Abstract
Objective: To evaluate the validity of 50 g oral Glucose challenge test(GCT) as a screening tool for gestational diabetes mellitus(GDM) in terms of its sensitivity, specificity, positive predictive value and negative predictive value in our population.
Study Design: Cross sectional comparative study.
Place and Duration of Study: Gynaecological and Obstetrical clinic, Shalamar Hospital, Lahore from Jul 2006- Jun 2007.
Methodology: Study was conducted on one hundred women carrying singleton pregnancy within 20-35 years of age booked in first trimester. Patients with risk factor of GDM or with established type I or II DM were excluded from study. After informed consent 50 g GCT was done between 24-28 weeks of gestation. Venom plasma glucose level was measured after 1 hour of glucose load using 140 mg/dl as a cut off value. Regardless of results of screening, all patients were tested with 100 g oral glucose tolerance test(OGTT) as a “gold standard” of diagnosis of GDM. Values used were of Carpenter and Coustan. Validity of 50g GCT was calculated in term of sensitivity, specificity, positive predictive value and negative predictive value. Data was analysed using computer software (SPSS) 16.
Results: Out of 100 patients, 19% were screen positive and 81% screened negative on applying 50g GCT. When patients were put to 100 g OGTT, true positive were 10 out of 19(52.6%) screen positive, and false positive were 9 out of 19(47.4%) screen positive. False negative were 3 out of 81(3.7%) screen negative, whereas true negative were 78 out of 81(96.3%) screen negative. Validity of 50 GCT has been calculated to be having specificity of 76.92%, specificity of 89.6%, positive predictive value of 52.6% and negative predictive value of 96.2%.
Conclusion: 50 g GCT is an effective screening tool for GDM when applied to our population between 24-28 weeks of gestation with adequate sensitivity and specificity.

Key words: GDM, OGTT, 50 g GCT, Screening tool for GDM.