Gestational diabetes laboratory findings

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Overview

Generally a test for gestational diabetes is carried out between the 24th and 28th week of pregnancy. If patient is at risk for gestational diabetes (see Risk Factors) he or she could prescribe a glucose test earlier in the pregnancy.

Often, gestational diabetes can be managed through a combination of diet and exercise. If that is not possible, it is treated with insulin(usually 15% need Insulin), in a similar manner to diabetes mellitus.

Laboratory Findings

A health care team will check the affected person's blood glucose level. Depending on the mother's risk and her test results, she may have one or more of the following tests.

• Fasting blood glucose or random blood glucose test
• Screening glucose challenge test
• Oral glucose tolerance test (OGTT)

Screening glucose challenge test

There are several tests intended to identify gestational diabetes in pregnant women. The first, called the Screening glucose challenge test, is a preliminary screening test performed between 26-28 weeks. If a woman tests positive during this screening test, the second test, called the Glucose Tolerance Test, may be performed. This test will diagnose whether diabetes exists or not by indicating whether or not the body is using glucose (a type of sugar) effectively. The Glucose Challenge Screening is now considered to be a standard test performed during the second trimester of pregnancy.

The glucose values used to detect gestational diabetes were first determined by O'Sullivan and Mahan (1964) in a retrospective study designed to detect risk of developing type II diabetes in the future. The values were set using whole blood and required two values reaching or exceeding the value to be positive. Subsequent information has led to alteration in O'Sullivan's criteria. For example: when methods for blood glucose determination changed from the use of whole blood to venous plasma samples, the criteria for GDM were also changed once whole blood glucose values are lower than plasma levels due to glucose uptake by hemoglobin (NDDG,1979).

The diagnostic criteria from the National Diabetes Data Group (NDDG) have been used most often, but some centers rely on the Carpenter and Coustan criteria, which set the cutoff for normal at lower values. Compared with the NDDG criteria, the Carpenter and Coustan criteria lead to a diagnosis of gestational diabetes in 54 percent more pregnant women, with an increased cost and no compelling evidence of improved perinatal outcomes.

Oral glucose tolerance test

Women who are considered at risk for gestational diabetes are given a screening test called a 50 gram glucose challenge between the 24th and 28th weeks of pregnancy (those with two or more risk factors may be tested earlier). The glucose challenge is performed by giving 50 grams of a glucose drink and then drawing a blood sample one hour later and measuring the level of blood glucose present. Women with a blood sugar level greater than 140 mg/dl may have gestational diabetes, and require a follow up test called a 3-hour oral glucose tolerance test (OGTT).

The test should be done in the morning after an overnight fast of between 8 and 14 h and after at least 3 days of unrestricted diet (>=150 g carbohydrate per day) and unlimited physical activity. The subject should remain seated and should not smoke throughout the test. The American Diabetes Association sets the following guidelines for results from the OGTT (oral glucose tolerance test)

Fasting blood glucose or random blood glucose test

A fasting plasma glucose level >126 mg/dl (7.0 mmol/l) or a casual plasma glucose >200 mg/dl (11.1 mmol/l) meets the threshold for the diagnosis of diabetes, if confirmed on a subsequent day, and precludes the need for any glucose challenge. In the absence of this degree of hyperglycemia, evaluation for GDM in women with average or high-risk characteristics should follow one of two approaches:

• One-step approach
• Two-step approach

The following are the values which the American Diabetes Association considers to be abnormal during the Glucose Tolerance Test:

• Fasting Blood Glucose Level≥95 mg/dl (5.33 mmol/L)
• 1 Hour Blood Glucose Level≥180 mg/dl (10 mmol/L)
• 2 Hour Blood Glucose Level≥155 mg/dl (8.6 mmol/L)
• 3 Hour Blood Glucose Level≥140 mg/dl (7.8 mmol/L)