Clinical Significance

   A 2-hour 75 g oral glucose tolerance test (OGTT / GTT) may also be used to diagnose diabetes (and acromegaly).

   This test shows how your body utilises a known amount of glucose (sugar) which will be given to you as a drink. A high blood glucose concentration approximately 2 hours after taking the glucose suggests that you have gestational diabetes (diabetes in pregnancy). You will be given instructions to fast before the test, a blood sample will be taken and you will then be given a drink containing a known amount of glucose. Another blood sample is taken 2 hours later and the glucose concentration in the blood measured.

Specimen

   Serum

Stability

   Refrigerated (preferred) : 7 days

   Frozen : 30 days

Interpretation

Diagnosis of Gestational Diabetes (OGTT)

   Either of below is used for diagnosis, however other thresholds are also in use.

       A fasting plasma glucose level of 5.6 mmol/L or above

       A 2-hour plasma glucose level of 7.8 mmol/L or above

   Some of the other diseases and conditions that can result in elevated glucose levels include:

       Acromegaly

       Acute stress (response to trauma, heart attack, and stroke for instance)

       Long-term kidney disease

       Cushing syndrome

       Drugs, including: corticosteroids, tricyclic antidepressants, diuretics, adrenaline, oestrogens (birth control pills and hormone replacement therapy [HRT]), lithium, phenytoin (Dilantin), aspirin

       Excessive food intake

       Hyperthyroidism

       Pancreatic cancer

       Pancreatitis

   Moderately increased levels may be seen with impaired glucose tolerance. This condition, if left un-addressed, can lead to type 2 diabetes mellitus. Low glucose levels (hypoglycaemia) are seen with:

       Adrenal disease (Addison disease)

       Drinking alcohol

       Drugs, such as: paracetamol and anabolic steroids

       Extensive liver disease

       Hypopituitarism

       Hypothyroidism

       Insulin overdose

       Overdose of glucose lowering medications

       Insulinomas (insulin-producing pancreatic tumours)

       Starvation

Limitation

   The OGTT results can be affected by carbohydrate intake and duration of fasting preceding the test, the time of day the test is performed and carbohydrate intake or activity during the test. During the 3 days preceding the OGTT, 150 g of carbohydrate should be eaten (approximately ten 40 g slices of bread per day).