Test Details
Methodology
Enzymatic
Result Turnaround Time
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.
Test Includes
100-g glucose load; plasma glucose measured fasting, and at one, two, and three hours
Use
Diagnose gestational diabetes
Special Instructions
The ADA 2011 standards1 vary from the ACOG recommendations.2 See Additional Information below.
Contraindications
Custom Additional Information
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) [101000].
Specimen Requirements
Specimen
Serum or plasma
Volume
1 mL serum or plasma each tube
Minimum Volume
0.5 mL serum or plasma each tube
Container
Gel-barrier tubes (4) or gray-top (sodium fluoride/potassium oxalate plasma) tubes (4)
Collection Instructions
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 (i.e., fasting, one-hour, two-hour, and three-hour).
Stability Requirements
Temperature | Period |
|---|---|
Room temperature | 14 days |
Refrigerated | 14 days |
Frozen | 14 days |
Freeze/thaw cycles | Stable x3 |
Reference Range
See table.
Specimen* | 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. | ||
Fasting | 70−94 mg/dL | 70−105 mg/dL |
One-hour | 70−179 mg/dL | 70−190 mg/dL |
Two-hour | 70−154 mg/dL | 70−165 mg/dL |
Three-hour | 70−139 mg/dL | 70−145 mg/dL |
Storage Instructions
Maintain specimen at room temperature.3
Patient Preparation
For three days prior to the test, patients should eat an unrestricted diet of 150 or more grams of carbohydrate and be permitted unrestricted physical activity. Patients should be instructed to eat and drink nothing except water for at least eight hours and not more than 14 hours before the test. Patients should also be advised to discontinue, whenever possible, all nonessential medication that can affect glucose metabolism at least three days before testing.
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)
Footnotes
LOINC® Map
| Order Code | Order Code Name | Order Loinc | Result Code | Result Code Name | UofM | Result LOINC |
|---|---|---|---|---|---|---|
| 102004 | Gestational Glucose Tolerance | 50586-7 | 102012 | Glucose - Fasting | mg/dL | 1558-6 |
| 102004 | Gestational Glucose Tolerance | 50586-7 | 102020 | Glucose - 1 hour | mg/dL | 1501-6 |
| 102004 | Gestational Glucose Tolerance | 50586-7 | 102038 | Glucose - 2 hour | mg/dL | 1514-9 |
| 102004 | Gestational Glucose Tolerance | 50586-7 | 102046 | Glucose - 3 hour | mg/dL | 1530-5 |
| 102004 | Gestational Glucose Tolerance | 50586-7 | 102047 | Note: | N/A | |
| 102004 | Gestational Glucose Tolerance | 50586-7 | 102049 | Gest Glucose Tolerance (4) | N/A | |
| Order Code | 102004 | |||||
| Order Code Name | Gestational Glucose Tolerance | |||||
| Order Loinc | 50586-7 | |||||
| Result Code | 102012 | |||||
| Result Code Name | Glucose - Fasting | |||||
| UofM | mg/dL | |||||
| Result LOINC | 1558-6 | |||||
| Order Code | 102004 | |||||
| Order Code Name | Gestational Glucose Tolerance | |||||
| Order Loinc | 50586-7 | |||||
| Result Code | 102020 | |||||
| Result Code Name | Glucose - 1 hour | |||||
| UofM | mg/dL | |||||
| Result LOINC | 1501-6 | |||||
| Order Code | 102004 | |||||
| Order Code Name | Gestational Glucose Tolerance | |||||
| Order Loinc | 50586-7 | |||||
| Result Code | 102038 | |||||
| Result Code Name | Glucose - 2 hour | |||||
| UofM | mg/dL | |||||
| Result LOINC | 1514-9 | |||||
| Order Code | 102004 | |||||
| Order Code Name | Gestational Glucose Tolerance | |||||
| Order Loinc | 50586-7 | |||||
| Result Code | 102046 | |||||
| Result Code Name | Glucose - 3 hour | |||||
| UofM | mg/dL | |||||
| Result LOINC | 1530-5 | |||||
| Order Code | 102004 | |||||
| Order Code Name | Gestational Glucose Tolerance | |||||
| Order Loinc | 50586-7 | |||||
| Result Code | 102047 | |||||
| Result Code Name | Note: | |||||
| UofM | ||||||
| Result LOINC | N/A | |||||
| Order Code | 102004 | |||||
| Order Code Name | Gestational Glucose Tolerance | |||||
| Order Loinc | 50586-7 | |||||
| Result Code | 102049 | |||||
| Result Code Name | Gest Glucose Tolerance (4) | |||||
| UofM | ||||||
| Result LOINC | N/A |