Oral Glucose Tolerance Test (OGTT)

Throughout the duration of the test you must not drink, eat or smoke.

  • You must remain seated in the waiting room, as exercise increases glucose metabolism.
  • You may go to the bathroom if necessary.
  • You must inform the healthcare professional if you feel unwell.
  • If you vomit the drink, the test may not be completed. The prescription will hence be given back to you and you will need to come back to take the test again.

Please ensure that the sampling centre is open for the full duration of the test!

What are the test proceedings?

  • A drop of blood is taken from a finger and blood sugar levels are measured. The healthcare professional will judge if the OGTT can be performed, according to the results. An OGTT should not be performed in case of a confirmed diabetes diagnosis.
  • If blood sugar levels are outside normal ranges, the test cannot be performed. In this case, it is customary to replace it with a timed glucose test (in the morning in a fasted state, at 11 am and at 3 pm).
  • To avoid multiple venepunctures, a catheter is placed in the arm
  • A sugary drink (75 g of glucose) is provided. It must be drunk within 5 minutes.
  • Blood is drawn from the catheter before glucose intake, then at 30 minutes, 60 minutes, 90 minutes and 120 minutes after intake. In certain cases (hypoglycaemia tests), samples will also be taken after 180 minutes and 240 minutes.

Can I eat and drink before having the blood test?

No, this is a “fasting blood test", you are hence not allowed to either eat or drink anything (other than water) for 8 hours before having the test.

Can I take medication before having the blood test?

Some medications may affect test results. If you are taking medication, please consult your doctor to find out if you can take it in the morning before the test.

The oral glucose tolerance test is performed to screen for gestational diabetes.

  • The procedure for pregnant women is the same as the one described above.
  • The test is generally performed between the 24th and the 28th week of pregnancy when insulin resistance usually begins. In the case of a very high risk of gestational diabetes, the test can be performed earlier.
  • 3 blood samples are necessary (before glucose intake, after 60 minutes and after 120 minutes).
  • Values are different for pregnant women:

Fasting blood glucose    < or = 0.92 g/L

Blood glucose 1H            < or = 1.80 g/L

Blood glucose 2H            < or = 1.53 g/L

  • Some years ago, OGTT was only performed if the O’Sullivan test was positive. However, due to its low sensitivity, the O’Sullivan test is no longer recommended.

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