A glucose challenge test (50 g, 1 hour) at 24–28 weeks of gestation has become the criterion standard for identifying women at risk of developing gestational diabetes mellitus (GDM).1 Different glucose thresholds during screening have been suggested to distinguish between normal and abnormal screening results within a range of 130–140 mg/dL.2–5 In general, there is an inverse relationship between the glucose threshold and the degree of sensitivity (ie, higher threshold, lower sensitivity). However, enhanced specificity lowers the likelihood of a false-positive test result. Women's risk factors for developing GDM, such as ethnicity, maternal age, and parity, may preclude the necessity for more stringent glucose thresholds to identify women at risk of GDM.5
Furthermore, there is a dearth of data and a lack of consensus in the management approach when screening results are 180–200 mg/dL or greater. Some researchers2,6,7 have suggested a more than a 95% probability of gestational diabetes with these screening results; others8 have suggested a lower rate of GDM with this threshold.
Because there is insufficient data to support the rate of GDM in different glucose tolerance threshold levels and no data related to Mexican-American women, we sought to determine the rate of GDM in different glucose tolerance thresholds in this target population.
MATERIALS AND METHODS
We conducted a prospective study during the period 1995–1999 of 6,857 predominantly Mexican-American women (85%). The study was approved by the institutional review boards of both institutions (St. Luke's-Roosevelt Hospital Center and University of Texas Health Science Center at San Antonio). The study population, drawn from maternal health clinics in metropolitan San Antonio, Texas, serves economically compromised inner-city residents. At the initial visit, a detailed history was taken that included a demographic profile, social history, and summary of past obstetrical and medical data, such as traditional risk factors associated with gestational diabetes and overall pregnancy complications.
Eligibility for the study was limited to women with singleton pregnancies screened at 24–28 weeks of gestation. Screening was performed during the morning hours, with no control for fasting time before the test. Exclusion criteria were a history of GDM and pregestational diabetes. All patients receiving prenatal care underwent a 50-g glucose challenge test at 24–28 weeks of gestation. A screening value of 130 mg/dL or greater was considered positive and was then followed by a 3-hour, 100-g oral glucose tolerance test (OGTT). Gestational diabetes mellitus was diagnosed by 2 or more abnormal values using the Carpenter-Coustan criteria.2 Obesity was defined as prepregnancy body mass index (BMI) of 27.3 kg/m2 or more.
For purpose of analysis, screening test results were categorized by 10-mg/dL increments. For each glucose challenge test category, the rates of 1 abnormal and 2 or more abnormal OGTT values were determined using the Carpenter-Coustan2 and National Diabetes Data Group9 criteria.
Sensitivity, specificity, and positive and negative predictive values for GDM diagnosis were calculated for 3 glucose challenge test thresholds (130, 135, and 140 mg/dL). Continuous data were analyzed by using Student t test, and categorical data were analyzed by using χ2 or Fisher exact tests.
Overall, a positive glucose challenge test screening result (130 mg/dL or more) was identified in 2,541 (37%) of 6,857 of the patients. Gestational diabetes mellitus was diagnosed in 469 (6.8%) of 6,857 patients, and 1 abnormal OGTT value was tested in 351 women (5.1%). Normal glucose challenge test results (threshold less than 130 mg/dL) were obtained in 4,316 of 6,857 women.
We stratified patients according to their screening results into 2 groups: those with levels equal to or greater than 130 mg/dL and those with levels less than 130 mg/dL. The positive glucose challenge test screening group consisted of significantly older, more obese women with a lower rate of nulliparity compared with women with a normal screening value (Table 1).
We further compared patient characteristics of subjects who had a positive screening in the 130–179-mg/dL range with those who had screening values of 180 mg/dL or greater. Patients with thresholds of 180 mg/dL or greater were older, more obese, and had a higher rate of multiparity. Additionally, the OGTT profile characteristics were significantly higher for the high screening group (Table 2).
When the incidence of GDM was analyzed in relation to the glucose challenge test categories, in the category of 130–139 mg/dL (n = 758), 18% of the patients were identified with abnormal OGTT (1 or more abnormal values). In the 5 categories from 140 to 190 mg/dL (n = 1,564), there were approximately 40% abnormal OGTT results. In the 2 glucose challenge test categories of 180–200 mg/dL (n = 219), approximately 60% of the patients were identified with abnormal OGTT results (Table 3).
Calculating the sensitivity, specificity, and positive and negative predictive values for GDM diagnosis using Carpenter-Coustan criteria for the different glucose challenge test thresholds (130, 135, and 140 mg/dL) revealed the highest sensitivity and the lowest specificity for a threshold of 130 mg/dL (Table 4).
A comparison between Carpenter-Coustan and the National Diabetic Data Group criteria for GDM diagnosis revealed that for each glucose challenge test threshold category, a lower rate of GDM is diagnosed with the National Diabetic Data Group criteria than with the Carpenter-Coustan criteria. In our study population, this resulted in an additional 243 (9.6%) GDM patients diagnosed only by the Carpenter-Coustan criteria (Table 5).
The results of this study, which are fairly specific to the Mexican-American population, revealed that, even in high glucose challenge test thresholds (more than 180 mg/dL), the predictive value for GDM was approximately 50%. Furthermore, thresholds of 130 mg/dL have the highest sensitivity for screening GDM in Mexican-American women. With the Coustan-Carpenter criteria for GDM diagnosis, a higher proportion of Mexican-American women are diagnosed than with the National Diabetic Data Group criteria.
Overall, we found that the rate of GDM in our population (6.8%) is higher than that reported to be the prevalent rate in the United States (2–5%).10 If we take into consideration the mild hyperglycemia group (patients with 1 abnormal value in the OGTT), the total rate of abnormal glucose tolerance test levels in our studied patients will increase to 12%. Our finding of an increased rate of carbohydrate intolerance during pregnancy can be explained by considering that the prevalence of GDM is directly related to the prevalence of non–insulin-dependent diabetes mellitus in a given population or ethnic group, which is appreciably high in Mexican-American women. We have previously demonstrated that patients with 1 abnormal value in the OGTT manifest complications similar to those of GDM patients, and with appropriate treatment, these patients will have a significant reduction in adverse pregnancy outcomes.11,12 Furthermore, it has been demonstrated that increasing carbohydrate intolerance in women without overt gestational diabetes was associated with a significantly increased incidence of cesarean delivery, preeclampsia, macrosomia, and need for phototherapy, as well as an increased length of maternal and neonatal hospital stay.13 In addition, Neiger and Coustan14 reported that approximately 40% of the patients diagnosed with 1 abnormal value in the OGTT at 24–28 weeks of gestation will have 2 or more abnormal values on the repeat test and will be classified as having GDM at 32–34 weeks of gestation. These data, along with our findings, ought to raise the question of whether these patients should be diagnosed and treated rather than left untreated during pregnancy.
Our finding of positive glucose challenge test results in 37% of our patients was higher than that previously reported.15 In general, the glucose challenge test is positive in 14–18% of women when we use a glucose cutoff value of 140 mg/dL and in 20–25% with a cutoff level of 130 mg/dL, with respective sensitivity rates of approximately 80% and 90% for the diagnosis of GDM.15 The sensitivity and specificity found in our study at these 2 values are practically the same (160 and 163, respectively); therefore, as a screening procedure, there is no difference between these threshold values. However, the high prevalence of GDM in Mexican Americans will direct us to select a lower threshold for screening. Our finding can be explained by the relatively high rate of obesity in our Mexican-American population. Obesity is one of the main components contributing to increased insulin resistance during pregnancy, which results in abnormal glucose tolerance.
In the glucose challenge test category 130–140 mg/dL, 56 (7.3%) of 758 patients were further diagnosed with GDM, making up 12% of the total GDM patients. Therefore, not using the lower threshold of 130 mg/dL will result in the failure to diagnose GDM in a substantial number patients. Furthermore, with the Carpenter-Coustan criteria, the highest sensitivity is achieved by using the glucose challenge test threshold of 130 mg/dL.
Moreover, because both the National Diabetic Data Group and Carpenter-Coustan criteria are accepted as definitive diagnostic tests for GDM,16 using the latter criteria will significantly increase the detection rate of GDM. Because of the relatively high prevalence of abnormal glucose tolerance during pregnancy in Mexican-American women and the fact that failure to identify and treat GDM may result an increase in perinatal morbidity, we believe that, in Mexican-American women, a glucose challenge test threshold of 130 mg/dL should be considered a positive screening.
Previous studies have suggested that, with a glucose challenge test threshold above 180 mg/dL, there is a greater than 95% probability for GDM.2,7 In these circumstances, an OGTT is not advised, and a diagnosis of GDM can be made based on the glucose challenge test results.7 In these studies, sample sizes were relatively small, which made it difficult to establish the role of GDM diagnosis for each glucose challenge test severity category.
We studied 6,857 women, the majority of whom were Mexican American. The large sample size enabled us to subdivide our study population into several groups, based on incremental levels of glucose challenge test results, and to evaluate the predictive value for diagnosis of GDM in each group. Although a gradual increase in the risk for GDM was identified with higher glucose challenge test thresholds, even in the high glucose challenge test category (more than 180 mg/dL), the predictive value for GDM was only 50–60%. Our data suggest that the glucose challenge test should be used only for screening purposes, and definitive diagnosis should be made only after completion of an OGTT. By adopting this approach, overdiagnosis (unnecessary treatment and GDM labeling) and underdiagnosis (failure to treat unidentified GDM) will be avoided, and more accurate identification of the high-risk population for GDM will be achieved.
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