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Glucose Tolerance Test


  • Suspected diabetes mellitus. An oral glucose tolerance test is not required if the diagnosis of diabetes is not in doubt or if a fasting venous plasma glucose is greater than 7.0 mmol/litre or a random venous plasma glucose is greater than 11.1 mmol/l.
  • In acromegaly, to establish the diagnosis and to follow patients after treatment with surgery or irradiation.
  • Suspected reactive hypoglycaemia.




Nausea and occasional vomiting


The subject should have been on a diet containing an adequate amount of carbohydrate (250g/day) for at least 3 days before the test
Overnight fast.
75g anhydrous glucose.
Fluoride oxalate tubes x 3 (grey top Vacutainers).
19g cannula.
Saline flush.
Syringes x3.


  • Diabetes
    Insert cannula.
    Take a baseline glucose at time 0.
    Give oral glucose load (75 g anhydrous glucose in 250-350ml water).
    Repeat blood samples at 60 and 120 min after glucose load.
  • Acromegaly
    See under "growth hormone" above.
  • Reactive hypoglycaemia
    Take blood glucose at -30, 0, 30, 60, 90, 120, 150 and 180 min.


WHO (established June 2000) for diabetes and impaired glucose tolerance
Plasma Glucose (mmol/l) Fasting 2 hrs after glucose load
Diabetes mellitus ³7.0 ³11.1
Impaired glucose tolerance >7.8 - 11.0
Impaired fasting glucose >6.1 - 7.0
Normal =6.1 =7.8
75 g oral glucose tolerance test.
In the absence of diabetic symptoms at least 2 abnormal values are necessary to establish a diagnosis of diabetes mellitus
Gestational diabetes: women who have IGT in pregnancy should be treated as if they have GDM.


These criteria were revised by the WHO in 1997 and remain arbitrary. Remember that acute illness (e.g. myocardial infarction) and drugs may affect glucose tolerance.
In acromegaly it is very rare for GH to suppress to the normal range with a glucose load. In fact there is often a paradoxical rise in GH. Some normals especially if stressed do not suppress. The definition of "cure" in acromegaly is very difficult. Patients may show dramatic clinical improvement but not suppress with glucose.


Keen H., Medicine International May, 2672-2675 (1989).
Diabetes Care, 21 S1, 5-19 (1998).