Could Fasting Plasma Glucose Be Used for Screening High-Risk Outpatients for Gestational Diabetes Mellitus? Free

The simplified oral glucose tolerance test (sOGTT) with 50 g is recommended for the screening of gestational diabetes mellitus (GDM) (1). However, because of the complexity of the test, the investigation of less expensive and easier methods to perform the screening has been extensively studied. Recently, in agreement with other studies (3), a population-based prospective study conducted on the general population of pregnant women in Brazil showed that fasting plasma glucose (FPG) was a good screening method for GDM (2).

With the purpose of determining an FPG value with good sensitivity and specificity that could identify pregnant women at risk for GDM, we conducted a study in 1994 with high-risk outpatient pregnant women in the State University Hospital of Rio de Janeiro, Brazil. During that year, 269 pregnant outpatients with at least one risk factor for GDM were included in the study. A screening test was performed on 261 pregnant women with 50 g of glucose load at fasting state. Positivity occurred at a 1-h plasma glucose (PG) ≥130 mg/dl. GDM was diagnosed according to Third International Workshop cutoff values (4). A receiver operating characteristics (ROC) curve was constructed to test the ability of the FPG at the OGTT to discriminate patients with a diagnosis of GDM and to determine the best cutoff value in predicting a GDM diagnosis in our population.

A positive sOGTT was obtained for 100 patients (38.3% [32.6–44.3]), and an OGTT was performed for 77 (77% [68.0–84.4]) pregnant women. A total of 23 patients (23% [155–320]) did not return to test OGTT. GDM was diagnosed in 25 pregnant women (9.6% [6.4–13.6]) (group 1), and in 52 pregnant women (19.9% [15.4–25.1]) (group 2) the OGTT did not meet diagnostic criteria.

By considering the GDM patients as a whole, we noted that 16 (64% [42.6–81.3]) had a diagnosis with altered PG at the 1st and 2nd hour; of these patients, 11 (64.7% [38.6–84.7]) had an FPG <105 mg/dl. PG values at OGTT did not meet any definite standards in nine (36.0% [38.6–84.7]) patients. Median FPG did not meet diagnostic criteria in either group but was higher in group 1 than in group 2 (98.0 mg/dl [72–178] vs. 83.5 [53–141], respectively; P < 0.001).

The area under the ROC curve for the ability of FPG to predict altered 1st- and 2nd-hour, 1st- and 3rd-hour, and 2nd- and 3rd-hour PG during the OGTT were 0.7743 (SE 0.0180) (Fig. 1), 0.7498 (0.0246), and 0.7683 (0.0202), respectively. No difference was observed between the greater and smaller area under the ROC curve (P = 0.21).

The best cutoff point for FPG occurred at 93 mg/dl for predicting altered 1st- and 2nd-hour PG (sensitivity 81.3% and specificity 74.4%) and 2nd- and 3rd-hour PG (75.0 and 69.4%) and at 97 mg/dl for predicting altered 1st- and 3rd-hour PG (85.7 and 81.3%) during the OGTT.

In our study, we could not complete diagnostic criteria in almost one-fourth of our patients (23%), because after the sOGTT they did not return for the OGTT. In terms of cost, the sOGTT is $3.00 (U.S.) and the OGTT is $9.00 (U.S.) per test, whereas the cost of the FPG test is half that of the sOGTT ($1.50; U.S.). During 1994, 2,571,571 term live births occurred in Brazil (5). Although this finding underestimates the total pregnancies for that year, we can infer that using the FPG as a screening method for GDM rather than the sOGTT, we could have saved more than $3,857,000.00 (U.S.).

There were no differences between the areas under the ROC curves, but altered 1st- and 2nd-hour PGs during OGTT showed the most accuracy in diagnosing GDM. FPG at a cutoff value of 93 mg/dl had good sensitivity and specificity to screen for GDM. Although our cutoff values were close to the 95 mg/dl value for FPG during the OGTT recommended for the diagnosis of GDM (1), we can infer that using an FPG to screen for GDM would probably be easier and cheaper than using an sOGTT and therefore should be considered, especially in developing countries such as Brazil. However, it must be emphasized that some positive cases will go undetected when using a screening method with a sensitivity <100%. In our country, this can be justified by greater savings in the health care system, whereas in developed countries, such a procedure may not be justified. Questions concerning the ideal screening trimester and cutoff value remain to be answered, and although our findings cannot be extended to other populations, we expect that they can be confirmed in future prospective studies.