How is a diagnosis of SIBO made?

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Welcome to part 3 of our series on small intestinal bacterial overgrowth (SIBO). As mentioned in the earlier articles, the symptoms of SIBO are similar to other gut conditions, so diagnosis can be complex. Your doctor will make a diagnosis of SIBO, after a series of tests. There is one test that differentiates SIBO from other conditions.

Patient sitting with her doctor discussing test results

Diagnosis of gut conditions

Depending on your symptoms and your medical history your doctor will decide which tests you require.

The tests are designed to assess your nutritional status, the structural and functional health of your intestines as well as the cause of your symptoms. You may have other tests that investigate your overall health.

It is possible that you may require the following preliminary tests:

TestFocus of investigation
Full blood examination (FBE)Identifies anaemia, changes to white blood cells (infection fighting cells), risks for thrombosis
Urea, Electrolytes & Creatinine (UEC)Checks electrolyte balance e.g. sodium, potassium and bicarbonate
ESR, CRPIdentifies the presence of widespread inflammation
Coeliac serologyScreens for coeliac disease
Iron studiesIdentifies iron deficiency
Faecal tests: conducted when diarrhoea predominant or mixed bowel motionsThere are specific tests for intestinal inflammation, as well as those for small intestinal bacteria and parasites
Pain abdominal x-ray: conducted when constipation predominant, or for mixed bowel motionsIdentifies obstruction or retained stool (faeces)

Dietary triggers

The majority of people who experience irritable bowel symptoms have a lower tolerance for FODMAPs (fermentable, oligo-,di and mono-sachharides and polypols).

FODMAPs are naturally occurring sugars such as lactose (milk and dairy), fructose (fruit), sorbitol (fruit) and mannitol (fruit and vegetables).

Sorbitol, mannitol and sucrose, for example, are also added to a range of processed foods.

In irritable bowel syndrome, these sugars are poorly absorbed and fermented by bacteria in the colon, producing excess gas, bloating, pain and diarrhoea. In SIBO, this would also occur in the small intestine.

Remember, in SIBO, the bacteria from the large intestine invade and take up residence in the small intestine.

Remember, in SIBO, the bacteria from the large intestine invade and take up residence in the small intestine.

During the fermentation process, bacteria produce a number of gases. Some of these are absorbed into our blood, transported to the lungs and then exhaled in our breath.

Breath tests

To help with the identification of dietary triggers, a number of breath tests are available. These tests measure the amount of hydrogen and/ or methane exhaled after ingesting a sample (test load) of the sugar being investigated.

Breath tests avoid an unnecessary restriction of foods, and they don’t have any adverse impacts on gut microflora. However, they do take around 3 hours to complete, and a breath test may need to be taken every 15 minutes.

Choice of breath tests is related to which foods trigger symptoms:

Breath testSymptoms Indicator
FructoseFruit
LactoseMilk/ dairy products
SorbitolStone fruits and food products with sorbitol added
SucroseWidespread in fruits, vegetables and processed foods
MannitolSome vegetables e.g. mushrooms, and it is used as sugar substitute in processed foods

Diagnosis of SIBO

The diagnosis of SIBO is assisted by a Glucose and Lactulose breath test. It is a very clever test in that it is designed to identify when the glucose and lactulose are fermented by the bacteria.

As explained in article one, the small intestine is approximately six metres long. In a healthy gut, the glucose would be fully absorbed in the small intestine, so no fermentation would occur. Fermentation of the lactulose would not occur until it reached the colon.

This test not only measures the amount of hydrogen and methane produced but also when the fermentation occurs.

A diagnosis of SIBO would occur if the hydrogen and methane levels rise to the expected levels, within specified time frames.

A diagnosis of SIBO would occur if the hydrogen and methane levels rise to the expected levels, within specified time frames.

A  person with SIBO may have two separate peaks for a rise in hydrogen, as the lactulose may undergo fermentation in both the small intestine and colon.

As you can see, the diagnosis of SIBO occurs after a thorough examination of gut health and function. If you experience any gut disturbances or digestive problems, I encourage you to see your doctor and undergo the appropriate tests and examinations.

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