reviewed by expert medical and health professionals and people living with cancer.

  • References

    Below is a sample of the sources used in our diabetes and cancer treatment information. If you would like more information about the sources we use, please contact us at

    Joint British Diabetes Societies for inpatient care (JBDS-IP). The management of glycaemic control in people with cancer. 2023.

    Joharatnam-Hogan, N; Chambers, P; Dhatariya, K; and Board, R. The Joint BritishDiabetes Society for Inpatient Care (JBDS), UK Chemotherapy Board (UKCB). A guideline for the outpatient management of glycaemic control in people with cancer. Diabetes Medicine. 2022; 39.1-11. Available from: [accessed Oct 2022].

    Shahid, R.; Shahid, A.; Duc, L; and Sunil, Y. Diabetes and Cancer, Risks, Challenges, Management and Outcomes. MDPI. 2021;13:1-21. Available from: [accessed Oct 2022].

Date reviewed

Reviewed: 01 September 2023
Next review: 01 September 2026
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