Glucose Tolerance for Gestational Diabetes

Gestational diabetes is defined as carbohydrate intolerance of varying degrees of severity with onset during pregnancy. Rather than predicting the development of diabetes later in life, as originally proposed, the main purpose of identifying gestational diabetes is to detect women at risk of adverse perinatal outcomes. Gestational diabetes affects ~14% of pregnant women. After a pregnancy with GDM, a woman has an increased risk of developing type 2 diabetes mellitus within 10 years postpartum.

Fetal macrosomia affects 40% of the offspring of women with GDM. Macrosomia is associated with increased risk of birth injuries as a result of the large size of the fetus. Infants of women with GDM are at higher risk of developing obesity, impaired glucose tolerance or diabetes mellitus at an early age. Keeping 1-hour postprandial blood glucose levels between 120 and 140 mg/dL minimizes the risk of macrosomia.

Recent guidelines do not recommend screening women who are:

Under 25 years of age
Normal body weight
No personal or family history of abnormal glucose metabolism
No history of poor obstetric outcome
Do not belong to an ethnic/racial group with a high prevalence of diabetes (Hispanic American, Asian American, African American, Pacific Islander)

Pregnant women who have a a fasting plasma glucose level >126 mg/dL or a casual plasma glucose level of >200 mg/dL during their initial prenatal visit meet the threshold for the diagnosis of diabetes. In the absence of unequivocal hyperglycemia, the diagnosis must be confirmed on a subsequent day. Confirmation precludes the need for any glucose challenge.

In the absence of overt hyperglycemia, the recommended screening test for women at high risk of gestational diabetes is the 50-gram 1-hour glucose tolerance test. Screening should be performed at 24 to 28 weeks' gestation, at any time of day, and without regard to previous meals.

The test is performed as follows:

The glucose-loading dose of 50 g is consumed over 5 minutes.
The patient should remain seated and can drink as much water as desired.
Blood is drawn at 1 hour after the glucose challenge.
The specimen should be refrigerated after collection.

A glucose value >140 mg/dL is considered a positive gestational diabetes screening test. This cutoff value identifies 80% of women with gestational diabetes. Sensitivity is increased to 90% if a cutoff of >130 mg/dL is used.

ACOG recommends further testing when the 1-hour result is >130 mg/dL. Nearly 25% of women will have a positive 1-hour test and require evaluation with the 3-hour test.

Most of the world uses the criteria of the World Health Organization (WHO) to diagnose GDM, which are based on a 75-g glucose load. However, physicians in the United States use the criteria endorsed by the National Diabetes Data Group (NDDG) and the ADA, which are based on a 100-g glucose load. An abnormal oral glucose tolerance test is defined as two or more blood glucose concentrations as high or higher than those listed in the following table.

Criteria for the Diagnosis of Gestational Diabetes Mellitus

	NDDG	ADA	ADA	WHO IGT	WHO GDM
Glucose Load	100	100	75	75	75
FPG	105	95	95	NA	140
1 h pp	190	180	180	180	NA
2 h pp	165	155	155	155	200
3 h pp	145	140	140	NA	NA

FBG = fasting plasma glucose; pp=postprandial

Specimen requirement is one gray top (potassium oxalate-sodium fluoride) tube of blood.




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