What is the false positive rate for a 1-hour glucose tolerance test (GTT) result of 160 mg/dL, indicating impaired glucose tolerance?

From the Guidelines

The false positive rate for a 1-hour glucose tolerance test (GTT) with a threshold of 160 mg/dL is not explicitly stated in the provided evidence, but based on the information given, it can be inferred that the test is not highly specific, and a significant portion of the population may test positive without actually having gestational diabetes. The 1-hour GTT is typically performed as part of the screening for gestational diabetes, where patients drink a 50g glucose solution and have their blood glucose measured one hour later 1. A result of 160 mg/dL or higher is considered positive and usually leads to a follow-up 3-hour GTT for confirmation. Some key points to consider when interpreting the results of a 1-hour GTT include:

• The test is a screening tool, not a diagnostic tool, and positive results require confirmation with a more definitive test, such as a 3-hour GTT 1.
• Normal physiological variations in glucose metabolism can temporarily elevate blood glucose levels above the threshold, leading to false positives 1.
• Factors that may increase false positives include eating before the test, stress, certain medications, or normal pregnancy-related insulin resistance.
• The test should be performed in a standardized manner, with the patient fasting overnight and drinking the glucose solution in the morning 1. It is essential to understand the limitations of the 1-hour GTT and to interpret the results in the context of the individual patient's clinical presentation and medical history. A false positive result can cause unnecessary anxiety and lead to further testing, which can be avoided by carefully evaluating the results and considering the patient's overall clinical picture.

From the Research

False Positive Rate for 1-Hour GTT 160

• The false positive rate for a 1-hour glucose challenge test (GCT) value of 160 mg/dL is not directly stated in the provided studies.
• However, a study by 2 found that a 1-hour GCT value of ≥200 mg/dL had a positive predictive value of 68.6% for gestational diabetes mellitus (GDM) using National Diabetes Data Group (NDDG) criteria, and 80.0% using Carpenter-Coustan (CC) criteria.
• This suggests that a significant proportion of women with a 1-hour GCT value of ≥200 mg/dL may not actually have GDM, implying a false positive rate.
• Another study by 3 found that a 1-hour plasma glucose (1-h PG) value of ≥11.6 mmol/L during an oral glucose tolerance test (OGTT) had a sensitivity of 0.92 and a specificity of 0.91 for detecting type 2 diabetes.
• However, this study does not provide information on the false positive rate for a 1-hour GTT value of 160 mg/dL.
• A study by 4 found that a 1-hour glucose value of <180 mg/dL had a similar negative predictive value, positive predictive value, and specificity for type 1 diabetes development as the standard 2-hour threshold of <140 mg/dL.
• This study suggests that a 1-hour GTT value of 160 mg/dL may be associated with a low risk of type 1 diabetes, but does not provide information on the false positive rate.
• Overall, the available evidence suggests that the false positive rate for a 1-hour GTT value of 160 mg/dL is not well established, and further research is needed to determine the accuracy of this test for diagnosing diabetes.