The 3-Hour Glucose Tolerance Test (GTT) is a diagnostic procedure that evaluates how the body processes sugar (glucose) over a defined period. This test is most commonly used to confirm a diagnosis of gestational diabetes following an abnormal one-hour screening test. It can also diagnose general impaired glucose tolerance or type 2 diabetes in non-pregnant individuals. Adhering to a specific protocol is required to ensure the results accurately reflect the body’s metabolic function.
Essential Preparation Requirements
Preparation for the 3-hour GTT begins several days before the appointment to ensure normal metabolic function. For three days prior, individuals are advised to maintain a diet including a minimum of 150 grams of carbohydrates daily. This high-carbohydrate intake prevents an artificially low glucose reading that could compromise the validity of the results.
The night before the test requires a strict fasting period, usually 8 to 14 hours, abstaining from all food and caloric beverages. Fasting outside this window can skew results; fasting for less than eight hours may cause high baseline glucose, while fasting for more than 14 hours can cause the body to start producing its own sugar. During the fast, avoid consuming substances that interfere with glucose metabolism, such as chewing gum, cough drops, and tobacco products. Patients should also consult their healthcare provider about any medications they are taking, as some drugs, vitamins, or cold preparations may need to be temporarily stopped.
Rules for Water Intake Before and During the Test
Plain water is permitted before and during the 3-hour glucose test, but only under specific conditions. During the mandated 8-to-14-hour overnight fast, drinking plain water is allowed and encouraged to maintain hydration. Pure water does not interfere with the fasting glucose measurement because it contains no calories or carbohydrates that would affect blood sugar levels.
Once the test begins, plain water can be consumed in moderation throughout the three-hour waiting period. Water is allowed primarily to prevent dehydration during the prolonged fast and lengthy test. It is necessary to avoid all other beverages, including coffee, tea, diet soda, and flavored water, even if they are labeled as sugar-free. Non-plain liquids often contain compounds like artificial sweeteners or caffeine that can stimulate insulin release or otherwise affect the accuracy of the blood glucose measurements. Water should be taken only as needed, and excessive consumption must be avoided to prevent blood dilution or stomach upset.
The Three-Hour Procedure
The 3-hour GTT is a four-hour commitment starting upon arrival at the laboratory or clinic. The first step involves a phlebotomist drawing a baseline blood sample to measure the patient’s fasting glucose level. This initial draw is important because it establishes the starting point against which subsequent measurements will be compared.
Following the initial draw, the patient receives a standardized glucose beverage, typically 100 grams of glucose dissolved in a liquid solution. The entire drink must be consumed quickly, usually within five minutes, and the completion time is precisely recorded to mark the start of the three-hour test. After consumption, the patient must remain sedentary within the lab environment for the duration of the test.
Physical activity is restricted because movement can lower blood sugar levels, leading to inaccurate results regarding glucose processing. Blood samples are drawn precisely at the one-hour, two-hour, and three-hour marks after the drink, resulting in four total blood draws. Patients should bring reading material or a quiet activity for the long waiting period. They should notify staff immediately if they experience nausea or lightheadedness, which can occur after drinking the concentrated glucose solution.