Oral Glucose Tolerance Test (oGTT) in Gestational Diabetes

Gestational diabetes mellitus (GDM) is the medical term for gestational diabetes. This form of diabetes occurs for the first time during pregnancy. Approximately 3-8% of pregnant women are affected.

Symptoms and complaints

Gestational diabetes does not show as clear symptoms as “real” diabetes mellitus. Occasionally, there are increased genital infections – for example, vaginitis (colpitids) – and/or urinary tract infections, as well as increased blood pressure (hypertension). However, these symptoms are relatively nonspecific and are sometimes not associated with possible gestational diabetes. The newborn may be found to be growing too quickly (macrosomia) or have increased amounts of amniotic fluid (polyhydramnios), which may be an indication of the mother’s condition.

Risk factors

  • Familial cases of diabetes mellitus
  • Pregnant women from the age of 30
  • Overweight mother
  • High birth weight of more than 4,000 g in previous pregnancies (macrosomia).
  • Previous premature births
  • Early infant death in previous pregnancies.

Consequential diseases for the newborn

  • Macrosomia – increased birth weight of more than 4,000 g.
  • Enlarged, immature internal organs of the child, e.g. cardiomyopathy – heart that is too large but not fully efficient.
  • Respiratory problems – due to surfactant deficiency.
  • Increased bilirubin levels in the blood
  • Blood formation outside the bone marrow
  • Hypoglycemia – lowered blood glucose level.
  • Hypocalcemia (calcium deficiency)
  • Increased mortality of the unborn child in untreated gestational diabetes due to pathological changes in the placenta (placenta) and thereby caused undersupply of the newborn (placental insufficiency).

Causes

The cause of gestational diabetes is said to be the change in hormonal metabolism and altered carbohydrate metabolism.During pregnancy, certain hormones that increase blood glucose levels, among other things, are produced more.The hormone that lowers blood glucose, insulin, must be produced in ever-increasing amounts by the pancreas to normalize the elevated blood glucose. This results in an imbalance and disturbed glucose metabolism in the body. If the pancreas is no longer able to normalize blood glucose, gestational diabetes develops. In most cases, gestational diabetes disappears after the end of pregnancy. However, in about 4% of those affected, the diabetes does not disappear but persists. In about 50% of women who once had gestational diabetes, a “real” diabetes disease occurs later in life.

Diagnostics

The glucose tolerance test – GTT for short (synonyms: glucose challenge test, GCT; 75-g-oGTT) – is used to detect gestational diabetes. Because symptoms are usually absent, this test is essential for diagnosing gestational diabetes. Indications

  • Screening or diagnosis of gestational diabetes mellitus (GDM).
  • Casual glucose measurement ≥ 200 mg/dl (11.1 mmol/l) or fasting glucose ≥ 92 mg/dl (5.1 mmol/l) and fasting glucose (second measurement): 92-125 mg/dl (5.1-6.9 mmol/l)

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)

The procedure

Material needed

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

Preparation of the patient

  • At least 14 days away from an acute illness.
  • No operations on the upper gastrointestinal tract.
  • No exceptionally heavy physical strain
  • Adhering to a fasting period of at least eight hours before starting the test.
  • Do not smoke before or during the test.
  • Test start between six and nine o’clock in the morning
  • During the test, the pregnant woman should sit and not make unnecessary movements.
  • No other tests are performed during the test.

Interfering factors

  • Confounding factors that may affect glucose tolerance:
    • Hunger state
    • Long bedriddenness
    • Hyperthyroidism (overactive thyroid gland)
    • Hypokalemia (potassium deficiency)
    • High-grade 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).
    • Stress
  • Discontinue interfering medications (if possible) three days before:
    • Benzodiazepines
    • Diuretics (especially thiazides)
    • Hormones
      • Hormonal contraceptives
      • Thyroid hormones
      • Steroids
    • Non-steroidal anti-inflammatory drugs (NSAIDs)
    • Laxatives
    • Nicotinic acid
    • Nitrazepam
    • Phenothiazines, phenacetin

Implementation

  • Timing: screening test in all pregnant women 24 + 0 to 27 + 6 SSW (week of gestation).
  • At least 14 days interval from acute illness.
  • 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-oGTT: To determine the fasting glucose serum level, blood is drawn 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. She then drinks 75 g of glucose dissolved in tea or a ready-to-use preparation: 75 g of dextrose, e.g. Dextro-Energen to 300 ml of water on an empty stomach. The glucose serum level of the pregnant woman is measured after 60 and 120 minutes.

Normal values

50-g glucose screening test (Glucose Challenge Test, GCT).

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

75-g-oGTT [recommendation: WHO, DGG].

Fasting 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

  • Complementary testing can be done for autoantibodies, which are found in about 10% of pregnant women with gestational diabetes and are an indication that there is a predisposition to diabetes mellitus.