50-gram one-hour glucose tolerance challenge
The patient need not be fasting. The one-hour screen requires a 50-gram oral glucose load or 50-gram "Fresh Test" product (https://www.labcorp.com/provider-services/programs/partners-pregnancy) followed by a plasma glucose determination one hour later. (Refer to Additional Information below.) See the online Endocrine Appendix: Glucose Tolerance: Gestational Diabetes Mellitus for further information regarding glucose tolerance testing.
Within 1 day
Turnaround time is defined as the usual number of days from the date of pickup of a specimen for testing to when the result is released to the ordering provider. In some cases, additional time should be allowed for additional confirmatory or additional reflex tests. Testing schedules may vary.
Serum or plasma
1 mL serum or plasma each tube
0.5 mL serum or plasma each tube
Gel-barrier tube or gray-top (sodium fluoride/potassium oxalate plasma) tube
Note: A fasting blood sample is not required. Draw one hour specimen after a 50-gram glucose challenge. The patient should remain seated and not smoke throughout the test. Submit 1 mL serum or plasma for one-hour specimen. Separate serum or plasma from cells within 45 minutes of venipuncture. Gray-top tubes only, may be submitted without centrifugation. Label tube with patient's name and collection time interval (i.e., one-hour).
Maintain specimen at room temperature.1
None; glucose challenge is administered without regard to time of day or time of last meal.
Frozen gray-top tube (frozen plasma from gray-top is acceptable); stressed patient (surgery, infection, corticosteroids) should not have GTT; specimen not labeled with collection time intervals (i.e., one hour)
Screening test for gestational diabetes
The American College of Obstetricians and Gynecologists (ACOG) recommends 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 suggests relying on the result of the 100-gram, three-hour oral glucose tolerance test for diagnosis (often referred to as a "two-step" method).2
The American Diabetes Association (ADA)3 released standards that vary from the ACOG recommendations. The ADA recommends 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) . A glucose tolerance screen glucose threshold >139 mg/dL after a 50-gram load identifies approximately 80% of women with gestational diabetes mellitus, while the sensitivity is further increased to approximately 90% by a threshold >129 mg/dL. Perform a diagnostic 100-gram oral glucose tolerance test (102004) on a separate day on women who exceed the chosen threshold on 50-gram screening.
|Order Code||Order Code Name||Order Loinc||Result Code||Result Code Name||UofM||Result LOINC|
|102277||Gest. Diabetes 1-Hr Screen||1504-0||102285||Gestational Diabetes Screen||mg/dL||1504-0|
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