Gestational Diabetes in Primary Care

Paul Hicks, MD

In This Article

Screening and Diagnosis

The American College of Obstetrics and Gynecology (ACOG) recommends universal screening between 24 and 28 weeks gestation for women of average risk (ACOG does not define what constitutes average risk; for low-risk women, such as teens, selective screening may be considered an alternative.[1] Recently, the Expert Committee on the Diagnosis and Classification of Diabetes Mellitus of the American Diabetes Association (ADA) advocated selective screening. They recommend that women younger than 25 years of age whose body weight is normal and who do not have a family history of diabetes, and who are not included in racial/ethnic groups considered to be at high risk, need not be screened for GDM.[15,16] The ADA recommendations are made on the basis of data collected since the 1994 ACOG guideline was written, but ACOG has not yet endorsed these screening recommendations or made revisions to their 1994 guideline.

How should a practicing clinician decide which screening strategy to adopt? It may be instructive to consider the results of several studies. Williams and colleagues[17] found that using the Expert Committee's criteria resulted in near universal screening, because 90% of the women they screened were either older than 25 years, had abnormal body weight, had a history of diabetes, or were in a racial ethnic group with higher risk. More importantly, such selective screening will miss 4% of cases of GDM. Solomon and coworkers[18] found that in the Nurses' Health Study II, 17% of women were not screened. Among these 17%, nearly 70% had 1 or more risk factors for GDM. Thus, if selective screening is employed, special care must be taken to include all women with risk factors.

Screening in the high-risk populations is essential. There is some evidence that early screening, before 24 weeks gestation, identifies a significant percentage of women with GDM -- 40% in one study.[19] Meyer and colleagues[20] recommend that women with risk factors be screened at one of the early prenatal visits (between 12 and 24 weeks gestation); if the test result is negative, they should be screened again per standard protocol at 24 to 28 weeks.

The standard screening test is the 50-g 1-hour glucose screening test (GST). It is not necessary for the woman to fast before the test or to perform the test at a specific time of day. The sensitivity to detect abnormal glucose tolerance is 80% to 84%. The threshold value for further testing is traditionally 140 mg/dL or greater. Coustan and coworkers,[21] however, found that when women were tested with both the 1-hour GST and 3-hour (100 g) glucose tolerance test (GTT), 10% of women diagnosed with GDM on the basis of the results of the 3-hour GTT had 1-hour GST values between 130 and 139 mg/dL (and would have been misdiagnosed as "normal" by the screening test, alone). Further, Nahum and Huffaker[14] have shown significant ethnic variations in GST results, leading to their recommendation for race-specific threshold values ( Table 2 ) for the 1-hour GST. ACOG recommends further testing when the 1-hour GST result is in the range of 130-140 mg/dL; one may then proceed directly to the 3-hour GTT or simply repeat the 1-hour test 1 week later.

Nearly 25% of women will have a positive 1-hour GST test result and require evaluation with the 3-hour test.[14] Traditionally, women have prepared for this test with a 3-day diet consisting of 75% carbohydrates followed by an 8- to 14-hour fast. However, Entrekin and colleagues[22] found that the results of the 3-hour GTT do not differ significantly when the preparatory diet is omitted. Table 3 lists the threshold values that determine a diagnosis of GDM. A test is considered positive when 2 or more values are met or exceeded. Note that there are 2 different sets of diagnostic criteria with slight variations in the threshold values. There is extensive debate in the literature and between professional organizations as to which criteria should be followed. ACOG leaves the choice to the individual provider. The Fourth International Workshop Conference on Gestational Diabetes Mellitus, held in Chicago, Illinois, March 14-16, 1997, advocated use of the Carpenter-Coustan criteria.[23] Currently, no strong data exist to support one set of criteria over the other. Several comparison trials[24,25] have found similar neonatal morbidity, maternal morbidity, and birth weights with both sets of criteria, suggesting that either set will diagnose most women with GDM. An interesting observation, however, is that although the diagnosis of GDM requires 2 elevated blood glucose levels, just 1 increased value using the O'Sullivan criteria has been repeatedly associated with increased fetal weight, suggesting at least subclinical glucose intolerance in those patients.[26,27,28]


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