Is the 50-gram glucose screening test ever diagnostic?

Abstract

The standard criteria for the diagnosis of gestational diabetes (GDM) is based on two abnormal values of a 3-h-100-g oral glucose tolerance test (GTT). Although a markedly elevated 1 h–50-g screen value has been suggested to support a diagnosis of GDM, limited data are available to substantiate this empiric observation. Our purpose was to examine the utility of various 50-g screen cutoff values in establishing the diagnosis of gestational diabetes.

We identified 422 gravidas with a positive 50-g screen (≥135 mg/dl) who underwent additional glucose testing. GDM was defined according to the National Diabetes Data Group (NDDG) standards for the 3-h GTT. An analysis employing the criteria of Carpenter and Coustan was performed for comparison. If a patient had an elevated 50-g value and no 3-h GTT was performed, a fasting serum glucose ≥140 mg/dl was considered evidence of gestational diabetes.

One hundred twenty four (29.4%) had GDM as defined by the NDDG criteria; this increased to 161 (38%) when the diagnosis was based on Carpenter and Coustan's criteria. The mean (≥SD) gestational age at screening was 24 ± 7 weeks. As expected, the prevalence of GDM increased in relation to an increasing 50-g value. All subjects with a 50-g screen < 216 mg/dl had evidence of gestational diabetes and required insulin for glycemic control.

Patients with a 50-g screen ≥220 mg/dl do not all require a 3-h GTT. Those with a fasting serum glucose of ≥140 mg/dl may begin diet therapy, glucose monitoring, and insulin as indicated. If the fasting serum glucose is < 140 mg/dl, a 3-h GTT should be performed for confirmation of GDM. This approach will facilitate rapid therapeutic intervention and reduce the cost of care in this subset of patients. Gravidas with a very high 50-g screen are at significant risk of requiring insulin to maintain euglycemia during pregnancy.

Key Words:

• Gestational diabetes
• 50-g glucose screen
• GTT