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Gestational Glucose Tolerance Diagnostic Test (Three-hour, ACOG Recommendations)
- Glucose Tolerance, Gestational Diabetes (Fasting, One-hour, Two-hour, and Three-hour)
100-g glucose load; plasma glucose measured fasting, and at one, two, and three hours
Diagnose gestational diabetes
Plasma or Serum Glucose Level
(Carpenter and Coustan Conversion)
Plasma Glucose Level
(National Diabetes Data Group Conversion)
*The diagnosis of GDM can be made based on the result of the 100-gram, three-hour oral glucose tolerance test, for which there is evidence that treatment improves outcome. Either the plasma or serum glucose level established by Carpenter and Coustan or the plasma level designated by the National Diabetes Data Group is appropriate to use. A positive diagnosis requires that two or more high thresholds be exceeded. The Carpenter and Coustan conversion is used to establish LabCorp reference intervals.
In 2013,2 the American College of Obstetricians and Gynecologists (ACOG) recommended that all pregnant women be screened for gestational diabetes mellitus (GDM)—whether by patient history, clinical risk factors, or with a 50-gram, one-hour loading test at 24 to 28 weeks of gestation to determine blood glucose levels—and suggested relying on the result of the 100-gram, three-hour oral glucose tolerance test for diagnosis (often referred to as a "two-step" method).
The American Diabetes Association (ADA)1 released standards that vary from the ACOG recommendations. The ADA standards recommend a simplified "one-step" approach to the screen and diagnosis of gestational diabetes mellitus with a 75-gram, two-hour glucose tolerance test. The LabCorp test according to the ADA recommendations is Gestational Glucose Tolerance Screening and Diagnostics Test (Two-hour, ADA Recommendations) .
Serum or plasma
1 mL serum or plasma each tube
0.5 mL serum or plasma each tube
Draw a fasting blood sugar before administering 100-gram glucose, and draw blood at one, two, and three hours. The subject should remain seated and not smoke throughout the test. Submit 1 mL serum or plasma for fasting, one-hour, two-hour, and three-hour specimens. Separate serum or plasma from cells within 45 minutes of venipuncture. Gray-top tubes only, may be submitted without centrifugation. Label each tube with patient's name and collection time interval (ie, fasting, one-hour, two-hour, and three-hour).
Maintain specimen at room temperature.3
Causes for Rejection
Frozen gray-top tube (frozen plasma from gray-top is acceptable); stressed patient (surgery, infection, corticosteroids) should not have GTT; specimens not labeled with collection time intervals (i.e., fasting, one-hour, two-hour, and three-hour)
The ADA 2011 standards1 vary from the ACOG recommendations.2 See Additional Information below.
|Order Code||Order Code Name||Order Loinc||Result Code||Result Code Name||UofM||Result LOINC|
|102004||Gestational Glucose Tolerance||102012||Glucose - Fasting||mg/dL||1558-6|
|102004||Gestational Glucose Tolerance||102020||Glucose - 1 hour||mg/dL||1501-6|
|102004||Gestational Glucose Tolerance||102038||Glucose - 2 hour||mg/dL||1514-9|
|102004||Gestational Glucose Tolerance||102046||Glucose - 3 hour||mg/dL||1530-5|
|102004||Gestational Glucose Tolerance||102047||Note:||N/A|
|102004||Gestational Glucose Tolerance||102049||Gest Glucose Tolerance (4)||N/A|