In a medical context, the acronym GTT stands for the Glucose Tolerance Test. This diagnostic procedure evaluates how effectively a person’s body processes glucose. By monitoring blood glucose levels after a controlled intake of a sugary drink, healthcare providers assess the body’s ability to produce and utilize insulin, the hormone responsible for moving glucose into cells for energy. The test provides a dynamic picture of glucose metabolism, which is a more comprehensive assessment than a single, random blood sugar measurement.
The Purpose and Need for GTT
The primary goal of the Glucose Tolerance Test is to screen for and diagnose disorders of carbohydrate metabolism, notably prediabetes and Type 2 diabetes. While a simple fasting blood glucose test measures sugar levels at one point in time, the GTT assesses the body’s functional response to a specific glucose challenge. This makes it a highly sensitive tool for detecting early stages of impaired glucose regulation.
The GTT is particularly important in obstetrics for identifying Gestational Diabetes Mellitus (GDM). Screening for GDM is routinely performed between the 24th and 28th week of gestation, though earlier testing may be ordered for individuals with certain risk factors. Timely diagnosis of GDM is important for managing the health of both the pregnant person and the developing fetus.
The test is often ordered when a patient shows risk factors for diabetes or when preliminary blood tests, such as a fasting plasma glucose or an HbA1c test, show inconclusive results. It provides the detailed data needed to confirm or rule out the diagnosis of impaired glucose tolerance. The GTT can also be used to investigate other conditions, such as reactive hypoglycemia. Its main application remains in diabetes diagnosis.
Preparing for the Test and Procedure
Preparation for a Glucose Tolerance Test begins in the days leading up to the appointment. Patients are instructed to maintain a diet containing at least 150 grams of carbohydrates per day for a minimum of three days prior to the test. This carbohydrate loading is necessary to prevent a false-positive result.
The patient must then begin a strict fast, typically lasting between eight and fourteen hours before the initial blood draw, during which only plain water is permitted. Patients should maintain a normal level of physical activity, as intense exercise can temporarily affect glucose metabolism. They should also discuss all medications and supplements with their healthcare provider, as some substances can interfere with the test’s outcome.
The procedure itself usually takes place in a laboratory setting and can last between two and four hours, depending on the specific test ordered. It begins with a blood sample taken to measure the patient’s fasting blood glucose level. Following this initial draw, the patient is given a precisely measured glucose solution to drink, which must be consumed completely within a short period, typically five minutes.
For the standard two-hour GTT, the drink contains 75 grams of glucose. For the three-hour GTT used for gestational diabetes, a 100-gram dose may be used. After consuming the solution, the patient must remain sedentary in the clinic for the duration of the test, as physical activity can affect the results, and blood samples are drawn at timed intervals. The standard test requires a final sample at the two-hour mark, while the three-hour test requires additional draws at one, two, and three hours after the drink.
Understanding the Results
The results of the Glucose Tolerance Test are reported as blood glucose concentrations in milligrams per deciliter (mg/dL) or millimoles per liter (mmol/L) at each timed interval. The values at the two-hour mark are most often used for diagnosing diabetes and prediabetes in non-pregnant adults. A two-hour plasma glucose level below 140 mg/dL (7.8 mmol/L) is considered a normal response to the glucose challenge.
A result between 140 mg/dL and 199 mg/dL (7.8 and 11.0 mmol/L) is classified as Impaired Glucose Tolerance (IGT). This finding indicates that the body is having difficulty clearing the glucose from the blood within the two-hour window. A two-hour glucose level of 200 mg/dL (11.1 mmol/L) or higher confirms a diagnosis of diabetes.
The diagnostic criteria are stricter for the three-hour GTT used to confirm Gestational Diabetes Mellitus (GDM). Using the 100-gram glucose load, GDM is diagnosed if two or more of the four blood glucose values meet or exceed specific thresholds:
- A fasting level of 95 mg/dL (5.3 mmol/L)
- A one-hour level of 180 mg/dL (10.0 mmol/L)
- A two-hour level of 155 mg/dL (8.6 mmol/L)
- A three-hour level of 140 mg/dL (7.8 mmol/L)
An abnormal result requires closer monitoring and management, often involving dietary changes or medication, to control blood sugar for the remainder of the pregnancy.
Other Contexts for the GTT Acronym
While the Glucose Tolerance Test is the primary meaning in modern diagnostics, the acronym GTT may appear in other, less common medical contexts. In pharmacology and nursing, “gtt” is an abbreviation derived from the Latin word gutta, meaning “drop.” This meaning is sometimes found in prescriptions or charts to indicate that a medication should be administered in drops, such as eye drops, or to describe the rate of an intravenous drip. GTT is also used in a specialized test for diagnosing acromegaly, where the glucose load is given to assess the suppression of growth hormone levels.