The Oral Glucose Tolerance Test (OGTT) is a common medical procedure designed to measure how efficiently your body processes glucose. This test is frequently used to screen for or diagnose conditions like gestational diabetes during pregnancy, as well as prediabetes and type 2 diabetes in non-pregnant individuals. The test assesses the body’s response to a specific, concentrated dose of glucose, providing healthcare providers with a detailed profile of glucose metabolism.
Standard Volumes and Glucose Concentrations
The amount of glucose administered during the test is standardized, but the corresponding volume of liquid can vary slightly depending on the concentration and manufacturer. For the initial screening test, especially during pregnancy, the standard is a 50-gram glucose load. This concentrated sugar is typically dissolved into a solution measuring between 8 and 10 fluid ounces (approximately 237 to 296 milliliters). This volume is consumed quickly to provide a sudden, measured spike in blood glucose.
For the more comprehensive diagnostic test, the glucose concentration increases. A 75-gram glucose load, commonly used to diagnose diabetes in non-pregnant adults, is generally mixed into a similar volume of 8 to 10 fluid ounces. The highest concentration is the 100-gram load, typically reserved for the three-hour diagnostic test for gestational diabetes, and is also often provided in a volume around 10 ounces.
The composition of the drink is primarily dextrose, purified water, and various flavorings like orange or lemon-lime to make it easier to consume. The high sugar concentration provides the necessary challenge to the body’s glucose regulation system.
Preparing for and Completing the Test
Proper preparation is necessary to ensure the accuracy of the OGTT results. For three days leading up to the test, you are instructed to maintain a normal diet that includes at least 150 grams of carbohydrates daily. This ensures that your body’s usual glucose processing mechanisms are fully active. You must fast for a minimum of eight hours before the test begins, allowing only sips of water.
The test procedure begins with a baseline blood draw to measure your fasting glucose level. Immediately after this first draw, you must consume the entire glucose solution within a short timeframe, usually no more than five minutes. This quick consumption standardizes the start of the glucose load and the subsequent metabolic response.
After drinking the solution, you must remain at the testing facility for the duration of the test, which can be one, two, or three hours. During this waiting period, you must stay seated and avoid strenuous physical activity, smoking, or eating, as these factors can interfere with the test results. Additional blood samples are drawn at precise intervals, often at the one-hour, and then at the two- and three-hour marks for the longer diagnostic tests.
Managing Common Side Effects
The rapid consumption of a highly concentrated sugar solution can sometimes cause temporary physical discomfort. Common side effects include nausea, stomach upset, or bloating. Some people may also experience lightheadedness or dizziness, particularly during the prolonged waiting period. These reactions are usually transient and result from the sudden influx of glucose into the bloodstream.
To help manage these symptoms, maintaining a seated and still position is recommended, as this reduces the risk of dizziness. If you feel nauseated, focusing on a distraction, such as reading or listening to music, can be helpful. Small sips of plain water are typically allowed and can help settle the stomach. If vomiting occurs shortly after drinking the solution, the test cannot be completed and must be rescheduled.