Oral glucose tolerance test
An oral glucose tolerance test (75 g glucose load) has no place in the diagnosis of type 1 diabetes and is used less often in diagnosing type 2 diabetes because glycated haemoglobin (HbA1c) testing and venous blood glucose concentrations are simpler to perform. However, the oral glucose tolerance test is the only test that can detect impaired glucose tolerance and can concurrently detect impaired fasting glucose.
An oral glucose tolerance test can be used to:
- diagnose type 2 diabetes when fasting venous blood glucose concentration alone can underdiagnose diabetes because of different patterns of hyperglycaemia (eg women with polycystic ovary syndrome or those taking a glucocorticoid)
- detect impaired glucose tolerance or diabetes in asymptomatic patients, particularly if the patient has prediabetes defined by impaired fasting glucose or elevated HbA1c—identifying patients with impaired glucose tolerance is important if it will affect management decisions (eg whether to start general lifestyle modification or a structured, intensive lifestyle intervention—see Management of people at risk of developing type 2 diabetes)
- confirm a diagnosis of gestational diabetes in pregnant women—specific diagnostic thresholds apply during pregnancy; see Detection and diagnosis of hyperglycaemia in pregnancy
- detect impaired glucose tolerance following hyperglycaemia in pregnancy (including gestational diabetes)—see Tests for postpartum screening of women with hyperglycaemia in pregnancy (including gestational diabetes).
The oral glucose tolerance test should not be used in patients who have already met the diagnostic thresholds for diabetes based on other tests.
If using an oral glucose tolerance test to diagnose diabetes, it must be performed in a standardised manner; see Procedure for performing an oral glucose tolerance test.
The two-hour 75 g oral glucose tolerance test used to screen for diabetes must be performed in a standardised manner.
- Instruct the patient to fast overnight for 8 to 12 hours (usually from 10 pm) and avoid tobacco smoking and drinking caffeine-containing drinks (eg coffee, tea, cola). Water is permitted until the glucose is taken [NB1] [NB2].
- Schedule the oral glucose tolerance test in the morning, starting before 10 am.
- Take a fasting venous blood sample just before the patient drinks the 75 g glucose drink. The glucose drink must be consumed within 5 minutes.
- Instruct the patient to remain seated and not eat or drink (including water) for the 2-hour duration of the test.
- For nonpregnant adults, take a venous blood sample 2 hours after the patient consumed the glucose drink. For children, adolescents and pregnant women only, take a venous blood sample at 1 hour and 2 hours after the glucose drink is consumed.
NB1: A carbohydrate-loading diet is no longer recommended before an oral glucose tolerance test.
NB2: Some patients have a hypoglycaemic response to the test, and some patients feel transiently lightheaded or weak after an oral glucose tolerance test. Advise the patient to take some food with them to the test, so that they can eat as soon as the last blood sample is taken.