Oral Glucose Tolerance Test (oGTT)

The oral glucose tolerance test (oGTT) is used to detect impaired glucose utilization and in the early diagnosis of diabetes mellitus (diabetes).

The procedure

Material needed

  • 1.0 ml NaF blood per blood draw for glucose or 1.0 ml venous whole blood with GlucoEXAKT (Sarstedt) per blood draw for glucose

Preparation of the patient

  • Before the test, the patient should have followed a high-carbohydrate diet for at least three days.
  • 12 hours before the test – no tobacco use, do not drink coffee, tea or alcohol.
  • Before starting the test Adhering to a fasting period of at least eight hours.
  • Test start between six and nine o’clock in the morning
  • Woman: at least three days apart menstruation
  • No other tests will be performed during the test.

Interfering factors

  • Confounding factors that may affect glucose tolerance:
    • Hunger state
    • Hypokalemia (potassium deficiency)
    • Stress
    • Long bedriddenness
    • Severe heart failure (cardiac insufficiency)
    • Hyperlipoproteinemia (lipid metabolism disorder).
    • Liver cirrhosis – irreversible (non-reversible) damage to the liver and a pronounced remodeling of liver tissue.
    • Metabolic acidosis (uremia).
    • Hyperthyroidism
  • Discontinue interfering medications (if possible) three days before:
    • Diuretics (especially thiazides).
    • Benzodiazepines
    • Hormones
      • Hormonal contraceptives
      • Thyroid hormones
      • Steroids
    • Non-steroidal anti-inflammatory drugs (NSAIDs)
    • Laxatives
    • Nicotinic acid
    • Nitrazepam
    • Phenothiazines, phenacetin

Patient (non-pregnant)

Implementation

  • At least 14 days interval from acute illness.
  • The patient has blood drawn for fasting serum glucose level determination in the morning on the day of examination – without having eaten or drunk anything in the last eight hours. Then he drinks 75 g of glucose dissolved in tea or a ready-made preparation on an empty stomach: 75 g of dextrose, e.g. Dextro-Energen to 300 ml of water.
  • The glucose serum level of the patient is measured fasting and after 2 hours.

Indications (areas of application)

  • Patients with signs of metabolic syndrome, that is, in all overweight individuals who have additional risk factors such as elevated blood pressure and/or elevated lipid (blood fat) levels
  • In case of impaired fasting glucose (100 to 125 mg/dl).
  • Relatives 1st degree of type 2 diabetics.
  • CHD patients and obese people.
  • Pregnant women to exclude gestational diabetes mellitus (GDM).

Contraindications

  • Manifest diabetes mellitus
  • Ketonuria (occurrence of supernormal amounts of keto bodies) without glucosuria (excretion of glucose through the urine by the kidney)
  • Acidosis (hyperacidity)
  • Febrile diseases
  • Hepatitis (inflammation of the liver)

Normal values

According to the German Diabetes Association, there are the following diagnostic criteria (AWMF guideline):

Postprandial, nonpregnant (oGTT- 2h value).

Plasma, venous Whole blood (capillary, hemolyzed) Assessment
< 140 mg/dl(< 7.8 mmol/l) < 140 mg/dl(< 7.8 mmol/l) Normal
140-199 mg/dl(7.8-11.0 mmol/l) 140-199 mg/dl(7.8-11.0 mmol/l) Impaired glucose tolerance
≥ 200 mg/dl(≥ 11.1 mmol/l) ≥ 200 mg/dl(≥ 111 mmol/l) Diabetes mellitus
  • Abnormal fasting glucose is present when the fasting value is between 100-126 mg/dl (5.6-7.0 mmol/l)

Confounding factors

  • False-positive results can occur with too little carbohydrate intake on previous days or medication use such as diuretics (dehydrating drugs) or laxatives (laxatives).
  • False-negative results may occur with antihypertensives (blood pressure-lowering medications) or physical activity during the test.
  • Other confounding factors see above

    • .

Further notes

  • Notice: If the two-hour value is elevated in the oGTT, one-third of the insulin-producing beta cells have already failed.
  • A study of 4,867 individuals and follow-up for a mean of 39 years examined whether type 2 diabetes and vascular complications developed and what causes of death occurred in relation to elevated one-hour glucose (> 8.6 mmol/l) or two-hour glucose (≥ 7.8 mmol/l). Results: 13% of participants developed type 2 diabetes. Among these, the one-hour glucose level was associated with a 3.4-fold increased future risk of diabetes. Furthermore, one-hour glucose was significantly associated with vascular complications and premature death.Conclusion: One-hour glucose is a stronger predictor of diabetes than two-hour glucose. It could probably be equally a very indirect indicator of insulin resistance with liver involvement.

Pregnant

Implementation

  • Time point: screening test in all pregnant women 24 + 0 to 27 + 6 SSW (week of gestation).
  • 50-g glucose screening test (Glucose Challenge Test, GCT): the test is performed by drinking 50 g of anhydrous glucose in 200 ml of water, regardless of food intake and time of day. The pregnant woman must not be fasting. The time of day is arbitrary.The glucose serum level of the pregnant woman is measured after 60 minutes. Blood glucose is measured from venous plasma.
  • 75-g-oGGT: To determine the fasting serum glucose level, blood is taken from the pregnant woman in the morning on the day of the examination, fasting – without having eaten or drunk anything in the last eight hours. Then she drinks 75 g of glucose dissolved in tea or a ready-made preparation on an empty stomach: 75 g of dextrose, e.g. Dextro-Energen to 300 ml of water.The glucose serum level of the pregnant woman is measured after 60 and 120 minutes. Blood glucose is measured from venous plasma.

Standard values

50 g glucose screening test

After 1 hour <135 mg/dl (7.5 mmol/l)

75-g-oGTT

Sober 92 mg/dl (5.1 mmol/l)
After 1 hour 180 mg/dl (10.0 mmol/l)
After 2 hours 153 mg/dl (8.5 mmol/l)

Interpretation

  • A blood glucose value of ≥ 135 mg/dl (7.5 mmol/l) on the 50-g glucose screening test one hour after the end of drinking the test solution is considered a positive screen and requires a subsequent 75-g diagnostic oGTT.
  • Provided any of the values on the 75-g oGTT are met or exceeded, the diagnosis of gestational diabetes is confirmed.

Further note