Glucose Challenge Test

The glucose challenge test is a screening test for gestational diabetes. A glucose load of 50 g is given orally without fasting, and venous plasma glucose is measured one hour later. A raised value identifies women who require a diagnostic test.

The test has a high sensitivity and low specificity. Most women with a positive screen do not have gestational diabetes, which is why a second test is needed.

Usage. It is used between 24 and 28 weeks in most women. It may be performed earlier in women with high risk.

Indications. Routine screening for gestational diabetes at 24 to 28 weeks, and earlier testing in women with a previous gestational diabetes, prior macrosomic infant, obesity, family history of diabetes, glycosuria, or a history of polycystic ovary syndrome.

Preparation and technique. Fasting is not required. The 50 g glucose solution is drunk over a few minutes, and blood is taken one hour after the start of drinking. The sample is taken as venous plasma, and it is analysed promptly or collected into a tube containing a glycolysis inhibitor.

Contraindications and precautions. Vomiting after the load invalidates the test, and the test is repeated. Women with gastric emptying problems may have false results. Pregnancy is the indication, so no other contraindication applies in general.

Interpretation.

• A plasma glucose under the local threshold is a negative screen.
• A plasma glucose at or over the local threshold, commonly around 7.8 mmol/L, is a positive screen and requires a diagnostic oral glucose tolerance test.
• Thresholds vary between services and assays, and the local threshold is applied.

Causes of a false positive screen. Stress, sedentary behaviour before the test, recent infection, and non-fasting variation.

Causes of a false negative screen. Vomiting, delayed sampling with glycolysis, and inadequate glucose ingestion.

Critical values and findings needing immediate action. No immediate critical value applies. A marked elevation in a symptomatic woman requires prompt review for hyperglycaemia.

Limitations and artefacts. The sensitivity is high but the specificity is low, so most positive screens are false positives in terms of the final diagnosis. Sample delay and sample handling change the glucose value.

Follow up tests. A diagnostic oral glucose tolerance test with fasting and timed plasma glucose measurements, using the local method. Early pregnancy glucose testing is used in high risk women where clinically indicated.

Special groups. Multiple pregnancy and obesity increase the rate of positive screens. Women with pre-existing diabetes do not undergo this test, since diagnosis is already established.