Glucose Challenge Test (GCT, O'Sullivan Test, 50-Gram 1-Hour Glucose Tolerance Test, Oral Glucose Screening)

Norm of Glucose Challenge Test (GCT, O'Sullivan Test, 50-Gram 1-Hour Glucose Tolerance Test, Oral Glucose Screening)

(Serum)

		SI Units
Normal	< 130 mg/dL	<7.2 mmol/L
Elevated	≥130 mg/dL	≥7.2 mmol/L has 90% sensitivity for gestational DM
Elevated	>180 mg/dL	>10 mmol/L has 50% sensitivity for gestational DM in Mexican-Americans

 

Usage of Glucose Challenge Test (GCT, O'Sullivan Test, 50-Gram 1-Hour Glucose Tolerance Test, Oral Glucose Screening)

Screening during pregnancy for gestational diabetes mellitus (GDM).
High-risk pregnant women should be screened at their initial visit; if negative, repeat at 24–28 weeks or if symptoms occur. High-risk characteristics include member of a high-risk ethnic group, age >30 years, obesity (>20% above ideal body weight), previous history of GDM, glucosuria, strong family history of diabetes (first-degree relative), previous impaired oral glucose tolerance test, previous baby with >9-lb birth weight.
Average-risk pregnant women should be screened at 24–28 weeks of gestation.
Increased
Acromegaly, anoxia, anxiety, brain tumor, cirrhosis, convulsive disorders, Cushing's disease, Cushing's syndrome, diabetes mellitus, dumping syndrome (after gastrectomy), hepatic disease (chronic), hyperlipoproteinemia, hyperthyroidism, infarction (myocardial, cerebral), lipoproteinemia, malnutrition, malignancy, nephritic syndrome, pancreatitis, pheochromocytoma, preeclampsia, pregnancy, sepsis, and stress (physical, emotional). Drugs include those administered in the Glucose tolerance test—Blood.
Decreased
Addison's disease, adrenal insufficiency, anterior pituitary insufficiency congenital adrenal hyperplasia, celiac disease, hepatic insufficiency, hyperinsulinism, hypoglycemia, hypopituitarism, hypothyroidism, insulinoma, islet cell adenoma, malabsorption syndrome, myxedema, steatorrhea, pancreatic islet cell hyperplasia, sprue, and von Gierke's disease. Drugs include those administered in the Glucose tolerance test—Blood.

 

Description of Glucose Challenge Test (GCT, O'Sullivan Test, 50-Gram 1-Hour Glucose Tolerance Test, Oral Glucose Screening)

Glucose is a monosaccharide formed from the digestion of carbohydrates and the conversion of glycogen by the liver and is the body's main source of cellular energy. The glucose challenge test is most commonly used to screen for gestational diabetes. It should NOT be confused with the oral glucose tolerance test. If absorption from the gut has been normal, pregnant women with a level below the cutoff have a low risk of developing GDM. Those with glucose levels at or above the cutoff are at higher risk and should have an oral glucose tolerance test for definitive diagnosis.

 

Professional Considerations of Glucose Challenge Test (GCT, O'Sullivan Test, 50-Gram 1-Hour Glucose Tolerance Test, Oral Glucose Screening)

Consent form NOT required.
Preparation

1. Tube: Gray top.
2. See Client and Family Teaching.

Procedure

1. Client should completely ingest 50 g of glucose solution within 5 minutes. (In some cases, jelly bean candy has been used instead of the glucose solution; however, this method can result in lower test sensitivity.)
2. Note the time the client completed drinking the glucose solution.
3. Draw a 5-mL venous blood sample.

 

Postprocedure Care

1. Label the tube “50-g glucose” and the time it was ingested.
2. Current administration of medications known to affect the test results should be noted on the laboratory requisition.
3. Send blood sample to the laboratory immediately or refrigerate.

 

Client and Family Teaching

1. It is not necessary to fast for this test.
2. No eating, smoking, drinking, or exercise is permitted during the test period. You will be asked to remain seated during the test period.
3. This test usually takes 60–90 minutes.

 

Factors That Affect Results

1. Eating, smoking, drinking, or exercise during the test period invalidate the results.
2. Caffeine interferes with the accuracy of the results. Sips of water may be allowed.
3. Stresses caused by acute illness, pregnancy, or surgery invalidate the results.
4. There is debate regarding the time of day that the GCT should be done. Some evidence indicates that pregnant women having the test in the afternoon were twice as likely to have a positive result than women tested in the morning.

 

Other Data

1. There is debate regarding whether all women should be screened at 24–28 weeks. Arguments for screening all women are that 1%–2% of women with no risk factors develop GDM. The American Diabetic Association indicates that women with low risk of gestational diabetes mellitus do not require glucose testing. Low-risk pregnant women meet all of the following requirements: age <25 years, body mass index of 25 or less, member of ethnic group with a low prevalence of GDM, no known diabetes in first-degree relatives, no history of abnormal glucose tolerance, no history of poor obstetric outcome.
2. There is debate regarding the most appropriate cut-off level for continued diagnostic criteria. The 130 mg/dL level provides more false-positive results and increased cost of continued testing; however, some clinicians feel that the maternal and fetal risk associated with GDM merits this level. Since the cost-benefit ratio of diagnosing GDM remains unresolved, either the 130 mg/dL or 140 mg/dL threshold is considered acceptable.