Why Is the OGTT Used in Pregnancy?

Julie Sease, PharmD


February 28, 2011


Why is the oral glucose tolerance test used to screen for gestational diabetes? What are the latest guidelines?

Response from Julie Sease, PharmD
Associate Professor, Presbyterian College School of Pharmacy, Clinton, South Carolina; Clinical Pharmacy Specialist, Good Shepherd Free Medical Clinic, Clinton, South Carolina

Criteria for diagnosing diabetes in nonpregnant individuals include an A1c level ≥ 6.5%, a fasting plasma glucose (FPG) level ≥126 mg/dL, a 2-hour plasma glucose level ≥ 200 mg/dL during an oral glucose tolerance test (OGTT) using a glucose load of 75 g, or classic symptoms of hyperglycemia plus a random plasma glucose level ≥ 200 mg/dL.[1] Barring the presence of overt hyperglycemia, any abnormal result should be verified with repeat testing.

Why is the OGTT the recommended screening strategy for gestational diabetes mellitus (GDM), which is diabetes diagnosed during pregnancy in a woman not previously known to have diabetes? First, the OGTT has been used for screening in studies on GDM and its treatment and outcomes.[2,3,4,5,6,7,8,9,10] The same cannot be said for other diagnostic criteria, including FPG.

Second, pregnancy is characterized by abnormal erythrocyte turnover, which increases the likelihood of inaccurate A1c results. For this reason, A1c measurement is not appropriate for diagnosis of GDM.[1] Therefore, the OGTT is the screening test recommended by such organizations and agencies as the American Diabetes Association (ADA), the American Congress of Obstetricians and Gynecologists, the World Health Organization, and the US Preventive Services Taskforce.

The 2011 ADA guidelines mirror recommendations from the International Association of Diabetes and Pregnancy Study Groups.[11] Screening for undiagnosed type 2 diabetes is recommended at the first prenatal visit in women who have risk factors.[1] At this point in pregnancy, standard diagnostic criteria (as noted above) are recommended. In all pregnant women who are not already known to have diabetes, screening should be done with a 75-g 2-hour OGTT between 24 and 28 weeks of gestation. The currently recommended diagnostic cut points for plasma glucose are a fasting level ≥ 92 mg/dL, a 1-hour level ≥ 180 mg/dL, and a 2-hour level ≥ 153 mg/dL.


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