Gestational diabetes diagnostic study of choice

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1];Associate Editor(s)-in-Chief: Trusha Tank, M.D.[2]

Overview

There are two main methods for diagnosing GDM. The One-step approach: 75-g Oral glucose tolerance test; and the Two-step approach with a 50-g (non-fasting) screen, followed by a 100-g OGTT for those who screen positive. Both methods can accurately diagnose GDM, but one-step strategy has been adopted internationally and has improved pregnancy outcomes with cost savings and may be the preferred method of diagnosing GDM.

Diagnostic recommendations

Following are the recommendations for the diagnostic test for gestational diabetes.

ADA Recommendation for diagnosis of gestational diabetes
Test for undiagnosed diabetes at the first prenatal visit in those with risk factors, using standard diagnostic criteria.
Test for gestational diabetes mellitus at 24–28 weeks of gestation in pregnant women not previously known to have diabetes.
Test women with gestational diabetes mellitus for persistent diabetes at 4–12 weeks postpartum, using the oral glucose tolerance test and clinically appropriate nonpregnancy diagnostic criteria.
Women with a history of gestational diabetes mellitus should have lifelong screening for the development of diabetes or prediabetes at least every 3 years.
Women with a history of gestational diabetes mellitus found to have prediabetes should receive intensive lifestyle interventions or metformin to prevent diabetes.
Adapted from ADA

Diagnostic criteria

If the screening test for GDM is positive, the next step is to confirm the diagnosis. There are 2 strategies for confirming the diagnosis of GDM.

• One-step approach: 75-g Oral glucose tolerance test (OGTT)

OR

• Two-step approach with a 50-g (non fasting) screen, followed by a 100-g OGTT for those who screen positive.

One-Step Strategy

Perform a 75 g glucose tolerance test in 24-28 weeks of pregnancy. Blood glucose level is measured 1 h and 2 h after glucose ingestion, fasting blood glucose is measured prior to the ingestion of the 75g glucose load. The OGTT should be performed in the morning after an overnight fast of at least 8 h. The diagnosis of GDM is made when any of the following plasma glucose values are met or exceeded:

• Fasting: 92 mg/dL (5.1 mmol/L)
• 1 h: 180 mg/dL (10.0 mmol/L)
• 2 h: 153 mg/dL (8.5 mmol/L)

Two-Step Strategy

In this approach, screening is done 1 h after a 50-g glucose load test (GLT), followed by a 3 h 100-g OGTT for those who screen positive.

The diagnosis of GDM is made when at least 2 out of 4 measurements of the 3 h 100-g OGTT is abnormal.

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The following table summarizes the diagnostic approach for gestational diabetes.

	Cut off (mg/dl)
	Fasting	1 Hour	2 Hour	3 Hour
One step test 2 hour 75 g glucose tolerance test	92	180	153	----
Two step test 1 hour 50 g screening test	----	140	----	----
3 hour 100 g test if screening test became positive Carpenter/Coustan approach	95	180	155	140
National Diabetes Data Group (NDDG) approach	105	190	165	145

• The glucose values used for the detection of gestational diabetes were first determined by O'Sullivan and Mahan (1964) in a retrospective study designed to detect the risk of developing type II diabetes in the future. The values were set using whole blood. Subsequent information has led to alterations in O'Sullivan's criteria. Following a change in the method for blood glucose evaluation from the use of whole blood to venous plasma samples, the criteria for GDM were also changed (whole blood glucose values have been shown to be lower than plasma glucose levels due to glucose uptake by hemoglobin).

• The diagnostic criteria from the National Diabetes Data Group (NDDG) have been used most often, but some centers rely on the Carpenter and Coustan criteria, which set the cutoff for normal glycemic status at lower values. Compared with the NDDG criteria, the Carpenter and Coustan criteria led to a diagnosis of gestational diabetes in 54 percent more pregnant women, with an increased cost and no compelling evidence of improved perinatal outcomes.