The Oral Glucose Tolerance Test (OGTT): A Guide to What to Expect

The Oral Glucose Tolerance Test (OGTT) is a crucial diagnostic tool used primarily to screen for and diagnose gestational diabetes during pregnancy and type 2 diabetes. If your doctor has recommended an OGTT, you might be feeling a bit apprehensive. This guide will walk you through the entire process, explaining what to expect before, during, and after the test. We'll also cover why the OGTT is performed and what the results mean.

Why is the OGTT Performed?

The OGTT measures your body's ability to process glucose (sugar). It's different from a standard fasting blood sugar test because it assesses how your body responds to a specific amount of glucose consumed orally. Here's a breakdown of common reasons for ordering an OGTT:

• Gestational Diabetes Screening: This is the most frequent use of the OGTT, typically performed between 24 and 28 weeks of pregnancy. Gestational diabetes can pose risks to both the mother and the baby, making early detection and management essential.
• Diagnosing Type 2 Diabetes: If a fasting blood glucose test or HbA1c test suggests prediabetes or potential diabetes, an OGTT may be performed to confirm the diagnosis.
• Investigating Insulin Resistance: Sometimes, even without meeting the diagnostic criteria for diabetes, the OGTT can help identify insulin resistance, a condition where the body's cells don't respond properly to insulin. This can be a precursor to type 2 diabetes.
• Diagnosing rarer conditions: In certain circumstances the OGTT can assist in identifying disorders of carbohydrate metabolism.

Preparing for Your OGTT

Proper preparation is key to ensuring accurate results. Your doctor will provide specific instructions, but here are some general guidelines:

1. Diet: For several days (usually 3 days) leading up to the test, you should maintain your normal diet. This should include at least 150 grams of carbohydrates per day. Do not drastically change your eating habits. Contact your doctor if you have concerns about following this diet.
2. Fasting: You'll need to fast for at least 8-14 hours before the test. This means no food or drinks (except water) after midnight. It's crucial to adhere to the fasting requirement, as any food or drink can affect your blood glucose levels.
3. Medications: Discuss all medications you are taking with your doctor. Some medications, like certain steroids or diuretics, can influence blood glucose levels and may need to be adjusted or temporarily stopped before the test. Do NOT stop taking any medication unless specifically instructed by your doctor.
4. Illness: If you're feeling unwell or have a fever, inform your doctor before the test. Illness can affect blood glucose readings, so it may be necessary to reschedule.
5. Avoid Strenuous Exercise: On the day before and the day of the test, avoid vigorous physical activity. This can deplete glycogen stores and affect your glucose levels.

What to Expect During the OGTT Procedure

The OGTT is typically performed in a doctor's office or a lab. Here's a step-by-step breakdown of what you can expect:

1. Initial Blood Draw: Upon arrival, a healthcare professional will draw a blood sample to measure your fasting blood glucose level.
2. Glucose Drink: You will then be asked to drink a liquid containing a specific amount of glucose. The amount varies depending on the type of test (usually 75 grams for a standard OGTT, and 50 grams for the gestational diabetes screening test during pregnancy if the healthcare provider chooses a 1-step approach) but the drink is normally a very sweet glucose solution. You'll need to consume the entire drink within a specified time frame (usually 5 minutes).
3. Waiting Period: After drinking the glucose solution, you'll need to remain at the facility and avoid eating, drinking (except water), or smoking for the duration of the test. This period typically lasts 2 hours, but may be longer depending on what the doctor orders. The technician may allow light activity (like walking around the room).
4. Subsequent Blood Draws: Blood samples will be drawn at specific intervals after you drink the glucose solution. The most common intervals are 1 hour and 2 hours after ingestion of the glucose. These blood samples measure how your blood glucose levels change over time in response to the glucose load.

Possible Side Effects During the Test:

Some individuals may experience mild side effects after drinking the glucose solution, including:

• Nausea
• Sweating
• Lightheadedness
• Feeling faint.

These symptoms are usually temporary and subside on their own. Inform the healthcare professional if you experience any of these side effects during the test. In extremely rare cases of more severe reactions (such as vomiting, fainting or severe dizziness), seek immediate medical attention.

Interpreting the OGTT Results

After the test, the blood samples will be sent to a laboratory for analysis. Your doctor will review the results and discuss them with you. The interpretation of the results varies slightly depending on the specific guidelines being followed. Here’s a general overview:

Gestational Diabetes:

• Different diagnostic criteria are used for diagnosing gestational diabetes, depending on the guidelines used. Generally, diagnosis is based on one or more elevated blood glucose values during the OGTT. Examples of the more common guidelines are the American Diabetes Association or the International Association of Diabetes and Pregnancy Study Groups.

Type 2 Diabetes:

The American Diabetes Association (ADA) defines diabetes based on the following thresholds:

• Fasting plasma glucose: ≥126 mg/dL (7.0 mmol/L)
• 2-hour plasma glucose during OGTT: ≥200 mg/dL (11.1 mmol/L)
• A1C: ≥6.5% (48 mmol/mol)
• Random plasma glucose: ≥200 mg/dL (11.1 mmol/L) in a patient with classic symptoms of hyperglycemia or hyperglycemic crisis

If one or more of your blood glucose levels exceed these thresholds, your doctor may diagnose you with diabetes.

Prediabetes:

• Prediabetes is when blood sugar levels are higher than normal, but not high enough to be diagnosed as type 2 diabetes. Prediabetes can be diagnosed with either the A1C, fasting blood sugar or OGTT.
• Fasting plasma glucose: 100 to 125 mg/dL (5.6 to 6.9 mmol/L)
• 2-hour plasma glucose during OGTT: 140 to 199 mg/dL (7.8 to 11.0 mmol/L)
• A1C: 5.7 to 6.4%

Important Note: These values are general guidelines and might vary based on specific lab procedures or individual patient factors. Always consult with your doctor for an accurate interpretation of your results.

What Happens After the OGTT?

The course of action after the OGTT depends entirely on the results.

• Normal Results: If your blood glucose levels are within the normal range, you may not need any further testing. However, your doctor may recommend periodic follow-up screenings, especially if you have risk factors for diabetes.
• Gestational Diabetes Diagnosis: If diagnosed with gestational diabetes, you'll work with your healthcare team to manage your blood glucose levels through diet, exercise, and sometimes medication (insulin). Regular monitoring and follow-up appointments are essential.
• Diabetes Diagnosis: If diagnosed with type 2 diabetes, your doctor will develop a treatment plan that may include lifestyle changes (diet and exercise), oral medications, or insulin injections.
• Prediabetes Diagnosis: If diagnosed with prediabetes, lifestyle changes, such as diet and exercise, are highly recommended to help prevent or delay the onset of type 2 diabetes. Your doctor may also recommend regular monitoring of your blood glucose levels.

Conclusion

The Oral Glucose Tolerance Test (OGTT) is an important diagnostic tool that helps identify diabetes and prediabetes. By understanding what to expect before, during, and after the test, you can approach the procedure with confidence. Remember to follow your doctor's instructions carefully and discuss any concerns or questions you may have. Early detection and appropriate management of diabetes are essential for maintaining good health and preventing long-term complications.