Gut tests: what they show and what they do not

Healthy state

A test is only as good as the question it is meant to answer. Two terms help you place it. Sensitivity says how many sick people the test finds; specificity, how many healthy people it correctly recognises as healthy. Added to this is pre-test probability: a test in someone with fitting symptoms says far more than the same test in someone without symptoms. That is why good gut tests are usually targeted rather than broad.

Faecal calprotectin Calprotectin A protein from white blood cells measured in stool. Raised levels point to inflammation in the gut. is a protein from immune cells and shows inflammation of the gut lining. It helps distinguish IBS Irritable bowel syndrome Recurring abdominal pain linked to bowel movements, without tests finding another cause. Today it is understood as a disorder of gut–brain interaction. More in the chapter “The gut-brain axis” from inflammatory bowel disease Inflammatory bowel disease Umbrella term for Crohn’s disease and ulcerative colitis: long-term, relapsing inflammation of the gut that needs medical treatment. More in the chapter “Immune system and inflammation”. A large review found that in adults, with a cut-off of 50 µg/g, sensitivity was about 93% and specificity about 94% for this distinction. A normal value makes chronic inflammation very unlikely. But it also rises with infections, with NSAID painkillers, and with polyps or tumours, so it is not a cancer test.

Coeliac Coeliac disease An autoimmune disease in which gluten damages the lining of the small intestine. It is diagnosed with a blood test and usually a biopsy while you are still eating gluten. More in the chapter “Lactose, fructose, gluten: making sense of intolerances” antibodies (against tissue transglutaminase, together with total IgA IgA (secretory IgA) The antibody of the mucous membranes. It is released into the gut in large amounts, where it holds germs in the mucus without causing inflammation. More in the chapter “IgA: the antibody that keeps the gut clean, and what vitamin A, D and omega-3 have to do with it”) in the blood point to coeliac disease and, according to the guideline, belong in the work-up of IBS symptoms. They only work while you eat gluten. Faecal elastase shows whether the pancreas makes enough digestive enzymes; with very watery stool the value is measured too low because of dilution.

Other established examinations: stool tests for specific pathogens (culture or PCR), which show an infection or parasites when there is a suspicion; tests for Helicobacter pylori, a stomach bacterium, by breath or stool test (no acid blockers or antibiotics for some weeks beforehand, otherwise it is falsely negative); tests for hidden blood in the stool for colorectal cancer screening; and endoscopy (gastroscopy or colonoscopy), which shows the lining directly and allows tissue samples.

Breath tests Breath test After drinking a sugar solution, hydrogen and methane in the breath are measured several times. This can reveal sugar intolerances and signs of bacterial overgrowth. measure gases that bacteria produce after you drink a test sugar solution. They are used for lactose, fructose and the question of small intestinal bacterial overgrowth Small intestinal bacterial overgrowth (SIBO) Too many bacteria in the small intestine, where normally few live. Bloating and diarrhoea are typical; it is usually diagnosed with a breath test. More in the chapter “When the small intestine is too densely colonised”. They are practical and harmless, but how informative they are depends strongly on preparation, test sugar and interpretation, see below.

Disrupted state: causes and effects

Commercial microbiome Microbiome, gut microbiota The community of all microbes in the gut, mainly bacteria, plus viruses, fungi and archaea. Most live in the colon. More in the chapter “Building microbial diversity” analyses promise to show you exactly what is going on in your gut. Researchers sent the same stool sample to seven providers: the results differed greatly, even for repeat tests from the same provider. One provider rated one of three identical samples as “unhealthy” and the other two as “healthy”. Reasons are differences in sample handling, DNA analysis and data evaluation.

The basic problem: it has not yet been defined what a healthy microbiome looks like. Without this benchmark, a single result can hardly be assessed. In addition, a stool sample mainly reflects the large intestine at the end of the digestive tract. It says little about the small intestine, the mucus layer Mucus layer A gel layer of mucins on the gut wall that keeps bacteria at a distance. In the colon, the inner layer is almost free of microbes. More in the chapter “The mucus layer and additives” or the folds of the lining. Condition on the day, the last few days’ diet, laxatives and antibiotics also change the result.

The zonulin test for “leaky gut” and IgG tests for foods are not suitable for a diagnosis. When many commercial tests were checked, zonulin in the blood was not reliably measured as what it promises. IgG antibodies against foods mainly show that you have eaten something. The DAO DAO (diamine oxidase) An enzyme in the gut wall that breaks down histamine from food before it reaches the blood. More in the chapter “DAO: the enzyme that intercepts histamine from food” blood test for histamine Histamine A messenger substance of the body that is also found in aged and fermented foods. Too much can cause headache, flushing or diarrhoea. More in the chapter “Histamine intolerance: much searched, little proven” intolerance is also unsuitable as sole proof: the values overlap strongly with those of healthy people. Expensive results then lead to expensive diets and supplements.

Established tests have limits too. A breath test can be falsely negative if the bacteria produce not hydrogen but methane or hydrogen sulphide; a three-gas test captures this better. It can be falsely positive if the test solution passes quickly through the small intestine and is already fermented in the large intestine. Small intestinal bacterial overgrowth is also hard to prove reliably; guidelines stress that values must be interpreted cautiously.

Also: a “normal” result reassures but does not rule out symptoms. In IBS most tests are unremarkable, because the disorder lies in sensitivity and movement of nerves and muscles, not in visible inflammation. On the other side, “abnormal” single values without symptoms often lead to unnecessary treatment.

Warning signs call for work-up without a detour via home tests: blood in the stool, unintended weight loss, night-time diarrhoea, persistent fever, anaemia, difficulty swallowing, and symptoms that are new after age 50 or occur with a family history of bowel cancer or inflammation.