OGTT protocol - Extended

Clinically significant dumping syndrome occurs in approximately 10% of patients after any type of gastric surgery and may be classed as "early" or "late." Late dumping occurs 1-3 hours after a meal and may be diagnosed by a characteristsic hyperinsulinaemic (reactive) hypoglycaemia upon glucose challenge (OGTT.)

 

Background

  • Gastric motility is regulated by the enteric nervous system, which is influenced by extrinsic innervation and by circulating hormones. Alterations in gastric anatomy after surgery or interference in its extrinsic innervation (vagotomy) may have profound effects on gastric emptying and are collectively referred to as the postgastrectomy syndromes.
  • Postgastrectomy syndromes include small capacity, dumping, bile gastritis, afferent loop syndrome, efferent loop syndrome, anaemia and metabolic bone disease. In the late dumping syndrome, rapid delivery of a meal to the small intestine results in an initial high concentration of carbohydrates in the proximal small bowel and rapid absorption of glucose. This is countered by a hyperinsulinemic response which is responsible for the subsequent hypoglycaemia.
  • Reactive hypoglycaemia is also thought to presage development to Type 2 diabetes. Further causes include an increased sensitivity to adrenaline production, a reduced synthesis or effectiveness of the hormone glucagon in addition to hereditary fructose intolerance.

Indications

  • Investigation of recurrent episodes of hypoglycaemia in non-diabetic patients
  • The late dumping syndrome is suspected in the person who has symptoms of hypoglycaemia in the setting of previous gastric surgery

Preparation

  • Glucose solution is best prepared in advance as glucose dissolves better in warm water but is more palatable when cold. On the day prior to the test, dissolve 75 grams of anhydrous glucose (82.5 grams of glucose monohydrate, obtainable pre-weighed from pharmacy) in warm water and store in a fridge overnight.
  • The patient must be fasted from midnight (sips of water only.)

Procedure

  • Blood samples are to be collected at "0" minutes for plasma glucose (yellow-top,) and serum insulin and c-peptide (white-top.)  Label tubes “0 minutes.”
  • Immediately afterwards give 75g oral glucose.
  • Place tubes in bag labelled as: “Extended Glucose Tolerance Test.”  Tubes are to be brought directly to the Biochemistry lab and handed to BMS staff directly for immediate processing.
  • Further blood samples are to collected at: 30', 60', 120' and 180' for glucose, insulin and c-peptide.  Label each tube clearly with the time at which it was taken.  Take labelled tubes to lab after drawing each set of samples.
  • If patient has symptoms of hypoglycaemia check glucose on finger prick. If <4 mmol/L take further blood for glucose, insulin and c-peptide.
  • If patient is very unwell with hypoglycaemia or has a finger prick glucose <2.2 mmol/L take blood for glucose, insulin and c-peptide immediately and terminate test with dextrose tablets, followed by a snack.
  • At the end of test, give the patient a snack.

The lab will measure each of the glucose levels and store the insulin and c-peptide tubes.  If any glucose measurements are <2.2 mmol/L all samples will be referred for insulin and c-peptide assay accordingly.

References

This investigation is taken from "Endobible: Practical guidance on endocrine diagnosis and management," K. Meeran et al. (http://www.endobible.com/investigation/prolonged-glucose-tolerance-test/)