Glucose Tolerance Test, Oral (OGTT)

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Subject: Glucose Tolerance Test, Oral (OGTT)

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Definition and Use

  • OGTT should be reserved principally for patients with “borderline” fasting plasma glucose levels. It is necessary for the diagnosis of impaired fasting glucose and impaired glucose tolerance. All pregnant women should be tested for gestational DM with a 50-g dose at 24–28 weeks of pregnancy; if that is abnormal, OGTT should be performed for confirmation. OGTT is the gold standard, and currently, its chief use is in the diagnosis of gestational DM (GDM).

  • Normal range: see Tables 16.35 and 16.36.

 
TABLE 16–35
Blood Test Levels for Diagnosis of Diabetes and Prediabetes
 
TABLE 16–36
Screening and Diagnostic Scheme for GDM

Interpretation

  • Criteria for the diagnosis of DM (males and nonpregnant females) (one of the following) (Table 16.35):

    • Symptoms of DM plus casual (random) plasma/serum glucose concentration ≥200 mg/dL. Casual is defined as any time of day without regard to time since the last meal.

    • Fasting plasma glucose (FPG) ≥126 mg/dL. Fasting is defined as no caloric intake for at least 8 hours.

    • A1C ≥ 6.5%. Test should be performed in laboratory using a method NGSP certified and standardized to the DCCT assay.

    • Two-hour postload glucose (PG) ≥200 mg/dL during an OGTT. The test should be performed using a 75-g glucose load.

      • In the absence of unequivocal hyperglycemia with acute metabolic decompensation, these criteria should be confirmed by repeat testing on a separate day. The third measure (OGTT) is not recommended for routine clinical use.

      • For diagnosis of DM in nonpregnant adults, at least two values of OGTT should be increased (or fasting serum glucose ≥140 mg/dL on more than one occasion) and other causes of transient glucose intolerance must be ruled out.

  • Criteria for the diagnosis of GDM (any degree of glucose intolerance with onset or first recognition during pregnancy), with the screening test for GDM:

    • A fasting serum glucose level >126 mg/dL or a casual plasma glucose >200 mg/dL meets the threshold for the diagnosis of DM if confirmed on a subsequent day, and it 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:

        • Perform a diagnostic 75-g oral glucose tolerance test (OGTT) without prior plasma/serum glucose screening (Table 16.36).

        • This approach may be cost-effective in high-risk patients or populations.

      • Two-step approach:

        • Perform an initial screening by measuring the plasma or serum glucose concentrations 1 hour after a 50-g oral glucose load (GCT) and perform a subsequent diagnostic OGTT on those women exceeding the glucose threshold value on the GCT.

        • A value of ≥140 mg/dL 1 hour after the 50-g load indicates the need for a full diagnostic, 100-g load, 3-hour OGTT performed in the fasting state (Table 16.36).

        • Two or more of the venous plasma concentrations must be met or exceeded for a positive diagnosis. The test should be done in the morning after an overnight fast of between 8 and 14 hours 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.

        • With either approach, the diagnosis of GDM is based on OGTT.

Limitations

  • Prior diet of >150 g of carbohydrate daily, no alcohol, and unrestricted activity for 3 days before test.

  • Test in morning after 10–16 hours of fasting. No medication, smoking, or exercise (remain seated) during test.

  • Not to be done during recovery from acute illness, emotional stress, surgery, trauma, pregnancy, inactivity due to chronic illness; therefore, is of limited or no value in hospitalized patients.

  • Certain drugs should be stopped several weeks before the test (e.g., oral diuretics, oral contraceptives, and phenytoin). Loading dose of glucose consumed within 5 minutes:

  • OGTT is not indicated in

    • Persistent fasting hyperglycemia (>140 mg/dL).

    • Persistent fasting normoglycemia (<110 mg/dL).

    • Patients with typical clinical findings of DM and random plasma glucose >200 mg/dL.

    • Secondary diabetes (e.g., genetic hyperglycemic syndromes, following administration of certain hormones).

    • OGTT should never be used for the evaluation of reactive hypoglycemia.

    • OGTT is of limited value for the diagnosis of DM in children.

Suggested Reading

Standards of Medical Care in Diabetes—2014 position statement. Diabetes Care.  2014;37(1): S14–S80.
 
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