Glucose tolerance test

Synonyms

Glucose screening test; glucose challenge test (GCT);

First author

As early as 1885, glucose could be detected in circulating blood. The oral glucose tolerance test (oGTT) was first described by Ivar Bang in 1917 and is still used today (Jagannathan 2020).

The GTT can be performed as an oral and as an intravenous test (Schäffler 2009).

Preparations before the test:

• Starvation state:

When performing the oGTT, make sure that the patient is not in a starvation state. He should consume ≥ 150 g carbohydrates / d at least 3 days before.

• no presence of febrile temperatures
• in women, not at the time of menstruation (from 3 d before to 3 d after, as otherwise false positive values may occur (Schäffler 2009)
• from 22.00 h of the previous day, patients should remain fasting (Herold 2020)
• a previous correction is necessary in:
  • Hypokalemia
  • Hypomagnesemia (Schäffler 2009)

Indication

Oral GTT (oGTT):

The oGTT is not recommended for routine clinical use. It is indicated for:

• all pregnant women between the 24th and 28th week of pregnancy (SSW)
• unclear cases (Herold 2020)
• for the diagnosis of impaired glucose tolerance
• in the context of the diagnosis of acromegaly (Herrmann 2008), as in this case
  • patients develop diabetes mellitus in up to 25% of cases
  • show a paradoxical increase in GH (groth hormones) in approx. 20 % after administration of glucose (Kasper 2015)
• Insulin resistance
• with disturbed beta cell function (Eyth 2021)
• postprandial hypoglycaemia together with further diagnostics such as:
  • reactive hypoglycaemia (reactive hyperinsulinism)
  • (late) dumping syndrome (Schäffler 2009)

Intravenous GTT (ivGTT):

This test is used for

• Prediabetes mellitus type 1 (to assess the early insulin response [Schäffler 2009])
• Patients with structural changes in the stomach or small intestine [Herold 2020].

The ivGTT is used exclusively in special cases; it is considered obsolete for routine diagnostics (Nawroth 2013).

Test performance

The test is performed according to the criteria of the WHO (Reinhardt 1994).

First, the fasting BG is determined by an i.v. blood sample. The patient then drinks 75 g of glucose or 50 g of glucose if gestational diabetes has been diagnosed (Schäffler 2009). In children, the dosage is based on weight: 1.75 g / kg KG, maximum dose 75 g (Eyth 2021). The drinking time should not exceed 5 min (Eyth 2021).

In the i. v. test, a bolus injection of glucose is given (Schäffler 2009).

Afterwards, the patient must continue to fast, avoid excessive fluid intake (Eyth 2021) and maintain physical rest (Herrmann 2007).

The blood glucose measurements in whole blood (Herrmann 2007) are carried out at precisely defined intervals, for which there are different options:

• 1. possibility:

This test is used for the clarification of gestational diabetes. The blood sample is taken after 60 minutes. If the result is positive, test options 2 or 3 are carried out immediately afterwards.

• 2nd possibility:

Serves to diagnose diabetes mellitus. In this case, the fasting blood glucose level is usually already elevated. The next blood sample is only taken after 120 minutes.

• 3rd possibility:

This is used to diagnose impaired glucose tolerance, a delayed reaction in the release of insulin from the pancreas and delayed absorption of insulin in the liver. Blood samples are taken at 30 min, 60 min, 90 min, and 120 min.

(Herold 2020 / Eyth 2021)

• 4th option:

This option is used to clarify postprandial hypoglycemia, insulin resistance or beta cell failure. In addition to glucose, insulin and C-peptide are also determined. Blood samples are also taken after 3 h and 5 h (Schäffler 2009).

Contraindication

The oral GTT cannot be used in patients with structural changes of the stomach or small intestine. In these cases, the test should be performed intravenously.

The i.v. GTT has not yet been investigated for pregnant patients.

If the fasting BG already shows elevated values, the oGTT is contraindicated (Herold 2020).

Interactions

False elevated blood glucose levels may occur with:

• prolonged bed rest
• Myocardial infarction
• medications such as:
  • Corticosteroids
  • Estrogens
  • Saluretics (Herold 2020)
• acute diseases with activation of stress hormones such as infections, pulmonary embolism, decompensated heart failure (Schäffler 2009)

Preparations

• Dextro O.G.T. (Schäffler 2009)

Test evaluation:

• normal:
  • Fasting- BG < 100 mg / dl (< 5.6 mmol / l).
  • BG after 2 h < 140 mg / dl (< 7.8 mmol / l)
• abnormal fasting glucose (impaired fasting glucose = IFG):
  • 100 - 125 mg / dl (5.6 - 6.9 mmol / l)
• impaired glucose tolerance (IGT):
  • BG after 2 h 140 - 199 mg / dl (7.8 - 11.0 mmol / l)

(Herold 2020)

• well-controlled diabetes type 1 or type 2:
  • Fasting BG 60 - 100 mg / dl (3.3 - 5.6 mmol / l)
  • BG after 1 h < 200 mg / dl (< 11.1 mmol / l)
  • BG after 2 h < 140 mg / dl (< 7.8 mmol / l)
• poorly controlled diabetes type 1 or type 2:
  • Fasting BG 100 - 125 mg / dl (5.6 - 6.9 mmol / l)
  • BG after 2 h 140 - 200 mg / dl (7.8 - 11.1 mmol / l)
• Initial diagnosis of diabetes type 1 or type 2:
  • Fasting BG > 126 mg / dl (> 7.0 mmol / l)
  • BG after 2 h > 200 mg / dl (> 11.1 mmol / l)
• Normal findings in pregnant women:
  • Fasting- BG < 90 mg / dl (< 5,0 mmol / l)
  • BG after 1 h < 130 - 140 mg / dl (< 7.2 - 7.8 mmol / l)
  • BG after 2 h < 120 mg / dl (6.7 mmol / l)
• Indication of gestational diabetes:
  • Fasting BG < 95 mg / dl (< 5.3 mmol / l)
  • BG after 1 h > 140 mg / dl (> 7.8 mmol / l)
  • BG after 2 h > 120 mg / dl (> 6.7 mmol / l)

(Eyth 2021)

• Prediabetes mellitus type 1:

After i. v. injection of glucose is usually found a biphasic maximum increase in insulin and C- peptide after 0 - 10 min (phase 1) and after 11 - 60 min (phase 2). If a reduction in insulin secretion is found in phase 1, this should be interpreted as a prediabetes mellitus type 1 (Schäffler 2009).

• Insulin resistance:

In this case, increased basal values and / or an excessive increase in both insulin and C-peptide after 2 h are found.

Normal values for C-peptide:

- fasting approx. 0.81 - 3.85 ng / ml

- after 1 h and after 2 h approx. 2.7 - 5.7 ng / ml (Schäffler 2009)

• Hyperinsulinemia:

There are also increased basal values and / or an excessive increase after 2 h of both insulin and C-peptide (normal values see above). In addition, there is hypoglycemia with values < 60 mg / dl after 2 - 5 h (Schäffler 2009).

• Beta cell failure:

In this case, there are reduced basal values after 2 h of both insulin and C-peptide. (Normal values see above).

(Schäffler 2009)

• Acromegaly:

Acromegalyis excluded if there is a suppression of GH (groth hormone) < 1 ng / ml under the oGTT or there is a condition after surgery of acromegaly.

A lack of suppression or a paradoxical increase in GH are indicative of acromegaly (Schäffler 2009).

The glucose concentration in the fasting state is approx. 5 - 10 % lower in venous blood than in arterial blood. Capillary blood is a mixture of venous and arterial blood. The glucose values of capillary blood are approx. 5 % higher than those of venous blood in the fasting state and approx. 10 % - 15 % higher postprandially (Schäffler 2009).

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