What makes you think your patient has gestational diabetes?
Patients whose body mass index (BMI) is 30 kg/m2 or higher prior to pregnancy or who have an immediate family member with diabetes are at increased risk for developing gestational diabetes mellitus (GDM). The American Diabetes Association (ADA) has recommended that patients at high risk of gestational diabetes be screened during their first prenatal visit for undiagnosed type II diabetes using standard diagnostic criteria. Those considered at average risk are typically screened during the second trimester (24-28 weeks of gestation).
Symptoms of gestational diabetes are typically mild and may include:
frequent infections (i.e., bladder, vagina, skin)
nausea and vomiting
weight loss in spite of increased appetite
What Tests Should I Request to Confirm My Clinical Dx? In addition, what follow-up tests might be useful?
In 2011, the ADA recommended a morning 75-gram (g) oral glucose tolerance test (OGTT) at 24-28 weeks of gestation, following an overnight fast of at least 8 hours for diagnosing GDM, with plasma glucose concentrations taken at fasting, 1 hour, and 2 hours. If any of the patent’s glucose measurements exceed one of the following measurements, the diagnosis of GDM is made: fasting plasma glucose greater than or equal to 92 mg/dL (5.1 mmol/L), a 1 hour plasma glucose measurement greater than or equal to 180 mg/dL (10.0 mmol/L), or a 2 hour plasma glucose greater than or equal to 153 mg/dL (8.5 mmol/L).
In 2010, two approaches were recommended for GDM screening and diagnosis. The one-step approach performs a diagnostic 100-g OGTT, following an overnight fast of 8 hours, in all women at 24-28 weeks of gestation. Using this criterion, at least two of the following plasma glucose values must be met for diagnosis of GDM:
fasting greater than or equal to 95 mg/dL (5.3 mmol/L)
1 hour greater than or equal to 180 mg/dL (10.0 mmol/L)
2 hours greater than or equal to 155 mg/dL (8.6 mmol/L)
3 hour greater than or equal to 140 mg/dL (7.8 mmol/L)
In the two-step approach, initial screening is performed at 24-28 weeks of gestation by measuring plasma glucose 1 hour following a 50-g OGTT. A value greater than or equal to 140 mg/dL (7.8 mmol/L) identified approximately 80% of cases of GDM, whereas decreasing the threshold to greater than or equal to 130 mg/dL identifies approximately 90% of cases. Women who exceed one of these 50-g load thresholds are tested on a subsequent day using a diagnostic 100-g OGTT.(Table 1)
|Glucose Measure||2010 Criteria (100-g OGTT)||2011 Criteria (75-g OGTT)|
|Fasting||≥95 mg/dL (5.3 mmol/L)||≥92 mg/dL (5.1 mmol/L)|
|1 hour||≥180 mg/dL (10.0 mmol/L)||≥180 mg/dL (10.0 mmol/L)|
|2 hour||≥155 mg/dL (8.6 mmol/L)||≥153 mg/dL (8.5 mmol/L)|
|3 hour||≥140 mg/dL (7.8 mmol/L)|
Are There Any Factors That Might Affect the Lab Results? In particular, does your patient take any medications – OTC drugs or Herbals – that might affect the lab results?
A drop in glucose levels may occur due to the continued metabolism of glucose if the specimen is not tested immediately, leading to inaccurate reporting of blood glucose levels. It is recommended that blood be drawn in a gray-top tube containing sodium fluoride, which will inhibit glycolysis within 1 hour. Blood samples drawn in a red-top tube for serum glucose or in a green-top tube for plasma glucose will demonstrate a 2-3% decrease in glucose concentration per hour. In addition, patients suffering from sepsis or conditions that result in high white blood cell counts, such as chronic lymphocytic leukemia, may have artificially low glucose concentrations because of an increased rate of glucose metabolism as a consequence of leukocytosis and activated neutrophils.
Mean fasting plasma glucose (FPGs) tend to be higher in the morning, thus, it is recommended that blood for this test be drawn in the morning. In addition, plasma glucose levels typically vary from day to day and can be affected by other factors, such as acute stress.
What Lab Results Are Absolutely Confirmatory?
Lab results from the OGTTs used for diagnosis are considered absolutely confirmatory. Patients who have an elevated glucose level exceeding those listed in
Table 1 should be tested again on a subsequent day to confirm diagnosis. In the case of two conflicting results (i.e., clinical symptoms of diabetes with only borderline values for the OGTT), testing should also be repeated. Laboratory tests used for diagnosis are also used for confirmation, and diagnosis is then based on the results of the repeated test.
Additional Issues of Clinical Importance
Table 2 outlines target maternal capillary glucose concentrations for women diagnosed with GDM.Table 3 outlines the recommended glucose concentrations for those patients diagnosed with type I or type II diabetes that become pregnant, providing such goals can be achieved without excessive hypoglycemia
|Glucose Measure||Target Concentration|
|Preprandial||≤95 mg/dL (5.3 mmol/L)|
|1 h post meal||≤140 mg/dL (7.8 mmol/L)|
|2 h post meal||≤120 mg/dL(6.7 mmol/L)|
|Glucose Measure||Target Range|
|Premeal, bedtime, overnight||60-99 mg/dL (3.3-5.4 mmol/L)|
|Peak postprandial||100-129 mg/dL (5.4-7.1 mmol/L)|
Complications of unmanaged GDM include risks to both the mother and the baby. Complications that may affect the baby include miscarriage, birth defects, excessive birth weight, preterm birth and respiratory distress syndrome, hypoglycemia, jaundice, and an increased risk of developing type II diabetes later in life. For the mother, complications include high blood pressure, preeclampsia, eclampsia, and future development of type II diabetes.
Errors in Test Selection
A1C is not recommended for the diagnosis of GDM.
Errors in Interpretation of Test Results
Symptoms of GDM may actually represent preexisting, undiagnosed type II diabetes. Women with risk factors for type II diabetes should be screened at their first prenatal visit using standard, nonpregnant diagnostic criteria. To classify a patient as having overt and not gestational diabetes, one of the following values must be met: FPG greater than or equal to 126 mg/dL (7.0 mmol/L), A1C greater than or equal to 6.5%, or a random plasma glucose greater than or equal to 200 mg/dL (11.1 mmol/L), in addition to confirmation by FPG or A1C testing using a DCCT/USPDS-standardized assay.
Those found to have diabetes at this visit should be diagnosed and treated for overt, not gestational diabetes. In addition, women diagnosed with GDM should be screened for diabetes at 6-12 weeks post-partum using nonpregnant OGTT criteria. Because women with GDM are at increased risk for developing diabetes, lifelong screenings for diabetes once every 3 years is recommended.
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- What makes you think your patient has gestational diabetes?
- What Tests Should I Request to Confirm My Clinical Dx? In addition, what follow-up tests might be useful?
- Are There Any Factors That Might Affect the Lab Results? In particular, does your patient take any medications - OTC drugs or Herbals - that might affect the lab results?
- What Lab Results Are Absolutely Confirmatory?
- Additional Issues of Clinical Importance
- Errors in Test Selection
- Errors in Interpretation of Test Results
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