Objective: To evaluate the rate of abnormal results of repeated screening tests for gestational diabetes mellitus when initial tests were normal and related factors.
Subjects: Six hundred women who had clinical risk factors for gestational diabetes mellitus (GDM) who attended the antenatal care clinic at Siriraj Hospital before 24 weeks of gestation, between January and June 2005 were recruited. All had normal screening test.
Material and Method: All subjects were followed throughout their pregnancies. All received repeated screening and confirmatory test at 28-32 weeks of gestation. All data of screening and confirmatory test results were collected and analyzed.
Results: Six hundred pregnant women who had normal screening test for GDM were enrolled. Eighty-seven cases failed to take the second screening test. Of the remaining 513 cases, 154 (30.0%, 95%CI 28.2%-36.3%) had abnormal results in repeated screening tests. Among them 20 cases (3.9%) were diagnosed as GDM. Pregnant women who were > 30 years old or had result of 50g GCT > 120 mg/dl had significant increased risk for abnormal repeated screening tests.
Conclusions: Pregnant women with clinical risk of GDM should receive repeated screening tests when they are 28-32 weeks of gestation. Higher risk was observed among women > 30 years old or those with a result of 50g GCT > 120 mg/dl.

Keywords: Gestational diabetes mellitus, Screening test