Question: How is GTT test taken?
Answer: The patient is instructed not to restrict carbohydrates intake in the days or weeks before the test.The test should not be done during an illness, as results may not reflect the patient's glucose metabolism when healthy. A full adult dose should not be given to a person weighing less than 42.6 kg (94 lb), or the excessive glucose may produce a false positiveresult. Usually the OGTT is performed in the morning as glucose tolerance can exhibit a diurnal rhythm with a significant decrease in the afternoon. The patient is instructed to fast (water is allowed) for 8–12 hours prior to the tests. Medication such as large doses of Salicylates, Diuretics, Anti-Convulsants, and oral contraceptives affect the glucose tolerance test
A zero time (baseline) blood sample is drawn.
The patient is then given a measured dose (below) of glucose solution to drink within a 5-minute time frame.
Blood is drawn at intervals for measurement of glucose, and sometimes insulin levels. The intervals and number of samples vary according to the purpose of the test. For simple diabetes screening, the most important sample is the 2 hour sample and the 0 and 2 hour samples may be the only ones collected. A laboratory may continue to collect blood for up to 6 hours depending on the protocol requested by the physician.
Dose of glucose and variations
75g of oral dose is the recommendation of the WHO to be used in all adults,and is the main dosage used in the United States.The dose is adjusted for weight only in children.The dose should be drunk within 5 minutes.
A variant is often used in Pregnancy to screen for gestational diabetes, with a screening test of 50 grams over one hour. If elevated, this is followed with a test of 100 grams over three hours.
In UK general practice, the standard glucose load was provided by 394ml of the energy drink with original carbonated flavour, but this is being superseded by purpose-made drinks.
In Portugal, the standard glucose load is provided by the clinical laboratory or hospital by 200 ml of fluid in a PET bottle. The recommendation is for a 75g oral dose for all adults, which is adjusted for weight in children. However, doses of 50g and 100g are also used, available in orange, lemon and cola flavor.
Substances measured and variations
If renal glucose urea (sugar excreted in the urine despite normal levels in the blood) is suspected, urine samples may also be collected for testing along with the fasting and 2 hour blood tests.
Fasting plasma glucose (measured before the OGTT begins) should be below 6.1 mmol/L (110 mg/dL). Fasting levels between 6.1 and 7.0 mmol/L (110 and 125 mg/dL) are borderline ("impaired fasting glycemia"), and fasting levels repeatedly at or above 7.0 mmol/L (>126 mg/dL) are diagnostic of diabetes.
A 1 hour GTT (Glucose Tolerance Test)glucose level below 10 mmol/L (180 mg/dL) is considered normal.
For a 2 hour GTT (Glucose Tolerance Test)with 75g intake, a glucose level below 7.8 mmol/L (140 mg/dL) is normal, whereas higher glucose levels indicate hyperglycemia. Blood plasma glucose between 7.8 mmol/L (140 mg/dL) and 11.1 mmol/L (200 mg/dL) indicate "impaired glucose tolerance", and levels above 11.1 mmol/L (200 mg/dL) at 2 hours confirm a diagnosis of diabetes.