Challenging the glucose challenge test

To reduce the number of patients needing oral glucose tolerance test (OGTT), screening options have been considered, balancing patient comfort, cost and risk of missed diagnosis. Australian Diabetes in Pregnancy Society (ADIPS) recommends glucose challenge test (GCT) as screening for gestational diabetes mellitus (GDM), while others suggest fasting plasma glucose (FPG). International Association of Diabetes and Pregnancy Study Group (IADPSG) recently recommended new diagnostic criteria for GDM using one-step OGTT.

(i) To determine how many GDM patients would be missed with GCT/OGTT or FPG/OGTT compared to OGTT alone. (ii) To assess GCT in screening for GDM using new IADPSG criteria.

Austin Pathology database was searched from 2005 to 2007; 8486 episodes of GCT and OGTT were found. Test characteristics were determined for: (i) Simulated GCT/OGTT, where the 60-min OGTT value was regarded as equivalent to 60-min GCT value; (ii) Simulated FPG/OGTT, investigating the utility of different FPG values to indicate need for OGTT.

Oral glucose tolerance test (one-step procedure): Of 5473 patients who had OGTT alone, 14% had GDM (ADIPS criteria). Actual GCT/OGTT: Of 2407 GCT, 17.3% were abnormal, with 75% having normal follow-up OGTT. Simulated studies: In the simulated GCT/OGTT, using ADIPS criteria, GCT had a sensitivity of 87%, specificity of 74% and would miss 13% of cases. Although simulated FG/OGTT had similar sensitivity of 82% for FPG ≥4.4 mmol/L, specificity was 42%. Using IADPSG criteria, 19% were diagnosed with GDM, screening GCT had a sensitivity of 83%, specificity of 75% and would miss 17% of cases.

Oral glucose tolerance test alone is the best procedure without prior preliminary testing.