The glucose screening and tolerance tests measure how effectively the body processes sugar (glucose) after ingesting a standardized sugary drink. These tests primarily identify conditions where blood sugar is abnormally high, such as gestational diabetes during pregnancy or pre-diabetes and Type 2 diabetes in other adults. Understanding what defines an abnormal or “failed” result can help alleviate the anxiety that often accompanies waiting for a diagnosis.
Understanding the Types of Glucose Tests
Testing for glucose intolerance often involves a two-step approach, especially during pregnancy, starting with a screening test. This initial procedure is the 1-Hour Glucose Challenge Test (GCT), which typically does not require fasting. The GCT involves consuming a 50-gram glucose solution, and a blood sample is taken one hour later to measure the body’s immediate response. The GCT’s function is to flag individuals who might have an underlying issue with glucose metabolism.
If the initial screening results are elevated, it indicates the need for a more definitive diagnostic procedure, the Oral Glucose Tolerance Test (OGTT). The OGTT is a longer, multi-sample examination requiring overnight fasting. Patients consume a higher-dose glucose solution, typically 75 grams for non-pregnant adults or 100 grams for pregnant individuals. Blood is then drawn at predetermined intervals (one, two, and sometimes three hours) to detail the body’s glucose clearance rate.
The diagnostic OGTT results are used to confirm or rule out a diagnosis because measuring multiple points in time provides a comprehensive assessment. A “failed” screening test simply means the patient must proceed to the diagnostic OGTT. This procedure ultimately determines the presence of a condition like gestational diabetes.
Thresholds for Abnormal Results
The designation of an abnormal result depends on the specific test and whether the person is pregnant. For the 1-Hour Glucose Challenge Test (GCT) used in pregnancy screening, the concerning threshold is generally 140 milligrams per deciliter (mg/dL). Some practices use a lower cutoff of 130 mg/dL, but values at or above 140 mg/dL almost always necessitate the follow-up diagnostic test.
The definitive diagnosis of gestational diabetes uses the 3-Hour Oral Glucose Tolerance Test (OGTT), requiring a fasting blood draw followed by three more draws after ingesting a 100-gram glucose load. Gestational diabetes is confirmed if two or more of the four blood sugar values meet or exceed specific thresholds. These widely accepted thresholds are:
- A fasting value of 95 mg/dL.
- A 1-hour value of 180 mg/dL.
- A 2-hour value of 155 mg/dL.
- A 3-hour value of 140 mg/dL.
For non-pregnant individuals, diagnosing pre-diabetes or Type 2 diabetes relies on different fasting and post-load values, typically using a 75-gram glucose solution measured up to two hours. Pre-diabetes, known as Impaired Glucose Tolerance (IGT), is indicated by a 2-hour post-load glucose level between 140 and 199 mg/dL. A reading of 200 mg/dL or higher two hours after the drink confirms a diagnosis of diabetes. Impaired Fasting Glucose (pre-diabetes) is defined by a fasting plasma glucose level between 100 and 125 mg/dL, while a fasting level of 126 mg/dL or greater on two occasions is diagnostic for diabetes.
What Happens After an Abnormal Result
A confirmed diagnosis, whether gestational diabetes or pre-diabetes, prompts medical and lifestyle adjustments. The first step involves consulting a healthcare provider or specialist, such as a registered dietitian or endocrinologist, to create a personalized management plan. This plan focuses heavily on nutritional changes designed to stabilize blood sugar levels throughout the day.
Dietary modifications typically involve reducing simple and refined carbohydrates while prioritizing complex carbohydrates, lean proteins, and healthy fats. Patients are instructed to monitor their blood glucose levels at home multiple times daily, recording values before meals and one or two hours after eating. This self-monitoring provides valuable data to adjust food intake and determine the effectiveness of the treatment plan.
If dietary and lifestyle adjustments alone are insufficient to keep blood sugar within the target range, medication may become necessary to achieve glycemic control. Treatment can include oral medications or, in some cases, insulin therapy to help the body process glucose more efficiently. For individuals with gestational diabetes, follow-up testing is recommended six to twelve weeks after delivery to ensure the condition has resolved. Regular screening every few years is advised for those diagnosed with pre-diabetes.