The Glucose Challenge Test (GCT) and the subsequent Glucose Tolerance Test (GTT) are standardized procedures used during pregnancy. These tests evaluate how effectively a pregnant person’s body processes sugar, a process that can be affected by pregnancy hormones. The primary purpose is to screen for and diagnose Gestational Diabetes Mellitus (GDM), a temporary condition that develops when the body cannot produce enough insulin to manage blood sugar levels. Preparing correctly for these tests is important to ensure the results accurately reflect the body’s true metabolic state. Following preparation steps can help prevent misleading results that might necessitate unnecessary follow-up testing.
Dietary Preparation in the Days Before the Test
The days leading up to a glucose test require a consistent and moderate carbohydrate intake to ensure the body’s insulin response is properly primed. Current guidelines recommend consuming an unrestricted diet that includes at least 150 grams of carbohydrates daily for a minimum of three days before the test. This preparation step is crucial because severely restricting carbohydrates, such as with a very low-carb diet, can cause a temporary state of carbohydrate intolerance. When the body is suddenly introduced to the high glucose load of the test drink, it may react poorly, leading to a false-positive high result. By maintaining a normal intake of carbohydrates, the pancreatic beta cells remain accustomed to producing and releasing insulin. Adequate hydration should also be maintained in the days before testing, as dehydration can elevate blood sugar readings.
Immediate Protocol on the Morning of the Test
The immediate protocol varies depending on whether the provider has ordered the one-hour screening or the diagnostic three-hour test. For the initial one-hour GCT, fasting is typically not required, allowing a person to eat a normal meal before the test. However, the three-hour GTT requires a strict fasting period, usually between 8 and 14 hours, to establish a baseline blood glucose level. During this window, only plain water is permitted; anything containing calories, including coffee, tea, or chewing gum, must be avoided as they can stimulate an insulin response and skew the fasting result. Individuals must remain at the testing site, usually seated, and avoid any strenuous physical activity, ensuring the measured glucose response reflects only the body’s insulin function.
Actions That Can Negatively Impact Results
Excessive Fasting and Exercise
Several common mistakes can inadvertently lead to inaccurate results, often resulting in an unnecessary failure of the screening test. A frequent pitfall is excessive fasting, where a person fasts for longer than the recommended 14 hours. Prolonged fasting can cause the body to release stress hormones and trigger the liver to produce glucose, a process known as gluconeogenesis, which can paradoxically elevate the fasting blood sugar level. Engaging in strenuous exercise immediately before the test can also interfere with accuracy. Physical activity can temporarily lower blood sugar levels, potentially masking a true issue or causing a rebound effect.
Dietary and Medication Interference
Trying to “game” the test by severely restricting carbohydrate intake in the days prior is counterproductive, as this lack of carbohydrate conditioning can lead to a false-positive result on the post-load measurements. Certain medications and supplements, particularly those containing high doses of iron or certain vitamins, can also impact the chemical assay of the blood sample. Patients should always consult their provider about medication adjustments in the days leading up to the test.
Understanding the Follow-Up Diagnostic Test
The Screening Result
A high result on the initial one-hour GCT screening test does not automatically mean a diagnosis of Gestational Diabetes. The one-hour test is intentionally designed to be highly sensitive, meaning it flags many people who do not have the condition, ensuring no cases are missed. An irregular result simply indicates the need for the more definitive, diagnostic three-hour GTT.
The Diagnostic Criteria
The three-hour test is the gold standard for diagnosis and involves four separate blood draws: one fasting sample, and then samples at one, two, and three hours after consuming a higher concentration of glucose solution, typically 100 grams. For a diagnosis of GDM to be made, two or more of the four blood glucose values must meet or exceed the established threshold levels. If only one value is elevated, the result is considered inconclusive, and the patient does not receive a GDM diagnosis based on that test. Passing the three-hour test confirms that the body is processing sugar effectively enough to avoid the diagnosis, providing reassurance and context for the initial screening result.