McDowell, G & Brydon, G. (2016) ‘Gastrointestinal Disorders and Malabsorption.’ In: Admed, N. (ed) (2016) Fundamentals of Biomedical Science Clinical Biochemistry. 2nd Edition. Oxford University Press. pp 420-444. (25 pages)
Learning objectives
After studying this chapter you should be able to:
- Explain the different functions of acid in the stomach and bile acids in the small intestine
- Identify the secretion and absorption processes within the small intestine
- Explain the functions of secretin and cholecystokinin in nutrient digestion
- Describe the symptoms and laboratory diagnosis of gut hormone secreting tumours
- Discuss the advantages and limitations of the faecal elastase test as a measure of exocrine pancreatic disease
- Outline the use of simple tests for initial investigation of patients with chronic watery diarrhoea
- Discuss the biochemical tests used to investigate malabsorption
- Outline the factors which have contributed to some gastrointestinal tests becoming obsolete
Introduction
The use of biochemical tests for the diagnosis of gastrointestinal disease has increased over recent years, as a result of several analytical developments.
Many older laboratory tests have become obsolete because of their poor predictive value and have been replaced by improved ones, which if used appropriately can facilitate or exclude a diagnosis. The advent of procedures such as endoscopy, where a flexible tube is passed through the mouth or the anus (colonoscopy) to visualize and sample tissues within the gastrointestinal tract, have transformed patient investigation. However, these are invasive, expensive procedures and some are not without risk.
Laboratory diagnosis of Helicobacter pylori, the microorganism which is the major cause of duodenal and gastric ulcers, has significantly reduced the upper endoscopy workload. Serological testing for coeliac disease, caused by sensitivity to gluten in the diet, has resulted in more frequent investigation for this disease and has revealed a much higher incidence than previously thought.
The importance of tests in faeces, the ultimate receptacle of gastrointestinal secretions, is now recognized. Some proteins released from fluids, sites of inflammation, or from bacteria, are stable in faeces and have significant diagnostic potential, providing direct information about gastrointestinal pathology. The introduction of specific commercial immunoassays to identify and quantify these proteins has greatly facilitated and standardized this process. In this chapter, you will be introduced to the basic biochemical and, where relevant, physiological processes in the gut, their optimization for digestion and absorption, and how diagnostic tests have evolved to investigate the breakdown of gut homeostasis.
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