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A glucose tolerance test (GTT) is a procedure that determines whether a patient can use and store glucose normally. The test is typically used to diagnose diabetes mellitus, insulin resistance, impaired pancreatic β-cell function, and, occasionally, reactive hypoglycemia or acromegaly, as well as rarer disorders of carbohydrate metabolism.[1] In the most commonly performed version, the oral glucose tolerance test (OGTT), a standard dose of glucose is ingested by mouth, and blood levels are measured 2 hours later.[2] Many variations of the GTT have been developed over the years for different purposes, with varying standard glucose doses, administration routes, sampling intervals, and measurements of additional substances beyond blood glucose.[3]

Insulin resistance and an insulin secretory deficiency induced by β-cell dysfunction are the 2 defects that characterize the transition from normal glucose tolerance to T2DM.[19] Reduced tissue sensitivity to insulin and pronounced compensatory hyperinsulinemia are features of insulin resistance.[20] Plasma glucose levels remain within the normal range initially. β-cell secretory capacity declines in patients who eventually develop T2DM. The first detectable glucose abnormality is a rise in postprandial glucose levels due to reduced 1st-phase insulin secretion. Fasting glucose levels increase as β-cell activity deteriorates over time, and diabetes eventually develops with further reduction of insulin secretion.

A GTT is typically ordered by a medical doctor, advanced practice nurse, or physician assistant. Interprofessional collaboration is required to ensure correct test performance.[50] The healthcare provider or nurse must give the patient clear instructions on how to prepare for the examination and what to expect during the test.[51] The GTT may be administered in several settings. The evaluation may be performed in a clinical office, equipped with the necessary instruments and staffing, or a laboratory. Although inpatient units are an atypical setting for the task, a hospital’s outpatient or clinical research department may provide staff with additional time to complete this examination. Nurses, medical assistants, or phlebotomists may perform the test. Clear communication of the healthcare provider’s order is essential, including the type of test, duration, and number of samples required. Personnel administering the test must be aware of all test requirements, including fasting and pretest dietary carbohydrate intake. Collaboration with laboratory staff is vital to ensure timely specimen processing, proper storage, and shipping if necessary. Laboratory personnel should work closely with providers to deliver accurate results efficiently.