the 2-Hour Oral Glucose Tolerance Test and Diabetes Screening

Oral glucose tolerance test at the 2-hour mark and others?

Hello, doctor.
I would like to ask a few questions:
1.
To check for diabetes, should I perform the 2-hour blood glucose test after a 75g oral glucose tolerance test, or should I do a 2-hour postprandial blood glucose test (checking after eating)?
2.
If I need to do the 2-hour blood glucose test after the 75g oral glucose tolerance test, should I first have my fasting blood glucose drawn and then proceed with the 75g oral glucose tolerance test followed by the 2-hour blood glucose check?
3.
Will taking psychiatric medications such as Olanzapine (Zyprexa) and Aripiprazole (Abilify) affect the blood glucose test results?
4.
If I have a fasting period of 12 hours (for lipid testing) versus 8 hours, will the fasting duration make a difference in the blood glucose test results?
5.
For fasting blood glucose testing, is the fasting period 8 hours, 12 hours, or can it be anywhere between 8 to 12 hours?
Finally, thank you for your response, doctor.

Putao, 30~39 year old female. Ask Date: 2015/06/25

Dr. Chen Qingbin reply Internal Medicine

The diagnostic criteria for diabetes mellitus are as follows: 1.
Fasting blood glucose (after fasting for more than 8 hours) > 126 mg/dL 2.
Postprandial blood glucose (2 hours after a meal) > 200 mg/dL 3.
Hemoglobin A1c > 6.5% 4.
Glucose tolerance test: blood glucose > 200 mg/dL at the 2-hour mark.
Confirmatory testing is required for all results.

Reply Date: 2015/06/25

More Info

The 2-Hour Oral Glucose Tolerance Test (OGTT) is a critical diagnostic tool for assessing glucose metabolism and diagnosing diabetes. Here’s a comprehensive overview addressing your questions and providing additional insights into the test and its implications.

1. OGTT vs. Postprandial Blood Sugar Test: To diagnose diabetes, the OGTT is preferred over a standard postprandial blood sugar test. The OGTT involves administering a specific dose of glucose (typically 75g) and measuring blood sugar levels at intervals, particularly at the 2-hour mark. This method provides a more controlled environment to assess how well your body processes glucose. In contrast, a postprandial blood sugar test measures blood sugar levels after eating, which can be influenced by various factors such as the type and amount of food consumed.

2. Procedure for OGTT: Yes, the procedure for the OGTT involves fasting for at least 8 hours before the test. Initially, a fasting blood sample is taken, followed by the ingestion of the glucose solution. After 2 hours, another blood sample is drawn to measure how effectively your body has processed the glucose. This structured approach helps in accurately determining your glucose tolerance.

3. Impact of Medications on Blood Sugar: Certain psychiatric medications, including those you mentioned (like Olanzapine), can indeed affect blood sugar levels. These medications may induce insulin resistance or alter glucose metabolism, potentially leading to elevated blood sugar levels. It’s essential to inform your healthcare provider about all medications you are taking, as they can influence test results and overall diabetes management.

4. Fasting Duration for Blood Tests: The standard fasting duration for blood tests, including glucose testing, is typically 8 to 12 hours. For the OGTT, an 8-hour fast is sufficient. However, if you are undergoing other tests, such as lipid panels, that require a 12-hour fast, it’s crucial to follow the specific instructions provided by your healthcare provider. Generally, maintaining a consistent fasting duration is important for accurate results.

5. Fasting Guidelines: For glucose testing, an 8-hour fast is generally acceptable. However, if you are also undergoing tests that require a longer fasting period (like lipid panels), it may be necessary to fast for up to 12 hours. It’s best to follow your healthcare provider’s recommendations regarding fasting times to ensure the accuracy of all tests performed.

Additional Insights on OGTT and Diabetes Screening
The OGTT is particularly useful for diagnosing conditions like prediabetes and gestational diabetes. According to the American Diabetes Association, the diagnostic criteria for diabetes include:
- Fasting plasma glucose ≥ 126 mg/dL
- 2-hour plasma glucose during OGTT ≥ 200 mg/dL
- A1C ≥ 6.5%
- Random plasma glucose ≥ 200 mg/dL in a patient with classic symptoms of hyperglycemia
If your 2-hour glucose level is ≥ 200 mg/dL during the OGTT, it indicates diabetes. If it’s between 140 mg/dL and 199 mg/dL, it suggests prediabetes, which is a critical stage where lifestyle changes can prevent the progression to diabetes.

Lifestyle Considerations
Regardless of the test results, maintaining a healthy lifestyle is crucial. This includes:
- Diet: Focus on a balanced diet rich in whole grains, lean proteins, healthy fats, and plenty of fruits and vegetables. Limit processed foods, sugars, and saturated fats.

- Exercise: Regular physical activity helps improve insulin sensitivity and glucose metabolism. Aim for at least 150 minutes of moderate aerobic activity each week.

- Monitoring: Regular monitoring of blood sugar levels can help you understand how your body responds to different foods and activities.

In conclusion, the 2-hour OGTT is a vital tool in diagnosing diabetes and assessing glucose tolerance. Understanding the procedure, potential medication effects, and the importance of lifestyle changes can significantly impact your health outcomes. Always consult with your healthcare provider for personalized advice and follow-up testing as needed.

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