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Glucose Tolerance Test, Oral (OGTT)


Definition and Use


  • OGTT should be reserved principally for patients with "borderline " ť 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.

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TABLE 16 " “35Blood Test Levels for Diagnosis of Diabetes and PrediabetesView LargeTABLE 16 " “35Blood Test Levels for Diagnosis of Diabetes and Prediabetes Hemoglobin A1c (%) Fasting Glucose (mg/dL) 2-Hour OGTT (mg/dL) Normal ≤5.6 ≤99 ≤139 Prediabetes 5.7 " “6.4 100 " “125 140 " “199 Diabetes ≥6.5 ≥126 ≥200
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TABLE 16 " “36Screening and Diagnostic Scheme for GDMView LargeTABLE 16 " “36Screening and Diagnostic Scheme for GDM One-Step (IADPSG*Consensus) 75-g OGTT State Plasma Glucose (mg/dL) Fasting ≥92 1 h ≥180 2 h ≥153 Two-Step (NIH Consensus) 50-g GCT (First Step) and 100-g OGTT (Second Step) Plasma Glucose (mg/dL) State 50-g GCT 100-g OGTT Carpenter/Coustan NDDG "   Fasting 95 105 1 h ≥140 " ˇ 180 190 2 h 155 165 3 h 140 145 *International Association of Diabetes and Pregnancy Study Groups. "  National Diabetes Data Group. " ˇAmerican College of Obstetricians and Gynecologists recommend 135 mg/dL for high-risk ethnic minorities with higher prevalence of 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


1Standards of Medical Care in Diabetes " ”2014 position statement. Diabetes Care.  2014;37(1): S14 " “S80.
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