Gestational Diabetes Mellitus (GDM) is diagnosed when a woman develops high blood sugar levels during pregnancy, usually resolving after childbirth. GDM affects 5% to 9% of pregnancies in the U.S. and is caused by pregnancy hormones creating insulin resistance. This resistance makes it difficult for the body to use insulin effectively, leading to glucose buildup in the blood. Like any diagnosis based on laboratory tests, GDM findings are subject to potential inaccuracies, meaning misdiagnosis can occur due to procedural errors or differing medical standards.
Understanding the Standard Diagnostic Tests
The standard method for diagnosing GDM involves two primary tests that measure the body’s response to a glucose load. The first, the Glucose Challenge Test (GCT), is a screening tool typically performed between the 24th and 28th week of pregnancy. During the GCT, the patient consumes a 50-gram glucose drink, and a single blood sample is drawn an hour later. A result above a threshold (commonly 130 mg/dL or 140 mg/dL) is considered abnormal and requires a definitive diagnostic test.
The definitive test is the Oral Glucose Tolerance Test (OGTT), which uses two different protocols. The traditional two-step approach uses a 100-gram glucose load and requires four blood draws: one fasting, and then at one, two, and three hours. The one-step approach uses a smaller 75-gram glucose load and involves three blood draws: one fasting, and then at one and two hours. These tests are the accepted benchmarks, but their accuracy depends heavily on meticulous execution.
Procedural Errors That Affect Results
The accuracy of the glucose tolerance test is vulnerable to errors occurring before, during, or immediately after the blood draw, known as pre-analytical errors. A significant source of error is improper patient preparation, such as failing to fast for the required 8 to 14 hours before the test. Consuming food or sugary drinks too close to the test time can artificially elevate glucose readings, leading to a false positive result.
Inaccurate results can also stem from the patient’s physiological state preceding the test. Factors like acute illness, severe stress, or lack of sleep can temporarily raise blood sugar levels, pushing a borderline patient over the diagnostic threshold. Errors in the lab setting also contribute to misdiagnosis, including improper storage of the blood sample, delayed processing, or using an incorrect amount of glucose in the drink. These procedural failures mean a diagnosis may reflect a testing anomaly rather than true glucose intolerance.
How Different Diagnostic Criteria Influence Diagnosis
A major reason for diagnostic variation, which can feel like a misdiagnosis, is the lack of a single, universally adopted standard for GDM. Two primary sets of criteria are used worldwide, each employing different thresholds and testing protocols. The Carpenter-Coustan criteria, often utilized in the United States, typically require two elevated values on the four-point, 100-gram OGTT to confirm a diagnosis. This standard tends to identify women with more pronounced glucose intolerance.
In contrast, the International Association of Diabetes and Pregnancy Study Groups (IADPSG) criteria use a one-step, three-point, 75-gram OGTT and require only a single abnormal value for diagnosis. The thresholds for the IADPSG criteria are lower than those for Carpenter-Coustan, which significantly increases the overall rate of GDM diagnosis, sometimes by 50% or more. A woman may be diagnosed under the stricter IADPSG criteria but considered normal under the Carpenter-Coustan standard. This disagreement exists because the IADPSG criteria were developed from a study that linked even mildly elevated glucose levels to adverse pregnancy outcomes, defining a broader population at risk.
Confirming or Refuting the Diagnosis
If a diagnosis is highly unexpected, or if the test results were borderline, patients have options to verify the finding. The Oral Glucose Tolerance Test is known to have poor reproducibility, meaning repeat testing is a valid course of action. This is especially true if a procedural error is suspected or if only one value was minimally elevated. Research indicates that a significant percentage of women initially classified with GDM were not diagnosed when retested later in pregnancy.
Continuous Glucose Monitoring (CGM) is an emerging tool that offers a more detailed picture of a patient’s real-world glucose control over a period of days or weeks. Unlike the snapshot provided by the OGTT, CGM provides thousands of data points on glucose levels in response to daily activities and meals. While CGM is not yet the standard for initial diagnosis, it can be used to triangulate the OGTT result with daily life data, helping to confirm or refute a questionable diagnosis. Open communication with the healthcare team is paramount to review the specific criteria used and to discuss whether retesting or supplemental monitoring is appropriate for verification.