Oral Glucose Tolerance Test (OGTT) to diagnose Gestational Diabetes Mellitus (GDM): Optimal Testing Recommendation

At 24-28 weeks gestation perform either a 1- or 2- step oral glucose tolerance test (OGTT)

  1. step 75g OGTT:
    • 0 (fasting), 1h, 2h collections

  2. step OGTT (accepted by ADA; recommended by ACOG):
    1. step non-fasting 50g OGTT with collection at 1 hour.
    2. step (if necessary): fasting 100g OGTT with collection at 0, 1, 2 and 3 hours

The American Diabetes Association (ADA) (1) recommends 1- step or 2-step format. The American College of Obstetrician-Gynecologists (ACOG) (2) recommends the 2- step format. Both formats are used widely in the United States.

Guidelines for Test Utilization

What does the test tell me?

This test is used to diagnose GDM. The diagnosis of GDM implies increased risk of negative outcomes for the mother and infant. For the infant, these include macrosomia, large for gestational age, shoulder dystocia, perinatal hypoglycemia, perinatal hyperbilirubinemia and increased risk for future diabetes.  Risks for the mother include pre-eclampsia and cesarian delivery as well as future development of diabetes. [back to top]

When should I order this test?

Test should be ordered at 24-28 weeks of gestation in patients who have no risk factors for diabetes OR who have risk factors but did not meet the standard criteria for diabetes at initial prenatal visit. [back to top]

When should I NOT order this test?

Do not order this test prior to the 24th week of gestation, or after the 28th week. Hemoglobin A1c should NOT be used for the diagnosis of GDM. [back to top]

How should I interpret the result?

  1. step 75g  OGTT 
    GDM is diagnosed if any of the plasma glucose values equal to or exceed those below:
    • Fasting - > 92 mg/dL
    • 1h - > 180 mg/dL
    • 2h - > 153 mg/dL
  2. step OGTT
    • Step 1: non-fasting 50g OGTT with collection at 1 hour.
      If 1- hour glucose is >140 mg/dL then proceed to step 2. If 1- hour glucose is < 140 mg/dL then patient does not have GDM. If more sensitivity for GDM is desired, a 1 h cutoff of 130 mg/dL can be used.<
    • Step 2 (if necessary): fasting 100g OGTT with collection at 0, 1, 2 and 3 hours.
      At least 2 values must meet or exceed the following values
      • 0 (fasting glucose) - > 95 mg/dL
      • 1 hr glucose - > 180 mg/dL
      • 2 hr glucose - > 155 mg/dL
      • 3 hr glucose - > 140 mg/dL

[back to top]

Is the test result diagnostic/confirmatory of the condition? If not, is there a diagnostic/confirmatory test?

Yes. This test identifies women with GDM who have increased risk of negative outcomes for themselves and their children. [back to top]

Are there factors that can affect the lab result?

Results from OGTT are influenced by duration of fast, age, activity, weight, illness, medications, and diet.  [back to top]

Are there considerations for special populations?

Patients diagnosed with GDM should be treated with lifestyle intervention (dietary and exercise counseling) and insulin therapy if necessary to achieve and maintain blood glucose levels as close to normal as possible. [back to top]

What other tests might be indicated?

Women who have risk factors for the development of type 2 diabetes at initial pre-natal visit should be screened for diabetes using the World Health Organization (WHO) 75g, oral glucose tolerance test (OGTT).

Women meeting classic diagnostic criteria for Type 2 diabetes (TD2)(1) at initial peri-natal visit are diagnosed as having diabetes complicating pregnancy rather than Gestational Diabetes Mellitus (GDM).

Women meeting the lower glycemic thresholds for the diagnosis of GDM(1) should be diagnosed with GDM and treated appropriately.

Women meeting neither threshold should be assessed for GDM between 24-28 weeks gestation (see above). [back to top]

References

American Diabetes A. 2. Classification and Diagnosis of Diabetes: Standards of Medical Care in Diabetes-2021. Diabetes Care. 2021;44(Suppl 1):S15-S33. Epub 2020/12/11. doi: 10.2337/dc21-S002. PubMed PMID: 33298413.

ACOG. ACOG Practice Bulletin No. 190: Gestational Diabetes Mellitus. Obstet Gynecol. 2018;131(2):e49-e64. Epub 2018/01/26. doi: 10.1097/AOG.0000000000002501. PubMed PMID: 29370047.


Last reviewed: March 2023. The content for Optimal Testing: the Association for Diagnostics & Laboratory Medicine’s (ADLM) Guide to Lab Test Utilization has been developed and approved by the Academy of Diagnostics & Laboratory Medicine and ADLM’s Science and Practice Core Committee.

As the fields of laboratory medicine and diagnostic testing continue to grow at an incredible rate, the knowledge and expertise of clinical laboratory professionals is essential to ensure that patients receive the highest quality and most useful laboratory tests. ADLM’s Academy and Science and Practice Core Committee have developed a test utilization resource focusing on commonly misused tests in hospitals and clinics. Improper test utilization can result in poor patient outcomes and waste in the healthcare system. This important resource geared toward medical professionals recommends better tests and diagnostic practices. Always consult your laboratory director to make sure these recommendations are appropriate for your patient population.