Gestational diabetes mellitus (GD) is a condition characterized by high blood sugar levels that first appear during pregnancy. This diagnosis often causes worry about the health of the mother and the developing baby. While screening and diagnostic tests are designed to be highly sensitive, they are not infallible. Certain circumstances, related to the testing procedure or the individual’s biology, can occasionally lead to a result that does not accurately reflect the patient’s glucose regulation.
Understanding the Standard Diagnostic Tests
GD diagnosis is generally performed between the 24th and 28th weeks of pregnancy, when insulin resistance naturally increases due to placental hormones. The screening process involves one of two globally recognized strategies, which contributes to diagnostic variation. The first approach, the “two-step” method, begins with a non-fasting 50-gram glucose challenge test (GCT).
If the one-hour blood glucose level from the GCT exceeds a certain threshold (usually 135 to 140 mg/dL), the patient proceeds to the definitive diagnostic test. This second step is the 100-gram, three-hour oral glucose tolerance test (OGTT), which requires overnight fasting. Blood is drawn four times: once while fasting, and then at one, two, and three hours after consuming the glucose drink.
Diagnosis using the three-hour OGTT is confirmed if at least two of the four blood sugar values meet or exceed established thresholds, such as the Carpenter and Coustan criteria. The alternative strategy, the “one-step” approach, uses a single 75-gram, two-hour OGTT. Diagnosis in the one-step approach is based on exceeding only one of three specific thresholds.
The existence of these two different testing protocols, which use different glucose loads and cutoff points, is a source of potential diagnostic ambiguity. The two-step method identifies fewer individuals with GD than the one-step method. This variation means a patient could be diagnosed under one set of guidelines but not the other, often resulting in a borderline result.
Why Test Results Might Be Misleading
Apparent misdiagnoses often stem from errors in patient preparation before the diagnostic test. The 100-gram, three-hour OGTT requires a specific fast, between eight and 14 hours. Fasting for longer than 14 hours can artificially elevate the initial fasting blood sugar reading because the body begins to release stored glucose (gluconeogenesis), potentially pushing the result over the diagnostic threshold.
A patient following a severely carbohydrate-restricted diet before the test may also experience a skewed result. When the body is accustomed to low-carb intake, it becomes less efficient at processing a sudden, large glucose load. This metabolic shock can cause an exaggerated glucose spike, resulting in a false positive diagnosis despite generally healthy glucose control.
Errors in the test administration itself can compromise accuracy. These include incorrect timing of the blood draws, which significantly alters the measured glucose concentration. Vomiting the glucose solution shortly after consumption is another source of error, as the full dose is not absorbed, necessitating a retest.
Physiological variations on the day of the test also play a role, especially when results are close to the cutoff point. High stress, lack of sleep, or a mild illness can transiently affect the body’s ability to process glucose. This biological variability can push an individual with normal glucose metabolism just over a single diagnostic threshold. Less commonly, errors can occur in the laboratory, such as issues with sample handling or equipment calibration.
Steps to Confirm or Challenge a Diagnosis
For patients diagnosed based on borderline results or who suspect a false positive, the first step is detailed communication with the healthcare provider. If a clear preparation error is suspected, such as an excessively long fast, the provider may approve an immediate OGTT retest. However, retesting is not always standard practice due to the risks of delaying necessary treatment.
The most practical way to confirm a diagnosis is through self-monitoring of blood glucose (SMBG) at home using a glucometer. This involves checking fasting and post-meal levels over several days or weeks. Consistently normal readings during this period, despite the positive OGTT result, suggest the initial diagnosis was an isolated event or a false positive.
If the OGTT results conflict significantly with the patient’s overall health profile, seeking a second opinion from a specialist, such as an endocrinologist, is reasonable. Specialists can review the raw data in the context of the patient’s full medical history. While a false positive diagnosis does not carry the risks of a missed diagnosis, it can lead to unnecessary dietary restrictions and emotional distress, making confirmation important.
In some cases, the healthcare team may forgo an immediate retest and instead treat the condition as “diet-controlled” gestational diabetes. This approach involves implementing a structured diet and monitoring blood sugar to keep levels within the target range. If home monitoring shows consistently controlled numbers without medication, it suggests the patient was either mildly affected or incorrectly diagnosed, but the monitoring provides a safeguard for mother and baby.