Glucose Tolerance Test (OGTT) protocol

The Glucose Tolerance Test is typically employed in the assessment of gestational diabetes and impaired glucose tolerance in pregnancy...


  • The glucose tolerance test is used in the diagnosis of diabetes, impaired glucose tolerance, insulin resistance and reactive hypoglycaemia. Please note: the OGTT is also used in the diagnosis of acromegaly - please see “Growth hormone suppression test” for further details(
  • Despite difficulties encountered with regards standardising the OGTT format, reproducibility of results, cost and inconvenience of the test overall it has been retained by the World Health Organisation (WHO) on the grounds that it remains the only means of assessing impaired glucose tolerance. Moreover, for diagnosis of diabetes the OGTT is selective for an additional cohort of patients that would have otherwise been missed through assessment of fasting glucose alone (indeed where the mode of diagnosis is by way of the two-hour plasma glucose result (as opposed to a lone fasting glucose reading) these patients tend to have worse outcomes in terms of cardiovascular morbidity and mortality*.) 


  • This test should NOT be performed in patients who fulfil the criteria for diabetes mellitus. These are 1) a fasting plasma glucose >7.0 mmol/L on two or more occasions or 2) clinical symptoms of diabetes e.g. polydipsia, polyuria, ketonuria and rapid weight loss with a random plasma glucose of >11.1 mmol/L.
  • This test should not be performed in patients who are under physical stress e.g. post-surgery, trauma or infection or extreme psychological stress as these may give misleading results.
  • This test should not be performed in patients with periodic hypokalaemic paralysis.


  • 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.)


  • Check that the patient has fasted from midnight.
  • Perform venepuncture taking 2mls of blood; test a small sample using a near patient glucose testing meter. Other baseline bloods should be taken if requested by referring clinician.
  • Place the remainder of the blood sample into a fluoride oxalate bottle and label with patient identification, date and “0” minutes in time on sample.
  • If the result on the glucose meter is greater than or equal to 11mmol/L, send the blood sample urgently to lab. If it is confirmed by biochemistry to be above 11mmol/L, there is no need to continue test, and the patient can go home.
  • If the result is less than 11mmol/L on meter, give the patient the glucose solution to drink (within 10 minutes.)
  • Collect a further blood sample in a fluoride oxalate bottle for glucose measurement two hours after the glucose solution has been given. Blood samples must be labelled with patient details, date and time of sample (0 and 120 minutes.)
  • Send samples all together with request form to laboratory.

Expected Results



[Glucose] 0'

[Glucose] 120'

Non-diabetic <6.0mmol/L <7.8mmol/L
Impaired glucose tolerance 6.1 - 6.9mmol/L 7.9 - 11.0mmol/L
Diabetic >7.0mmol/L >11.1mmol/L


This investigation is taken from "Endobible: Practical guidance on endocrine diagnosis and management," K. Meeran et al. (

*"Definition and diagnosis of Diabetes Mellitis and intermediate hyperglycaemia" World Health Organisation, 2006