Oral Glucose Tolerance Test (OGTT) – What do you need to know?

If you’re expecting a baby, you’re probably familiar with an oral glucose tolerance test (OGTT). This diagnostic assessment is often carried out between the 24th to 28th week of pregnancy or earlier if the mother-to-be has risk factors for gestational diabetes. Read on to find out everything you need to know about an oral glucose tolerance test. 

What is an oral glucose tolerance test (OGTT)?

Generally, an oral glucose tolerance test refers to a type of blood test used to measure how well your body processes glucose or sugar. This diagnostic procedure involves drinking a concentrated glucose solution and having your blood samples taken at various intervals. 

How is the oral glucose tolerance test (OGTT) done?

During pregnancy, the OGTT procedure works by firstly fasting overnight for at least a few hours, followed by taking in a glucose drink containing about 75 grams of glucose. Blood samples are then taken at intervals to measure blood glucose levels, usually before and 2 hours after consumption.

What is the purpose of the oral glucose tolerance test (OGTT)?

• Diabetes screening: OGTT is commonly used to screen for type 2 diabetes, gestational diabetes mellitus (GDM) during pregnancies, and impaired glucose tolerance (a precursor to diabetes).

• Assessment of insulin resistance: OGTT can also help diagnose conditions related to insulin resistance, such as polycystic ovary syndrome (PCOS).

Importance of doing oral glucose tolerance test (OGTT)

Early detection of diabetes:

OGTT can detect abnormalities in glucose metabolism before symptoms of diabetes appear, allowing for early intervention and management.

Gestational diabetes screening:

Identifying gestational diabetes early in pregnancy is crucial for reducing the risk of complications for both the mother and baby.

Personalised treatment: 

OGTT results help healthcare providers tailor treatment plans, including lifestyle modifications, medication, and monitoring, to manage diabetes effectively.

Risk Assessment: 

OGTT helps assess an individual's risk of developing diabetes or other metabolic disorders, allowing for targeted preventive measures.

For pregnant women, the criteria for a diagnosis of gestational diabetes usually include meeting or exceeding one or more of the following blood glucose thresholds:

• Fasting blood glucose level: 5.1 mmol/L or higher

• Two-hour blood glucose level: 7.8 mmol/L or higher

FAQs

Where can oral glucose tolerance test (OGTT) be done?

The oral glucose tolerance test (OGTT) is typically conducted in healthcare facilities, such as hospitals, clinics, or specialised diagnostic centres. 

What are the procedures of OGTT?

1. Preparation: Before the test, you will need to fast overnight (around 8-12 hours) to ensure accurate results. For fasting, avoid consuming any food or drinks except water.

2. Arrival: Upon arrival at your hospital or clinic on the day of the test, make sure to inform your healthcare provider of any medication taken, as some drugs can affect the results.

3. First Blood Sample: A healthcare provider will draw a blood sample from your arm to measure your fasting blood glucose level.

4. Glucose Drink: Following the taking of the initial blood sample, you will be given a sugary drink containing a specific amount of glucose to consume within a few minutes.

5. Waiting Period: You will wait for a specified time (usually two hours) without eating or drinking anything other than water. Remain seated and avoid strenuous activity.

6. Second Blood Sample: Another blood sample will be drawn to measure your blood glucose level again. This sample will determine how well your body processes glucose over time.

7. Results: The OGTT results will show if your body effectively manages glucose or if there are signs of impaired glucose tolerance, which may suggest prediabetes or diabetes.

What are the risks and side effects of OGTT?

While the OGTT is generally considered safe, there are possible side effects associated with it.

• Nausea: Some may feel nauseous after taking the glucose solution if they have a sensitive stomach or if the solution is particularly sweet.

• Dizziness or light-headedness: Drinking a concentrated solution on an empty stomach can cause an increase in blood sugar levels, leading to feeling dizzy or lightheaded.

• Blood draw discomfort: Some may feel discomfort or bruising at the blood draw site.

• Allergic Reaction: Although uncommon, some may see an allergic reaction to the glucose solution or materials used, such as the needle or tape used to fasten the IV line.

• Hypoglycemia: In rare cases, the body may produce too much insulin, causing blood sugar levels to drop too low. This can result in sweating, trembling, confusion, or fainting.

• False Positive or False Negative Results: While the OGTT is a valuable diagnostic tool, false-positive results (indicating a condition when it's not present) or false-negative results (missing a condition when it is present) can occur, leading to unnecessary worry.

When should I do my oral glucose tolerance test (OGTT)?

Doing an oral glucose tolerance test (OGTT) depends on your age, medical history, diabetes risk factors, and whether or not you're pregnant. Here are some key guidelines:

Non-pregnant adults:

Type 2 Diabetes screening: If you have no known risk factors, an OGTT is unneeded. Instead, you may be recommended periodic fasting blood glucose or A1C tests starting at age 45.

Prediabetes assessment: If you have risk factors for diabetes (e.g., obesity, family history, sedentary lifestyle) or if fasting blood glucose or A1C tests are borderline, an OGTT may be recommended to you

Diabetes Diagnosis: If fasting blood glucose or A1C tests indicate diabetes, an OGTT may be used to confirm the diagnosis.

Pregnant women:

Gestational diabetes (GDM) screening: An OGTT is performed in 24-28 weeks of gestation. This is the recommended time for screening gestational diabetes, but it may be done sooner in a pregnancy if you have gestational diabetes risk factors or present diabetic symptoms. 

Other situations:

Polycystic Ovary Syndrome (PCOS): You may be recommended for an OGTT if you have PCOS and are at risk of diabetes and insulin resistance.

Assessment of Insulin Resistance: An OGTT may be used to assess your insulin resistance in certain situations, such as suspected metabolic syndrome or acanthosis nigricans.