Unlocking the Secrets of SIBO Testing: A Comprehensive Guide for Accurate Diagnosis

You may wonder which SIBO testing methods provide accurate results.

Unfortunately, SIBO continues to be misdiagnosed and under diagnosed (1).

In this article, we will discuss when you should have Small intestinal bacterial overgrowth (SIBO) testing, SIBO testing methods and the advantages and limitations of each method.

When should you have SIBO testing?

You may be wondering whether you should have SIBO testing. Perhaps you have read some online articles and feel your symptoms fit the SIBO profile.

Unfortunately, SIBO symptoms overlap with multiple other gut conditions, such as IBS, coeliac disease and even bowel cancer.

If you have digestive symptoms, then your doctor will need to first do some routine investigations to rule out these other conditions.

SIBO testing is not routinely carried out. However, your doctor may test for SIBO if certain factors in your assessment make you a higher risk.

Indicators of when to test for SIBO;

• Diagnosed with IBS, but routine IBS management does not work 
• Constipation is resolved, but bloating remains consistent 
• Having a history of conditions such as bariatric surgery or bile acid malabsorption which increase the risk of SIBO
• Being on anti-acid medications which increase the risk of SIBO

The types of SIBO testing

There are two main diagnostic tests for SIBO – via endoscopy or a breath test (2). 

Alternatively, some doctors may trial antibiotics to see if symptoms subside (3). The idea being that a positive diagnosis is made if your symptoms resolve. 

Your doctor may refer you to a gastroenterologist who may carry out SIBO testing.

SIBO Testing via Endoscopy

The ‘gold standard’ for diagnosing SIBO is doing an endoscopy (4).

An endoscopy is a long tube with a camera placed down your throat, through your stomach and into your small bowel.

To diagnose SIBO, a sample of fluid, called jejunal aspirate, is collected from the small bowel. This sample is sent to the laboratory to check for abnormally high bacteria levels (5).

A SIBO diagnosis is confirmed if the concentration of the bacteria in the aspirate is more than 10³ – 10⁵ CFU in 1 ml (6).

Limitations

This method is highly invasive and expensive, so it is rarely used outside a research setting (7).

There is also a lack of standardisation regarding what counts as a ‘high amount’ of bacteria (8). Although most experts have accepted a bacterial count of 10⁵ CFU in 1mL or more to diagnose SIBO (9, 10). 

But bacterial concentrations in the middle part of the small bowel (where the tests are taken) are lower than at the end of the small bowel. So, this threshold may be too high to test positive for SIBO (11).

Thus, bacteria often will not be equally distributed along your small bowel. The test can then give a wrong result if the sample is taken from an area without bacterial overgrowth (12).

Another issue is that many small bowel bacterial species cannot be grown in a lab. This is known as bacteria culturing (13). 

A big enough sample sample size is needed to identify the bacteria species. However, current culturing techniques may not be able to replicate the gut environment to stimulate gut bacteria growth,  so they cannot be studied (14, 15).

This can result in a false negative where you have SIBO, but your test results show negative for SIBO.

Lastly, the endoscope can be contaminated with bacteria when it passes through the digestive tract. This can then affect the test results (16).

Side effects and risks

The risks of the SIBO testing endoscopy procedure are low, but it can cause (17):

• Bleeding
• Perforation of the digestive tract by the camera
• A reaction to the sedative

SIBO Testing: Breath test

A more common, less invasive, and inexpensive test for SIBO is a hydrogen and/or methane breath test (18).

Breath tests can be performed in a clinical environment or at home. You can read more about the hydrogen breath test here.

How to prepare for a SIBO breath test

You will be asked to follow specific instructions beforehand, as certain medications, activities, and food can affect the results (19). Otherwise, you may be asked to reschedule the test.

These instructions can vary slightly depending on the testing site. The Rome Consensus Conference Expert Group has published general guidance on how to prepare for the breath test (20):

2 – 4 weeks before the test (21):

• Do not undergo any tests that involve colon cleansing, such as a colonoscopy, colonic irrigation, or barium enema
• No take any probiotics, antibiotics, laxatives or prokinetics

The evening before the test:

• Avoid complex carbohydrates which can be fermented in the bowel, such as pasta, bread, fibre cereals and beans
• Overnight fasting

Before the test:

• Do not smoke or exercise

Interpretation of results

Hydrogen levels are reported as a concentration in parts per million (ppm) (22).

Normal hydrogen levels in a digestive system are lower than 16 ppm. If your baseline levels exceed this, you may be asked to reschedule the test (23).

There has not been an agreed threshold for a positive SIBO result. Based on the Rome Consensus Conference Expert Group, a positive SIBO result can be indicated if (24):

• A rise in hydrogen by 10 – 12 ppm or more above the baseline
• A rise in methane by 10 ppm or more above the baseline

Risks and side effects

The breath test is non-invasive and considered very safe. However, the sugar solution can cause digestive symptoms such as (25):

• Abdominal pain
• Bloating and gas
• Diarrhoea

Limitations

There are concerns about the validity of the breath test (26). Conditions that alter the speed of your small bowel transit can affect the test accuracy.

For example, you can have a false negative result if you have gastroparesis (slow stomach emptying). A false negative is you have SIBO, but your test results are negative.

This result can be caused by the sugar solution not reaching the small bowel within the time provided (27).

On the other hand, the sugar solution can reach the large bowel if your bowels are moving rapidly. This is where bacteria are usually found, resulting in a false positive (28). A false positive is you do not have SIBO, but your test results are still positive.

Another issue is the lack of standardisation in preparing and performing the test and the amount of gas produced for a positive result (29). This makes it difficult to compare results from a research perspective (30).

The types of sugar used in the test affect the accuracy.

Glucose is completely absorbed in the first part of the small bowel. So, if you have bacteria in the end part of your small bowel, this may not be picked up on the test (31).

Lactulose has a laxative effect, speeding up your bowel transit time. This means it can reach the colon in less than the testing time, creating a false positive result (32).

At-Home SIBO testing

You can order an at-home SIBO testing kit. The kit contains everything you need to perform a breath test, which is helpful for those who cannot travel or live in remote areas (33).

Once you finish the test, you will return the breath samples to the testing company or lab. They will analyse the gases to give you the results. 

The functional gut clinic offers at-home SIBO testing kits. The results will be sent to you and your doctor, who will explain them in a follow-up consultation. Click here for more information.

It’s important to note that all SIBO testing is not a definitive diagnosis. It can give you an idea if you might have SIBO, but you should still consult a doctor to confirm the results and discuss further steps or treatments if needed.

SIBO Diagnosis via Antibiotic Trial

In practice, access to a breath test may be limited, or doctors may not consider it accurate enough to use it (34). 

An increasingly popular option in this scenario is to trial a course of antibiotics such as Metronidazole or Rifaximin (35).

These medications act by killing off bacteria in your small bowel (36). If symptoms subside, this suggests that you have SIBO.

However, there are no set guidelines for this approach. There is also a growing concern about the overuse of antibiotics causing antibiotic resistance, leading to the emergence of “superbugs” (37, 38).

Summary

There are two main diagnostic tests for SIBO; endoscopy with jejunal aspiration or a breath test. 

The gold standard for diagnosing SIBO is endoscopy. However, it is highly invasive, expensive and lacks a standardised protocol, so it is rarely used outside a research setting.

A breath test is a more common, less invasive, and inexpensive test for SIBO.

The validity of the test can be affected by your adherence to the preparation instructions, the speed of your small bowel transit, and the type of sugar solution used. 

Due to the lack of access to a breath test, an increasingly popular alternative is trialling a course of antibiotics.

However, once again, there is a lack of guidelines for this and concerns over the rise of antibiotic resistance.

Article written by Leeona Lam, Registered Associate Nutritionist, MSc, ANutr, and reviewed by Kirsten Jackson, Consultant Dietitian BSc Hons, RD, PG Cert