Oral Glucose Tolerance Test and it's implications

Patients with impaired fasting glucose should undergo tolerance test. This test is also useful in diagnosis of GDM. Current WHO recommends 75g of anhydrous glucose to be used. For 3 days before the test, subject should be on unrestricted weight maintaining diet and should exercise normally. The subject should fast for at least 10h and should remain seated and not smoke during the test. OGTT are not recommended for sujects with fasting hyperglycaemia or hospitalized, ill or immobile patients.

Screening for diabetes

Screening has been recommended by ADA for asymptomatic subjects as follows.

·         Age ≥45 years, particularly if BMI >25 kg/m²; if the result is normal, screening should be repeated at 3-yearly intervals.

·         Overweight individuals (BMI>25 kg/m²) irrespective of age and those having any of the following additional risk factors:

§  Physically inactive

§  A first degree relative with diabetes

§  History of IFG or IGT

§  Polycystic ovary syndrome

§  Hypertension (>140/90 mmHg)

§  Dyslipidaemia (e.g. HDL-C<40 mg/dl or Tg>150 mg/dl

§  History of gestational diabetes or delivery of baby weighing >4 kg

§  A history of vascular disease.

If the screening is positive then it should be followed by diagnostic test. For the standard OGTT of 2 hr. or the mini OGTT can be done. Mini OGTT includes, after ingestion of 75 g glucose blood sample is taken at 0 hr. before glucose load and after 2 hours. If 2 hour glucose is ≥200 mg/dL then is confirmed on a subsequent day by either an increased random or fasting glucose, the patient is diagnosed with diabetes.

Standard OGTT - After patient preparation fasting blood is collected plus the urine to see glycosuria. Then 70 mg glucose (in pregnant women to exclude diabetes 100 gm is given) is given in 300 ml water in 5 minutes time 1.75 g/kg for pediatric. After then blood and urine are taken at 60, and 120 min.

a.      Normal response:

Fasting <100 mg/dL

1 hr <160 mg/dL

2 hr <140 mg/dL

Negative benedict test in all urine samples

b.      Impaired glucose tolerance:

Fasting <100 mg/dl

1 hr >200 mg/dl

2 hr 140-199 mg/dl

1 hr and 2 hr urine sample has positive benedict test

This may be caused due to hyperthyroidism, cushing syndrome, diabetes, intercurrent illness, etc.

c.       Diabetic curve:

Fasting >126 mg/dL

1 hr >200 mg/dl

2 hr >200 mg/dl

All urine samples has positive benedict test

d.      Gestational curve:

Fasting ≥105 mg/dl

1 hr ≥180 mg/dL

2 hr ≥ 155 mg/dL

1 hr urine sample are positive

e.      Renal glycosuria:

Blood glucose levels are normal but urine glucose (1 hr) is positive. In this case there is renal tubular defect in glucose absorption or there is decreased renal threshold. GTT is the only test useful in the diagnosis of this inherited renal tubular defect.

f.        Lag curve:

Some individuals show an abnormal sudden rise in blood glucose after an oral load of glucose, but the level quickly falls and 2 hour sample is within normal limits. This phenomenon probably results from an increased rate of glucose absorption from the gut (sometimes in hyperthyroid). The increase in blood glucose is due to delay in insulin action. (Insulin function lagging behind, hence called lag curve) There is also transient glycosuria (1 hr sample).

g.      Flat curve:

Fasting blood is ≤80 mg/dL. All samples show low blood glucose. Urine glucose is negative. Flat curves are seen in patients with hypoactivity of other endocrine organs, e.g. in hypopituitatism and Addison’s disease, malabsorption.

Extended GGT – Instead ending at 2 hours after taking glucose, GTT is sometimes extended upto 5 hours. If glucose values tend to drop below 60 mg/dL after every 30 minutes then it may be due to insulin secreting tumors of pancreas or may be due to Simmond’s disease which cause hypoglycemia (<60 mg/dL).