What is the 1-hour glucose challenge test (GCT)?

The 1-hour glucose challenge test or GCT, measures the effect of a glucose solution on blood glucose levels. The GCT does not necessarily diagnose diabetes but it screens women who may be at risk and who may need a 3-hour glucose tolerance test or GTT. A blood sample is taken 1-hour after ingesting a glucose solution. The glucose solution, which is typically flavored, contains 50 - 100 grams of glucose (sugar).

ACOG supports the "2-step" approach to diagnosing diabetes in pregnancy

1. Step 1: 24 – 28-week 1-hour venous glucose measurement following 50g oral glucose solution (1-hour GCT)
2. Step 2: a 3-hour oral glucose tolerance test (3-hour OGTT) if step 1 is positive and the GCT was elevated

Note: Diagnosis of GDM is based on 2 abnormal values on the 3-hour OGTT
ACOG recommends that currently there is insufficient evidence to diagnose GDM based on only one abnormal value
Patients with only one elevated value may require additional surveillance

Typically, the 1-hour glucose tolerance test is given first with the 3-hour glucose tolerance test reserved for women who fail the initial test. 

This screening test is done to identify who may be at risk of diabetes and normally given in the 28th week of pregnancy. It does not usually diagnose diabetes but is performed to identify those who require additional testing for gestational diabetes.

If the test results are below 130 mg/dl (or below 135/140 mg/dl) then no additional testing is required. If the test is above 130 mg (135 or 140 mg) then a 3-hour glucose tolerance test or GTT is ordered. The 3-hour glucose tolerance test uses a 100-gram glucose solution.

If non-fasting glucose levels are greater than 126 mg/dL or normal daily glucose levels are greater than 200 mg/dL, glucose tolerance testing is not necessary as these measurements indicate diabetes.

A glucose threshold value >140 mg/dL identifies approximately 80% of women with pregnancy diabetes (GDM), and the yield is further increased to 90% by using a cutoff of >130 mg/dL. Women whose serum glucose is above the threshold value on the GCT (130-140 mg/dL) have a diagnostic 3-hour GTT done to find out if they have diabetes.

The 3-hour GTT consists of drinking 100g of glucose and having four blood samples drawn: fasting, and then 1, 2, and 3 hours after the drink. Cutoff values for the 3-hour GTT are as follows:

• Fasting: 95 mg/dL
• 1-hour: 180 mg/dL
• 2-hours: 155 mg/dL
• 3-hours: 140 mg/dL

Pregnant women with two or more elevated values are considered to have gestational diabetes mellitus (GDM), though even one elevated value increases certain risks.