Objective
The objective of the study was to compare perinatal outcomes among women diagnosed with gestational diabetes by the National Diabetes Data Group (NDDG) criteria with women meeting only Carpenter-Coustan criteria.
Study Design
This was a 14 year retrospective cohort. Women who screened positive with 1 hour glucose load 140 mg/dL or greater underwent a diagnostic 3 hour oral glucose tolerance test. We report adjusted prevalence ratios (aPRs) of perinatal outcome risk.
Results
Of the 4659 screen-positive women with diagnostic testing, 1082 (3.3%, of 33,179) met NDDG criteria; 1542 (4.6%, of 33,179), or 460 more, met Carpenter-Coustan criteria. These 460 untreated women had greater risk of preeclampsia than women diagnosed by NDDG criteria (aPR, 1.70; 95% confidence interval [CI], 1.23–2.35). They had a greater risk of cesarean delivery (aPR, 1.16; 95% CI, 1.04–1.30) and infants greater than 4000 g (aPR, 1.25; 95% CI, 1.01–1.56) than women not meeting either diagnostic criteria.
Conclusion
The 42.5% additional women diagnosed only by Carpenter-Coustan criteria are at greater risk for some adverse outcomes. Cost-effectiveness of a change remains to be determined.
Gestational diabetes mellitus (GDM) is diagnosed in 4-7% of pregnancies, and the prevalence is likely to continue increasing given the epidemic of obesity in the United States. Uncontrolled hyperglycemia in pregnancy is associated with adverse perinatal outcomes. Although strict glycemic control of women with GDM improves perinatal outcomes, screening and diagnostic criteria remain controversial.
The American Congress of Obstetricians and Gynecologists recommends that all pregnant women be screened for GDM using a random 50 g 1 hour glucose load test, followed by a diagnostic fasting 100 g, 3 hour oral glucose tolerance test (OGTT) if their screening test is positive. Two diagnostic criteria for the 3 hour OGTT currently exist. The National Diabetes Data Group (NDDG) criteria stipulate using fasting, 1, 2, and 3 hour plasma glucose levels of 105, 190, 165, and 145 mg/dL, respectively, for GDM diagnosis.
Carpenter-Coustan (CC) criteria are more inclusive with lower threshold values of 95, 180, 155, and 140 mg/dL. By both criteria, any 2 values at or above the established thresholds diagnose GDM. Debate continues with regard to the most appropriate criteria to apply, and both NDDG and CC criteria remain common in the United States.
Applying Carpenter-Coustan’s lower thresholds, as opposed to the NDDG criteria used at University of North Carolina Center (UNC) hospitals during the study period, would increase the number of women labeled as having gestational diabetics and thus offered treatment. A change to the more inclusive Carpenter-Coustan criteria may be warranted if these women who are currently undiagnosed and thus untreated have an increase in adverse perinatal outcomes compared with women with GDM and treatment by NDDG criteria with women who did not meet either diagnostic criteria.
To answer this question, we assessed perinatal outcomes among all women screened for GDM at our institution over a 14 year period to evaluate the potential impact of diagnosing GDM by Carpenter-Coustan compared with the current practice of diagnosing GDM by National Diabetes Data Group criteria.
Materials and Methods
Study cohort
We performed a retrospective analysis of all women who were eligible for GDM screening and delivered at UNC Women’s Hospital (Chapel Hill, NC) between April 1, 1996 and May 31, 2010. We excluded women who delivered prior to 24 weeks’ gestation, those with pregestational diabetes mellitus, and those without a documented GDM screening test result. For multiple gestations, we used neonatal data for the firstborn. University of North Carolina Institutional Review Board approval was obtained for this study.
Gestational diabetes diagnosis
GDM screening was performed between 24 and 28 weeks’ gestation using a 50 g, 1 hour glucose load test, with plasma glucose values 140 mg/dL or greater considered screen positive. Diagnostic testing was offered to these women and performed using a 100 g, 3 hour OGTT. Women meeting NDDG criteria were diagnosed with GDM and received nutritional counseling and instruction for glucose self-monitoring.
Women monitored capillary blood glucose with goals set as fasting less than 105 mg/dL and 1 hour postprandial less than 140 mg/dL or 2 hour postprandial less than 130 mg/dL. Adequate glycemic control at our institution was defined as 50% or more of blood glucose levels at goal levels. Medical therapy was initiated (subcutaneous insulin or oral glyburide) if adequate glycemic control was not achieved with diet control alone as determined by the primary obstetrical provider.
Women who screened positive (1 hour glucose load 140 mg/dL or greater) but did not meet NDDG diagnostic criteria received routine prenatal care. Three hundred twenty women who had elevated 1 hour glucose load results that prompted a GDM diagnosis by their primary provider, and thus did not undergo a 3 hour OGTT, were excluded from this analysis.
The 3 study groups for this analysis included the following: (1) women who would be diagnosed with GDM only by Carpenter-Coustan criteria (CC only); (2) women diagnosed and treated for GDM by NDDG criteria (NDDG), regardless of subsequent treatment (diet control vs medical management with insulin or glyburide) required; and (3) women who screened positive but had a negative 3 hour OGTT and were not diagnosed with GDM by either criteria (negative OGTT).
Data abstraction
Clinical providers prospectively record perinatal data from all deliveries at UNC . Trained abstractors enter all information into and maintain the UNC Perinatal Database. Prior to analysis, outliers and clinically implausible values (eg, maternal age >50 years or birthweight >6000 g) were identified by exploratory analysis and corrected when possible by review of original paper charts and electronic medical records. A random sample of 200 patient records was cross-referenced with original paper charts and electronic medical records to assess accuracy of key variables.
We abstracted maternal demographic data and pregnancy diagnoses. Race/ethnicity was self-reported from choices in the prenatal record (white, African American, Hispanic, or Asian) and was collected to assess the potential relationship between race/ethnicity and GDM diagnoses and outcomes.
Perinatal outcomes
We examined perinatal outcomes shown to improve with treatment of mild GDM in randomized controlled trials or be statistically significant in retrospective studies. Measured outcomes included the following: gestational age at delivery, preterm birth less than 37 weeks, mode of delivery (spontaneous vaginal delivery, vacuum-assisted vaginal delivery, forceps-assisted vaginal delivery, or cesarean delivery), third- or fourth-degree perineal laceration, gestational hypertension, preeclampsia (composite of mild, severe, eclampsia, and/or HELLP [hemolysis, elevated liver enzymes, and low platelet count] syndrome), birthweight (grams), macrosomia greater than 4000 g, shoulder dystocia (abstracted from provider notation in perinatal record), neonatal intensive care unit (NICU) admission, and NICU stay longer than 48 hours.
Statistical analysis
We compared women who would have been diagnosed with GDM only by the more inclusive Carpenter-Coustan criteria (CC only) with each of the other 2 study groups: women diagnosed with and treated for GDM by NDDG criteria (NDDG); and women who screened positive but had a negative OGTT by both diagnostic criteria (negative OGTT). The bivariate analyses included Student t test and Wilcoxon rank-sum test for continuous and Pearson’s χ 2 for categorical variables. Means with standard deviation (SD) and medians with interquartile ranges (IQRs) were reported for continuous variables with normal and nonnormal distributions, respectively.
We compared the prevalence of dichotomous adverse outcomes using unadjusted and adjusted regression models. Significant variables in the bivariate analysis and those known to be strong clinical risk factors for the outcome of interest were considered for inclusion in the adjusted models. We considered continuous, dichotomous, linear, and squared terms of potential confounders and report the most parsimonious models.
We fit Poisson regression models with robust standard errors to account for the fact that some women contributed data on more than one pregnancy during the study period. We report adjusted prevalence ratios (aPRs) with 95% confidence intervals (CIs).
We also fit a linear regression model for birthweight as a function of gestational age at delivery within each group, allowing for a nonlinear relationship between the two. A P < .05 and CIs that excluded the null were considered statistically significant. Stata 10 (StataCorp, College Station, TX) was used to perform all analyses with the exception of the linear regression models, for which we used SAS 9.1.3 (SAS Institute, Inc, Cary, NC).
Results
Between April 1, 1996, and May 31, 2010, 33,179 women were screened for GDM and thus met initial study inclusion criteria. A total of 1082 women were diagnosed by NDDG criteria, and 1542 would be diagnosed by Carpenter-Coustan criteria. This represents a 42.5% increase in GDM diagnoses, from 3.3% (1082 of 33,179) to 4.6% (1542 of 33,179), using the more inclusive criteria. On average, an additional 33 women would be diagnosed with GDM per year in our cohort.
Of the 33,179 women screened, 5454 screened positive for GDM based on a 50 g, 1 hour glucose load 140 mg/dL or greater and were neither diagnosed with GDM based solely on this result nor excluded based on established criteria. Eighty-five percent (4659 of 5454) underwent a diagnostic 100 g, 3 hour OGTT and had results available in our database to confirm or exclude GDM diagnosis ( Figure 1 ) .
