What percentage of pregnant women fail the 1-hour glucose tolerance test (GTT)?

From the Guidelines

Approximately 15-20% of pregnant women fail the 1-hour glucose tolerance test (GTT). This estimate is based on the anticipated increase in the incidence of GDM diagnosed by the new criteria, as stated in the 2014 standards of medical care in diabetes 1. The 1-hour glucose tolerance test is a screening tool used to identify pregnant women at risk for gestational diabetes, typically performed between 24-28 weeks of pregnancy.

Key Points to Consider

• The test involves drinking a glucose solution and having blood drawn one hour later to measure blood sugar levels.
• A result of 140 mg/dL or higher is considered failing the test, according to the NIH consensus, although some experts recommend a lower threshold of 130-135 mg/dL in high-risk ethnic minorities 1.
• Failing this initial screening doesn't necessarily mean a woman has gestational diabetes, but rather indicates the need for the more definitive 100-g OGTT.
• The relatively high failure rate of the 1-hour test reflects its design as a sensitive screening tool intended to catch most potential cases of gestational diabetes, even at the cost of some false positives, as noted in the 2014 standards of medical care in diabetes 1.

Implications of Failing the 1-Hour GTT

• Women who fail the 1-hour test will undergo further testing to confirm the diagnosis of gestational diabetes.
• Early identification and treatment of gestational diabetes can help reduce the risk of complications for both mother and baby, such as large-for-gestational-age births and neonatal macrosomia, as discussed in the 2014 standards of medical care in diabetes 1.

From the Research

Percentage of Women Failing the 1-Hour Glucose Tolerance Test

• The percentage of women who fail the 1-hour glucose tolerance test (GTT) can vary depending on the population and the criteria used to define a failed test.
• According to a study published in 1997 2, approximately 12.8% of pregnant women (76 out of 595) had a positive 1-hour 50-g OGTT, indicating that they failed the initial screening test.
• Another study published in 2021 3 mentions that approximately 15% to 20% of screened women require the 100 g, 3-hour oral glucose tolerance test, implying that this percentage of women fail the initial 1-hour glucose challenge test.
• A study from 1996 4 found that 6.1% of the study patients (20 out of 329) were diagnosed with gestational diabetes, and 40% of these cases were detected with early screening protocol, but it does not directly provide the percentage of women who failed the 1-hour GTT.

Factors Associated with Failing the 1-Hour Glucose Tolerance Test

• Factors such as maternal age over 30 years, black race, and the presence of risk factors for diabetes are associated with an increased risk of failing the 1-hour glucose tolerance test 4.
• Poor pregnancy weight gain is also associated with the late development of glucose intolerance 4.

Implications of Failing the 1-Hour Glucose Tolerance Test

• Women who fail the 1-hour glucose tolerance test are at an increased risk for adverse pregnancy outcomes, including macrosomia, large for gestational age, neonatal hypoglycemia, and pregnancy-induced hypertension 5.
• Early detection and treatment of gestational diabetes can help reduce the risk of these adverse outcomes 4.