The Practice of Early Gestational Diabetes Screening Evaluated

The U.S. Centers for Disease Control and Prevention estimates that each year, gestational diabetes mellitus (GDM) affects between 2 percent and 10 percent of pregnancies. Experts say there has been a steady rise in the prevalence of gestational diabetes as risk factors for the disease are also increasing (e.g., rates of obesity and increasing age of women at the time of pregnancy).

Despite American College of Obstetricians and Gynecologists recommendations for GDM screening, questions remain, particularly about early screening in practice, including concerns about diagnostic thresholds and the utility of early screening in obese women.

Several studies presented at the Society for Maternal-Fetal Medicine's annual Pregnancy Meeting (Las Vegas; Feb. 11-16) by a group of researchers from University of Alabama at Birmingham sought to answer these practice-based questions related to GDM screening.

Diagnostic Thresholds for Early GDM Screening
Early screening (14 to 20 weeks) for GDM may require using lower diagnostic cutoffs than are used for regular GDM screening at 24 to 28 weeks, according to a study presented at the conference.

The researchers randomized 912 obese women to either early GDM screening (14 to 20 weeks) or to routine screening (24 to 28 weeks). GDM screening used a 50-g, 1-hour glucose challenge test followed by a 100-g, 3-hour glucose tolerance test if test results were ≥135 mg/dL. Test characteristics were assessed, comparing early and regular screening, as well as the ideal cutoff. Lastly, the incidence of a composite of adverse perinatal outcomes was evaluated above and below these cutoffs.

Analysis showed that just over one-third of women (35 percent) had a glucose challenge test available at both time points and 9.4 percent were diagnosed with GDM after 24 wks.. The mean gestational age at first screen was 17.3 weeks and 26.2 weeks at second screen. Results from glucose challenge tests performed at 14 to 20 weeks were closely associated with GDM at 24 to 28 weeks (area under the curve, 0.80), with an optimal cutoff of 130 mg/dL. Lowering the threshold from 135 mg/dL to 130 mg/dL increased the sensitivity from 63 percent to 70 percent and increased the number of glucose tolerance tests performed by four per 100 patients.

Early glucose challenge test results of 130 mg/dL or above were associated with a significantly higher incidence of adverse perinatal outcome. Using the Liu method, ideal cutoffs for early diagnosis of GDM using the glucose tolerance test were 97 (fasting), 155 (1-hour), 127 (2-hour), and 95 (3-hour). Applying these cutoffs yielded an additional 37 women with an early diagnosis of GDM, 43 percent of whom were diagnosed with GDM at 24-28 weeks and 75 percent of whom had the adverse composite outcome.

The authors say that while evidence suggests lower diagnostic cutoffs may be needed for early GDM screening, outcomes with these lower levels should be assessed before implementation.

HbA1C Should Not Be Used for Early Screening
The University of Alabama researchers used data from the same randomized controlled to identify 501 women who had both a normal HbA1c in early pregnancy and screening for GDM at 24-28 weeks. The relationship between HbA1c and GDM was assessed, as well as ideal test characteristics for HbA1c.

The researchers found that the average gestational age at HbA1c measurement was 17.5 weeks and the average HbA1c level was 5.3 percent. The gestational age at repeat testing was 26.3 weeks, and 51 women (10.2 percent) were diagnosed with GDM at 24 to 28 weeks. The receiver operator characteristics curve had an area under the curve of 0.63, indicating only a moderate association between early HbA1c and a diagnosis of GDM, the authors say.

At the selected cutoff of 5.4 percent, sensitivity and specificity were both poor (60.8 percent and 60 percent, respectively). Further, HbA1c measurements greater than the cutoff point were not associated with the primary adverse perinatal composite outcome.

"HbA1c for early screening for GDM in obese women performs poorly and should not be used for this purpose," conclude the authors led by Elizabeth Ausbeck,

Rationale for Early GDM Screening Questioned
Early screening for gestational diabetes may not be linked with better perinatal outcomes in obese women, according to a large study presented at the conference.

Researchers used data from the same randomized controlled to assess the benefits of early screening in terms of a composite of perinatal outcomes. In addition to the glucose challenge test, HbA1c was also measured for all patients. Women negative for GDM at early screening were rescreened at 24 to 28 weeks.

The researchers found that of the 454 women randomized to early screening, 15.2 percent were diagnosed with GDM—6.4 percent before 20 weeks and 8.8 percent after 24 weeks. Approximately 12 percent of the 458 women randomized to routine screening had GDM.

However, early screening did not reduce the incidence of the composite of adverse perinatal outcomes, which was actually nominally higher in the early screening group (59.0 percent versus 53.3 percent). Further, hypertensive disease of pregnancy trended higher in the early screening group (13.5 percent versus 9.6 percent in the routine screening group) and use of insulin was significantly increased in the early screening group (2.6 percent versus 0.7 percent in the routine screening group).

"Recommendations for early GDM screening need to be reassessed in light of these findings," conclude the authors, led by Lorie Harper, who also call for additional large studies in diverse populations to validate these findings.

Takeaway: New evidence may clarify some unanswered questions about the practice of early GDM screening, particularly in obese women.