Oral Glucose Tolerance Test. (OGTT)

Also known as Glucose Tolerance Test (GTT).

This test measures the body's ability to metabolise glucose.

What is the purpose of this test?
It is mainly used to screen pregnant women for gestational diabetes between 24 and 28 weeks of pregnancy. Occasionally it is used alongside the Fasting Blood Sugar test to confirm a diagnosis of diabetes.

How the test is done?
After fasting overnight a blood sample is taken. The patient is then give a solution of 75g of glucose in 300ml of water, which they have to drink within 5 minutes. Blood is then taken at half hour intervals for 2hours and tested for the amount of glucose present.

Sometimes urine is also collected before the test and after 1 and 2 hours and tested for sugars.

In pregnant women 100g of glucose may be given and the blood tested over a 3 hour period.

Understanding the results.
The levels of glucose for a non-diabetic with the 75g test would look like this:-
1. Fasting: 3.3-6.1mmol/L ( 60 to 110 mg/dL).
2. One hour: less than 11.1mmol/L (200 mg/dL).
3. Two hours: less than 7.8mmol/L (140 mg/dL).

Between 7.8 & 11.1mmol/L (140-200 mg/dL) is considered impaired glucose tolerance or pre-diabetes and is at risk of developing diabetes.

Greater or equal to than 11.1mmol/L (200 mg/dL) is positive for a diagnosis of diabetes mellitus.

The levels for the 100g used to screen for gestational diabetes would be:-
1. Fasting: less than 5.2mmol/L (95 mg/dL)
2. One hour: less than 10mmol/L (180 mg/dL)
3.Two hour: less than 8.6 mmol/L (155 mg/dL)
4. Three hour: less than 7.8 mmol/L (140 mg/dL)

Results of this test can be affected by:-
1. Any illness, even so mild a one as a cold.
2. Smoking or coffee prior to test.
3. Inactivity - such as being in bed in hospital.
4. Strenuous exercise for 8 to 14 hours prior to the test.
5. Medications such as corticosteroids, certain diuretics, anticonvulsants, oral contraceptives, nonsteroidal anti-inflammatory drugs (NSAIDs), and certain high blood pressure medications.

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