The acronym GTT stands for Glucose Tolerance Test, a common diagnostic procedure in medicine. This test is designed to measure how effectively a person’s body processes and clears sugar, or glucose, from the bloodstream after consuming a standardized dose. By tracking the concentration of glucose over a period of time, healthcare providers can assess the body’s ability to regulate blood sugar levels. The test is a highly specific tool used to check for conditions where glucose metabolism is impaired, most notably in the diagnosis of prediabetes, diabetes mellitus, and gestational diabetes.
The Purpose and Mechanism of the Glucose Tolerance Test
Maintaining stable blood glucose is a fundamental biological function regulated by the hormone insulin. After eating, carbohydrates break down into glucose, the body’s primary energy source. The pancreas responds to rising blood sugar by releasing insulin, which facilitates the movement of glucose from the blood into cells for use or storage.
The GTT challenges this system by introducing a large, measured dose of glucose. In a person with normal metabolism, insulin production quickly increases, causing blood sugar to spike and then return to a healthy baseline within two to three hours. If the body produces insufficient insulin or cells are resistant to its action, blood glucose levels remain elevated longer. This prolonged elevation signals a failure in glucose processing.
Preparing For and Undergoing the Procedure
Proper preparation ensures accurate GTT results, beginning with dietary guidelines. Patients must consume a normal diet containing at least 150 grams of carbohydrates daily for three days before the procedure. This ensures the body’s glucose-handling mechanisms are active.
Fasting is required, meaning the patient must abstain from eating or drinking anything except water for 8 to 12 hours before the test. Patients should confirm with their provider if they need to temporarily stop any medications that could interfere with blood glucose levels. The test is usually scheduled for the morning to accommodate overnight fasting.
The procedure begins with a phlebotomist drawing a baseline blood sample to determine the fasting glucose level. The patient then consumes a measured, syrupy glucose solution, typically 75 grams, which must be consumed within five minutes. The patient must remain seated quietly for the duration of the test, usually two hours, without eating, drinking anything other than sips of water, or smoking. Additional blood samples are collected at precise intervals, usually one hour and two hours after the drink, to track the body’s response curve.
Understanding and Interpreting the Results
The results of the GTT provide a detailed picture of glucose clearance using specific numerical thresholds for diagnosis. For non-pregnant adults undergoing the standard two-hour, 75-gram oral GTT, a two-hour blood glucose level below 140 milligrams per deciliter (mg/dL) is considered normal. This indicates that the body efficiently processed the glucose load.
A two-hour reading between 140 mg/dL and 199 mg/dL is classified as impaired glucose tolerance (IGT), often referred to as prediabetes. This suggests the body is struggling to clear glucose, increasing the risk of developing diabetes mellitus. A result of 200 mg/dL or higher at the two-hour mark confirms a diagnosis of diabetes.
The fasting blood sugar reading, taken before the glucose solution, also provides diagnostic information. A fasting level between 100 mg/dL and 125 mg/dL indicates impaired fasting glucose (IFG), another category of prediabetes. If the fasting level is 126 mg/dL or higher, it is diagnostic of diabetes.
Key Variations of the Test
The Oral Glucose Tolerance Test (OGTT) is frequently modified for screening gestational diabetes during pregnancy. Healthcare providers often use a two-step approach, starting with an initial one-hour glucose challenge test. This screening involves the patient drinking a 50-gram glucose load without prior fasting, followed by a single blood draw one hour later.
If the one-hour screening is elevated, a diagnostic three-hour OGTT is performed, requiring fasting and a 100-gram glucose load. Gestational diabetes is diagnosed if at least two of the four blood sample readings meet or exceed established thresholds:
- Fasting
- One-hour
- Two-hour
- Three-hour
Alternatively, a one-step approach uses a 75-gram glucose load and requires only one abnormal value at fasting, one-hour, or two-hours for diagnosis.
A less common version is the Intravenous Glucose Tolerance Test (IVGTT), where glucose is administered directly into the vein rather than ingested orally. This method bypasses the digestive system and is rarely used to diagnose diabetes. Instead, the IVGTT is primarily employed in research settings to assess insulin sensitivity or for diagnosing conditions like acromegaly.