Gestational Glucose Tolerance Diagnostic Test (Three-hour, ACOG Recommendations)

Methodology: 
Enzymatic
Performed: 
Monday - Friday
Use: 

This test is used to diagnose gestational diabetes. The test includes100-g glucose load and plasma glucose measured fasting at one, two and three hours.

Specimen Requirements: 
Type: Serum or plasma
 
Container/Tube: Gel-barrier tubes (4) or gray-top tubes (sodium fluoride/potassium oxalate plasma) (4)
 

Sample Volume: 1 mL serum or plasma (each tube)

Minimum Volume: 0.5 mL serum or plasma (each tube)

Storage: Specimens muct be stored at room temperature after collection.1

Stability (collection to time of analysis/testing): 
  • Ambient: 14 days
  • Refrigerated: 14 days
  • Frozen: 14 days
  • Freeze/Thaw cycles: Stable (x3)
Rejection Criteria:
  • Stressed patients (surgery, infection, corticosteroids) should not have GTT.
  • Specimens not labeled with collection time intervals (i.e., fasting, one-hour, two-hour, and three-hour)

Contraindications: Patients with a known history of diabetes mellitus

Reference Values: 

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

65 − 94 mg/dL

65 − 105 mg/dL

One-hour

65 − 179 mg/dL

65 − 190 mg/dL

Two-hour

65 − 154 mg/dL

65 − 165 mg/dL

Three-hour

65 − 139 mg/dL

65 − 145 mg/dL

CPT Code (s): 

82951, 82952

Notes: 

UFHPL Test #: 28085

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.
 

Footnotes

  1. American Diabetes Association. Standards of Medical Care in Diabetes—2015: Summary of revisions. Diabetes Care. 2015 Jan; 38(Suppl 1):S4. PubMed 25537706
  1. American College of Obstetricians and Gynecologists. Committee on Practice Bulletins—Obstetrics. Practice Bulletin N° 137: Gestational diabetes mellitus. Obstet Gynecol. 2013 Aug; 122(2 Pt 1):406-416. PubMed 23969827
Collection procedure: 

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.

Collection

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).
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