Glucose Tolerance Tests

Glucose screening and tolerance tests during pregnancy

Common Use of glucose screening and tolerance tests during pregnancy: To evaluate blood glucose levels to assist in diagnosing gestational diabetes mellitus (GDM). ACOG recommends, all pregnant patients should be screened for GDM, whether by the patient’s medical history, clinical risk factors, or laboratory screening test results to determine blood glucose levels.

  • A glucose screening test is a routine test during pregnancy that checks a pregnant woman’s blood glucose level. Also known as “50-g Oral Glucose Challenge Screening test”, is a preliminary screening test performed between 24-28 weeks of pregnancy. No preparation is required prior to the test. During the test, the patient is asked to drink a sweet liquid (containing 50 gram glucose) and then will have blood drawn one hour from having the drink, as blood glucose levels normally peak within one hour. No fasting is required prior to this test.
  • An oral glucose tolerance test (OGTT) is done if a glucose screening test result is higher than normal (2 step approach). It is used to diagnose gestational diabetes.  Patient will not be permitted to eat or drink anything but sips of water for 8 to 14 hour prior to the test. Blood is drawn to measure her baseline “fasting blood glucose level”. Then patient is asked to drink a liquid that contains glucose (100-g glucose load). Following that, blood is drawn and tested every hour for the next three hours.

Specimen: Plasma (1 mL) collected in a gray-top (sodium fluoride) tube. Serum (1 mL) collected in a gold-, red-, or red/gray-top tube or plasma collected in a green-top (heparin) tube is also acceptable, but plasma is recommended for diagnosis. It is important to use the same type of collection container throughout the entire test.

Method: Spectrophotometry.


One step and Two steps approach in screening for and diagnosis of GDM:

GDM diagnosis can be accomplished with either of two strategies:

1. “One-step” 75-g OGTT or
2. “Two-step” approach with a 50-g (nonfasting) screen followed by a 100-g OGTT for those who screen positive


(1) One-step screening Approach

It involves fasting 75-g oral glucose tolerance test.

ADA Threshold Recommendations for Gestational Diabetes (2012) (75-g glucose load):

ADA Threshold Recommendations for Gestational Diabetes (2012) (75-g glucose load)Threshold for abnormal result (Conventional Units)Threshold for abnormal result
(SI Units: Conventional Units × 0.0555)
Fasting sample 92 mg/dL 5.1 mmol/L
1-hr sample180 mg/dL 10 mmol/L
2-hr sample153 mg/dL8.5 mmol/L

According to recommendations of the ADA, GDM is diagnosis when ANY of these thresholds are met or exceeded. 
Plasma glucose values are reported to be 10% to 20% higher than serum values.
ADA = American Diabetes Association.

(2) Two-step screening Approach:

It involves a 50-g (nonfasting) screen followed by a 100-g OGTT for those who screen positive.

50-g Oral Glucose Challenge Screening Test for GDM:

Oral Glucose Challenge Screening TestThreshold for abnormal result (Conventional Units)Threshold for abnormal result
(SI Units: Conventional Units × 0.0555)
ACOG Standard gestational screen (50-g glucose load)*140 mg/dL*7.8 mmol/L
*The ACOG recommends a lower threshold of 135 mg/dL (7.5 mmol/L) in high-risk ethnic populations with higher prevalence of GDM; some experts also recommend 130 mg/dL (7.2 mmol/L).
Plasma glucose values are reported to be 10% to 20% higher than serum values.

 

ACOG Threshold Recommendations for Gestational Diabetes (2011); either Carpenter and Coustan or National Diabetes Data Group (100-g glucose load): 

ACOG Threshold Recommendations for Gestational Diabetes (2011); either Carpenter and Coustan or National Diabetes Data Group (100-g glucose load)Carpenter and CoustanNational Diabetes Data Group
Fasting sample95 mg/dL
(SI: 5.3 mmol/L)
105 mg/dL
(SI: 5.8 mmol/L)
1-hr sample180 mg/dL
(SI: 10 mmol/L)
190 mg/dL
(SI: 10.5 mmol/L)
2-hr sample155 mg/dL
(SI: 8.6 mmol/L)
165 mg/dL
(SI: 9.2 mmol/L)
3-hr sample140 mg/dL
(SI: 7.8 mmol/L)
145 mg/dL
(SI: 8 mmol/L)
According to recommendations of the ACOG or National Diabetes Data Group, the diagnosis of gestational diabetes is made if any two of the four thresholds (measured fasting and 1 h, 2 h, 3 h after the OGTT) are met or exceeded.
Plasma glucose values are reported to be 10% to 20% higher than serum values.
ACOG = American Congress of Obstetricians and Gynecologists.