Colonic transit study: how it works and what to expect

The large bowel, or colon, transports food residue from the small intestine to the anus. Cambridge University Hospitals explains that a colonic transit study looks at how your large bowel is working and checks how quickly food moves through it. It helps to diagnose conditions affecting bowel function.

It uses small markers that show up on X-ray. Alder Hey Children’s Hospital describes capsules containing small radio-opaque markers which, once swallowed, can be seen on an X-ray. From the X-ray, the doctor can look at how quickly food is moving by counting the markers seen, and can see areas where there may be a hold-up or blockage.

The NHS describes constipation as passing poo less than 3 times in a week or less often than usual, with poo that is hard, lumpy, large or small, straining or pain, or a feeling of not having fully emptied. Most constipation is dealt with by fibre, fluids, activity, toilet routine and laxatives from a pharmacist.

The NHS advises seeing a GP if constipation is not getting better with treatment, is regular, comes with blood in the poo, unexplained weight loss, tummy pain or sudden changes in bowel habit. When simple measures have not worked, a specialist may want an objective measure of how fast the bowel is moving. A transit study is one such test. Which tests are used, and in what order, is for the specialist to decide from your symptoms.

In the Cambridge adult leaflet, six capsules are sent in advance. You swallow two a day for three days with water at breakfast, there are no capsules on days 4 and 5, and the X-ray is on day 6, at an appointment between 8.30am and 5pm. It says you can eat and drink normally throughout, and should not use laxatives before or during the study so that the results are accurate.

Alder Hey’s paediatric leaflet uses a different schedule for children: one capsule with breakfast every day for 6 days, with an X-ray on day 7, and capsules can be opened and mixed with yoghurt or soft food if swallowing is difficult. Schedules differ between hospitals, so follow the instructions you were sent.

The Cambridge leaflet notes that the capsules contain gelatine, which is an animal product, so vegetarians and vegans who do not wish to take them should contact the referring consultant. It also says not to take them if you are allergic to gelatine or barium sulphate and to ask for an urgent GP appointment if you have an allergic reaction such as a rash, itching or redness. Swelling of the face, lips, tongue or throat needs 999 or A&E.

The leaflet describes the X-ray as quick and painless. It only takes a few minutes, but you should allow at least 30 minutes in the department. You may be asked to change into a hospital gown, and you lie flat on your back on the X-ray table.

The X-ray involves ionising radiation. The leaflet says the dose can be equivalent to a few weeks, months or years of natural background radiation depending on the test, and that the risk of this causing harm is considered low. If there is any chance you could be pregnant, tell the department before taking the capsules.

After the X-ray you can restart any laxative medicines you stopped for the study, according to the leaflet.

The doctor or specialist who referred you receives the report and tells you the results, in the Cambridge leaflet by letter, with results also available online to that trust’s patients. The radiologist counts the markers and notes where they are sitting.

Alder Hey says the number of markers seen shows how quickly food is moving, and the pattern can show where there may be a hold-up or blockage. The interpretation depends on the protocol used, so this guide does not give figures. Ask the referring clinician what the result means for your treatment.