How to interpret a 3-hour glucose tolerance test result in pregnancy, measured in milligrams per deciliter (mg/dL)?

From the Guidelines

To interpret a 3-hour glucose tolerance test result in pregnancy, a value of 140 mg/dL or less at the 3-hour mark is considered normal, according to the Carpenter-Coustan criteria 1. The complete diagnostic criteria for gestational diabetes using the 3-hour 100g glucose tolerance test includes:

• Fasting levels below 95 mg/dL
• 1-hour below 180 mg/dL
• 2-hour below 155 mg/dL
• 3-hour below 140 mg/dL If two or more values are elevated, gestational diabetes is diagnosed 1. This test is typically performed between 24-28 weeks of pregnancy in women who had an abnormal glucose challenge test. The test measures how efficiently your body processes glucose after fasting overnight and then consuming a glucose solution. Proper interpretation is important because gestational diabetes can lead to complications for both mother and baby, including macrosomia, increased risk of cesarean delivery, and neonatal hypoglycemia 1.

The American Diabetes Association recommends using the IADPSG diagnostic criteria to optimize gestational outcomes, as these criteria are based on pregnancy outcomes rather than end points such as prediction of subsequent maternal diabetes 1. The expected benefits of using IADPSG criteria for offspring include reduced rates of large-for-gestational-age births and preeclampsia 1. If diagnosed with gestational diabetes, treatment typically involves blood glucose monitoring, dietary modifications, exercise, and sometimes insulin therapy.

Key points to consider when interpreting the results include:

• The test is performed after an overnight fast of at least 8 hours 1
• The glucose solution used is 100g 1
• The test measures plasma glucose levels at fasting, 1-hour, 2-hour, and 3-hour marks 1
• The diagnosis of gestational diabetes is made when at least two of the four plasma glucose levels are met or exceeded 1

From the Research

Interpreting 3-Hour Glucose Tolerance Test Results in Pregnancy

To interpret a 3-hour glucose tolerance test result in pregnancy, measured in milligrams per deciliter (mg/dL), the following points should be considered:

• The test is used to diagnose gestational diabetes mellitus (GDM) in pregnant women 2, 3, 4, 5, 6.
• The American College of Obstetricians and Gynecologists (ACOG) recommends a 2-step approach for screening and diagnosing GDM, which includes a 50-g glucose challenge test followed by a 100-g oral glucose tolerance test (OGTT) for those with abnormal results 2, 4.
• The 3-hour OGTT measures plasma glucose levels at fasting, 1 hour, 2 hours, and 3 hours after consuming a 100-g glucose solution 2, 3, 4, 5, 6.
• The diagnosis of GDM is typically made when two or more of the following plasma glucose values are met or exceeded:
  • Fasting: 95 mg/dL
  • 1 hour: 180 mg/dL
  • 2 hours: 155 mg/dL
  • 3 hours: 140 mg/dL 2, 3, 4, 5, 6
• A study found that a threshold value of 135 mg/dL for the 3-hour GTT accurately diagnoses almost all women with GDM while eliminating unnecessary GTT 2.
• Another study suggested that even with relatively mild degrees of glucose intolerance at 2 hours, no treatment is associated with large for gestational age (LGA) babies 3.
• The third-hour plasma glucose measurement of the OGTT is essential for diagnosing GDM in some cases, and omitting it may lead to a substantial decrease in sensitivity 5.
• Women with one abnormal value on the 3-hour OGTT are at an increased risk for adverse pregnancy outcomes, including macrosomia, LGA, neonatal hypoglycemia, and pregnancy-induced hypertension 6.

Abnormal Values and Associated Risks

The following abnormal values on the 3-hour OGTT are associated with increased risks:

• Fasting glucose > 95 mg/dL: associated with increased risk of macrosomia and neonatal hypoglycemia 3, 6
• 1-hour glucose > 180 mg/dL: associated with increased risk of LGA and macrosomia 3, 6
• 2-hour glucose > 155 mg/dL: associated with increased risk of LGA and macrosomia 3, 6
• 3-hour glucose > 140 mg/dL: associated with increased risk of macrosomia and neonatal hypoglycemia 3, 5, 6

Clinical Implications

The results of the 3-hour glucose tolerance test should be interpreted in the context of the individual patient's risk factors and medical history 2, 3, 4, 5, 6. Women with abnormal results should be closely monitored and managed to reduce the risk of adverse pregnancy outcomes 6.