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Investigation of gastrointestinal function 

Investigation of gastrointestinal function

Chapter:
Investigation of gastrointestinal function
Author(s):

Julian R.F. Walters

DOI:
10.1093/med/9780199204854.003.150304

November 30, 2011: This chapter has been re-evaluated and remains up-to-date. No changes have been necessary.

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date: 30 March 2017

Many blood measurements of absorbed dietary components (such as iron, folate, vitamin B12, cholesterol, and triglycerides) or their metabolic products (such as haemoglobin or albumin) can be abnormal when gastrointestinal function is impaired, and their serial changes can be used to follow progress of disease. Specific assessment of the function of the gastrointestinal tract complements the findings of imaging, endoscopy, and histology in the diagnosis of digestive diseases.

Measures of nutritional status (e.g. body mass index, anthropometry, body density), nutrient intake (e.g. dietary history) and faecal output (e.g. stool weight and volume, faecal fat) provide indications of the net effects of absorption.

In patients with possible malabsorption an important clinical decision is whether digestion is at fault (such as pancreatic exocrine insufficiency) or whether absorption is the problem (as in coeliac disease). Often this is rapidly established by employing tests with high positive predictive values for common individual diseases, such as tissue transglutaminase serology for coeliac disease, or imaging studies for chronic pancreatitis.

When a more systematic approach is required to reach a diagnosis, there are many tests that indirectly measure digestion and absorption and can help to differentiate between disorders affecting the two processes, e.g. reduced levels of faecal elastase in pancreatic insufficiency. Increased excretion of breath hydrogen after administration of lactose is found in lactase deficiency, and rapid excretion of hydrogen occurs after administration of glucose or lactulose when there is small-bowel bacterial overgrowth.

Evidence for infection, structural damage, or loss of barrier functions of the gut can also be obtained, e.g. (1) Helicobacter pylori infection of the stomach can be detected by the urea breath test—13C-labelled urea is given by mouth; the urease activity of H. pylori in the stomach metabolizes this to CO2, which is then exhaled; a standard amount of CO2 is collected in a breath sample and the activity of 13C determined by mass spectrometry.

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