This sheet explains how to complete your bacterial overgrowth test. If you would like further information, or have any worries, please do not hesitate to ask your nurse.

Please read this information carefully before starting your test. You should complete your bacterial overgrowth test within two weeks of receiving the kit.

What does the bacterial overgrowth test involve?

The test involves taking samples of your breath at 15-minute intervals before and after taking a sweet tasting drink called lactulose. This test allows your clinical team to check for small intestinal bacterial overgrowth (SIBO), a condition where an excess of healthy gut bacteria is found in the small bowel. The bacterial overgrowth test should take about two and a half hours.

How do I prepare for the test?

Four weeks before the test

Please ensure that you do not take antibiotics in the four weeks leading up to your test. If you have taken antibiotics during this time, please contact us as soon as possible.

One week before the test:

In the week before your test, you should not undertake a colonoscopy (a test to check inside your bowels) or use any full bowel cleansing preparations, including colonic irrigation. Motility enhancing drugs such as antidiarrheal, laxatives, fibre supplements, or poo (stool) softeners like milk of magnesia, ex-lax and Imodium, should also not be used. We also recommend that the use of probiotics is stopped at least one week prior to the test as taking probiotics introduces bacteria into the upper gut.

If you have had any colon cleansing treatment in the week prior to the test, please contact us as soon as possible.

24 hours before your test:

In the 24 hours leading up to your test, you should avoid fermentable food and drinks that are high in complex carbohydrates. These include:

  • sugary drinks like fruit juices or squash
  • fizzy or soft drinks
  • alcohol
  • dairy milks
  • vitamins
  • chewing or bubble gum.

During this time, the following foods are permitted:

  • Baked or grilled seafood, chicken, turkey, lean beef, or pork
  • Eggs
  • Plain white rice
  • Plain coffee or tea or water
  • Minimal oils for cooking, salt and pepper.

All other foods are not allowed during the 24-hour preparatory diet. You can contact the physiology unit if you have special dietary requirements, the team can discuss this with you.

12 hours before your test:

In the 12 hours leading up to your test you should not:

  • Eat any food but you may drink still water
  • Smoke tobacco products or use e-cigarettes
  • Undertake vigorous exercise.

You may brush your teeth and have a glass of warm water on the morning of the test.

If you have any questions or would like to speak to someone about any of the instructions above, please contact the Louise Ryan unit on 020 7830 2923.

How to take the bacterial overgrowth test

Before you begin the test, please ensure that you have completed the pre-test checklist and that you have all the items you will need to hand:

  • Sachet of lactulose (this will have been sent to you)
  • A cup or glass
  • 300ml of cold water
  • A small spoon
  • Results sheet
  • Sample bag with sealing clips
  • A clock or alarm.

Empty the sachet of lactulose into a cup or glass and add 300ml of water and stir well. Leave the drink to one side while you take the first sample and write the date on your results sheet.

A step-by-step guide to taking the test

Part of the test involves taking breath samples. These should be taken in a resting, seated position. We recommend that you try a practice breath before completing the test.

Step one

Take a deep breath in, hold for 15 seconds and then slowly and gently blow out into the open air until you are about halfway through your breath.

Step two

Pick up sample bag number one and breathe out the remainder of your breath by putting the tube at the open end of the bag up to your lips and breathing out into the bag so that the bag inflates.

The gas that is needed for analysis is the very last part of your breath as you breathe out.

The bag should be at least three quarters full, and you will have your lungs completely emptied. If this is not the case, or you are not happy with the sample in any way, please empty the bag and start again.

The sample bags which you have been sent are numbered, please use them in numerical order, starting with bag one. A spare bag will also be included with your testing kit.

Step three

After completing the breath sample, snap the white clip (near to your mouth) shut to close the bag and prevent the sample escaping, and then remove the tube from your mouth.

Step four

Squeeze the bag gently for several seconds to check that there is not a leak, and it does not deflate. You can use the spare bag if it is needed. Record the time of the breath sample on your results sheet.

Step five

Drink all the mixed lactulose and water solution and write the time you finish the drink on your results sheet. More water can be added if you find it difficult to drink or need to rinse around the cup or glass. You do not have to drink the solution in one go but do not take more than two to three minutes.

Step six

Write in the times for your remaining breathe tests on your results sheet. For example, if you finish the drink at 9am and your next sample says ‘+15minutes’, you will write 9.15am as the time to take your second sample.

Do not worry if you are slightly delayed in taking your first sample breath. Do not adjust the time of the next sample; you should still take this at the planned time. You may find it useful to use a clock or alarm to remind you when to take your samples.

Step seven

Repeat the breath sample process (steps one to four) as indicated on your results sheet.

How to return your samples

Place all sample bags (used and unused) and your results sheet into the box provided. Return your box as soon as possible, or within seven days of completing your test.

In person

GI physiology unit in the Louise Ryan unit at the Royal Free Hospital, lower ground floor. We are open Monday to Friday, 9am to 4.30pm.

By post

If you choose to post your samples back to the team, you will need to pay for return postage. Please use the self-address return label enclosed in your box and send it by normal post (not recorded delivery). Address the return label to:

Louise Ryan Unit, GI Physiology Unit, Lower Ground Floor, Royal Free Hospital, Pond Street, NW3 2QG.

What happens after my test?

The samples will be analysed and your test results and any appropriate treatment will be discussed with you. In some circumstances you may be required to undertake another breath test. If this is necessary, the team will discuss this with you.