Glucose tolerance test


The diagnosis of diabetes mellitus or other carbohydrate metabolism impairment is verified by classical oral glucose tolerance test (OGTT). OGTT determines the state of carbohydrate metabolism and is used to recognize an early stage of diabetes mellitus. Upon glucose load, the concentration of glucose rises; OGTT determines the time needed for the concentration of glucose to return to normal. This test simulates the physiologic intake of food under standard conditions. Therefore, the intrinsic factors such as intestinal absorption, hormones, liver function, etc., influence the level of blood glucose. For these reasons, this test cannot be performed in patients with gastrointestinal disturbances.
The concentration of glucose is determined in all blood samples obtained. Analytical result is expressed as a numerical value and graphically as a curve.

Glucose and ketone values in OGTT test

Time (min)306090120180240300
Urine glucoseNormalnegnegnegnegnegneg 
Mild diabetes(+)+++++(+)
Severe diabetes++++++++++++++++++++
Urine ketonesNormalnegnegnegnegnegnegneg
Mild diabetesnegnegnegnegnegnegneg
Severe diabetes(+)(+)(+)(+)(+)(+)(+)

The shape of the curve does not only depend on insulin secretion, but also on the secretion of other hormones, rate of glucose absorption, and some other compensatory mechanisms. The possible types of OGTT curve are:
1. normal curve
2. diabetic curve
3. precipitated curve
4. flat curve
5. renal glycosuria curve

1. Normal curve: according to Fajans-Conn, a normal OGTT curve contains the values listed in table.

Samplefasting (0 min)60 min90 min120 minUnit
whole blood (venous)
whole blood (capillary)-10.0-6.7mmol/L
plasma / serum6.410.39.07.8mmol/L

2. Diabetic curve: at 2 hours, the concentration of glucose is at least 6.1 mmol/L in venous blood, 6.7 mmol/L in capillary blood, or 7.8 mmol/L in plasma or serum. The maximal concentration is abnormally high, while fasting glucose levels need not always be elevated. If the maximal glucose concentration exceeds renal threshold, then glucosuria is also present.
3. Precipitated curve: in some patients, the maximal concentration of glucose is achieved already at 30 minutes, to decline to normal or even lower after 2 hours. This curve type is observed in patients with reactive hypoglycemia, after gastrectomy or gastrojejunostomy, in severe liver diseases (reduced glycogenesis), etc.
4. Flat curve: glucose load results in only a slight increase in blood glucose concentration. This curve type is observed in patients with glucocorticoid or growth hormone deficiency, however, it may quite frequently be found in healthy individuals.
5. Renal glycosuria curve: OGTT is completely normal, however, glucosuria is found in some or even all urine samples.