Preferred Sample Type

Glucose Tolerance Test (OGTT)

Suitable Specimen Types

  • Fluoride Oxalate
  • Li Hep Plasma
5-10 mL vacutainer (minimum sample volume 50 uL

Sample Processing in Laboratory


Sample Preparation


Turnaround Time

1 day

Sample Stability


Glucose Tolerance Test (OGTT)

General Information

The fasting blood glucose level (collected after an 8 to 10 hr fast) is used to screen for and diagnose diabetes. An oral glucose tolerance test (OGTT / GTT) may also be used to diagnose diabetes. To be certain of a diagnosis of diabetes two tests (either the fasting glucose or the OGTT) should be done at different times. The OGTT involves a fasting glucose measurement, followed by the patient drinking a glucose drink to 'challenge' their system, followed by another glucose test two hours later.

Gestational diabetes is a temporary type of hyperglycaemia seen in some pregnant women, usually late in their pregnancy. Almost all pregnant women are screened for gestational diabetes between their 24th and 28th week of pregnancy using a test for glucose in the urine. If glucose is detected in the urine, they are considered at risk of developing gestational diabetes and they will undergo further testing.

The OGTT is used for the diagnosis of Diabetes Mellitus and when previous investigations have indicated a fasting glucose of 6.1-6.9 mmol/L.


Patient Preparation

  • Please ensure that stressful procedures are not performed within 48 hours before the GTT.
  • The patient should be on a normal carbohydrate diet (more than 150g) for at least 3 days before the GTT. Any carbohydrate restriction ordered for dietary or other reasons must be reversed before the test.
  • The patient is fasted overnight and allowed water or tea without sugar but no food or other liquids. No smoking is allowed during the morning of the test.

See the Clinical Guidelines section of the Laboratory Medicine websire for more information.


Appointments for glucose tolerance tests can be made via Diabetes Centre on 0121 424 0234. A service is available at Solihull - for information contact Laboratory Reception at Solihull on 0121 424 5531 / 0121 424 5526.

As of 25.07.19, GTTs are reported against pregnancy cut-offs

Reference Range

Levels are influenced by many factors including previous carbohydrate diet, drugs, gastric emptying time and gastrointestinal surgery, as well as circulating insulin levels.

The following criteria for adults and children are based on recommendation made by the World Health Organisation and have been adopted by Diabetes UK.


Glucose concentration in mmol/L

Venous plasma

Capillary plasma

Diabetes mellitus

Fasting value



2h after glucose load



Impaired Glucose


Fasting Value

< 7.0

< 6.1

2h after glucose load

7.8 & <11.1


Impaired Fasting



6.1 & <7.0

5.6 & <6.1

2h after glucose load

< 7.8

< 7.8


NOTE. If the fasting blood glucose concentration is in the diabetic range on more than one occasion, the oral GTT is superfluous.

The same criteria are used for pregnant patients but those fulfilling the criteria of impaired glucose tolerance and diabetes mellitus have gestational diabetes mellitus (GDM).

'Diabetic' levels of glucose are not infrequently found in stressed individuals.








  • EQA Scheme?: Yes

The laboratories at Heartlands Hospital, Good Hope Hospital and Solihull Hospital form part of the services provided by University Hospitals Birmingham and are UKAS (United Kingdom Accreditation Service) accredited to the ISO 15189:2012 standard. For a list of accredited tests and other information please visit the UKAS website using the following link:

  • Heartlands, Good Hope and Solihull Hospital pathology laboratories are a UKAS accredited medical laboratory No.8217
  • United Kingdom Health Security Agency laboratory is a UKAS accredited medical laboratory No.8213

Tests not appearing on the UKAS Schedule of Accreditation currently remain outside of our scope of accreditation. However, these tests have been validated to the same high standard as accredited tests and are performed by the same trained and competent staff.

For further test information, please visit the test database:

Protection of personal information - Laboratory Medicine comply with the Trust Data Protection policy and have procedures in place to allow the Directorate and its employees to comply with the Data Protection act  1998 and associated best practice and guidance.

For further information contact Louise Fallon, Quality Manager, 0121 424 1235