The Oral Glucose Tolerance Test (OGTT) is a diagnostic procedure used to assess how the body processes glucose, the primary source of energy derived from food. It is frequently performed to screen for gestational diabetes during pregnancy or to identify prediabetes (impaired glucose tolerance) or diabetes in non-pregnant individuals. The OGTT measures the body’s response to a high dose of sugar over a timed period, providing a detailed picture of metabolic function.
Essential Preparation Steps
Proper preparation in the days leading up to the test is necessary to ensure accurate results. For at least three days before the appointment, you must maintain your regular diet, ensuring a normal intake of carbohydrates. This means consuming a minimum of 150 grams of carbohydrates daily. Carbohydrates are found in foods like bread, cereals, fruits, and milk. Maintaining this typical intake prevents the body from adapting to a low-carb state, which could incorrectly elevate test results.
Avoid any strenuous physical activity for at least three days before the test, including the morning of the procedure. Intense exercise can alter glucose metabolism and lower blood sugar levels, potentially leading to misleading results. You must also speak with your healthcare provider about any medications you are taking, as some prescriptions and over-the-counter drugs can interfere with glucose readings.
You must fast for a minimum of eight hours, but generally no more than 14 hours, immediately before the initial blood draw. Fasting means absolutely no food, no calorie-containing liquids, smoking, or chewing gum (even sugarless varieties). Only plain water is permitted during this time, and staying well-hydrated is recommended. This overnight fast establishes a baseline measurement of your body’s glucose level before the sugar challenge begins.
Navigating the Test Day Procedure
The procedure begins immediately upon arrival with a blood draw to measure your fasting glucose level. This initial draw serves as the reference point against which all subsequent measurements will be compared. Following this baseline test, you will consume a specific, measured amount of concentrated glucose solution, typically 75 or 100 grams of glucose. This liquid must be consumed entirely within a short timeframe, usually five minutes.
After drinking the solution, you must remain at the testing facility for the duration of the test, typically two or three hours. During this waiting period, you are required to remain seated and avoid physical activity, as movement influences how quickly your body uses the glucose. You must also refrain from eating, smoking, or chewing gum, and only small sips of water are permitted until the test is complete. If you vomit the glucose drink, the test will be invalidated and must be rescheduled.
At precisely timed intervals—typically one, two, and possibly three hours after finishing the drink—additional blood samples will be drawn. These serial draws track how effectively your body, through the action of insulin, clears the large glucose load from your bloodstream. Once the final timed sample is collected, the test is finished, and you are free to leave and resume normal activities.
Why Accuracy Matters Most
Attempting to manipulate the OGTT by crash dieting, extreme fasting, or severely restricting carbohydrates beforehand will only produce an inaccurate result. Such actions can cause the body to overcompensate when challenged with the high glucose load, making a normal result look abnormal or masking a true condition. The objective is to obtain an accurate reflection of your natural glucose metabolism.
An inaccurate result can lead to a missed diagnosis of conditions like prediabetes or gestational diabetes, which carry significant health risks. For a pregnant person, an unmanaged diagnosis of gestational diabetes increases the risk of complications such as preeclampsia, premature birth, and the baby being born with high birth weight. Missing a diagnosis of prediabetes means losing the opportunity for early intervention through lifestyle changes that could prevent the progression to Type 2 diabetes.
Understanding Your Results
Once the laboratory analyzes your blood samples, your healthcare provider interprets the results by comparing your glucose values at each timed interval against established thresholds. Results are categorized as normal, suggestive of impaired glucose tolerance (prediabetes), or indicative of diabetes. The criteria for diagnosing gestational diabetes are distinct from those used for diagnosing Type 2 diabetes in non-pregnant adults, and the specific numerical values can vary slightly.
If results fall within the normal range, it indicates your body is handling glucose effectively, and standard health monitoring will continue. An abnormal result requires a follow-up consultation to discuss the next steps. If the test suggests prediabetes, the discussion will focus on lifestyle modifications, such as dietary changes and increased physical activity, to help prevent the onset of diabetes.
A diagnosis of diabetes or gestational diabetes prompts a comprehensive treatment plan, which may involve further monitoring, nutritional counseling, and medication. Your provider will consider your complete medical history and other factors to determine the best course of action.