An oral glucose tolerance test (OGTT) is rarely indicated in making the diagnosis diabetes in childhood and adolescence. If doubt exists, it is easies to monitor the child or adolescent regularly with urine testing for glycosuria and to perform random blood tests to detect hyperglycaemia.

If the decision is made to proceed with diagnostic testing then an OGTT may be indicated if symptoms are present and a random venous plasma glucose concentration taken a least 2 hours after eating is 5.5 - 11.0 mmol/L and if in asymptomatic individuals the fasting venous plasma glucose is 5.5-7.7 mmol/L.

The dose of glucose used in the OGTT is 1.75 g/kg of body weight to a maximum of 75 g. The glucose load or partial hydrolysate of starch of equivalent carbohydrate content is administered in 250 - 300 ml water over a minute period.

The OGTT should be performed strictly under the following conditions:

• The child or adolescent should not be suffering from any obvious intercurrent illnes or be on any medications known to affect blood glucose levels.
• Venous plasma glucose should be used instead of whole blood glucose measurement (which are 10-15% lower).
• Ward testing of blood glucose levels should not be used for the OGTT as they are not sufficiently accurate.
• Blood specimens should be collected using sodium oxalate, sodium/potassium EDTA or heparin as anticoagulants and centrifuged immediately or kept on ice if there is delay.
• The test should be done in the morning after fasting overnight.
• The diet in the preceding 3 days should be high in carbohydrate

Interpretation of OGTT
The criteria necessary for a diagnosis of diabetes mellitus in children and adolescent are as follows :

1. elevated fasting venous plasma glucose (>7.8 mmol/L) OR
2. elevated venous plasma glucose at 2 hours (> 11.1 mmol/L)

Children with a fasting venous plasma glucose < 7.8 mmol/L and 2 hour values between 7.8 and 11.0 mmol/L are classified as having impaired glucose tolerance. A number of these children will revert to normal glucose tolerance or progress to over diabetes.

In borderline diganostic situations measurement of glycosylated haemoglobin and the presence of autoimmune markers such as antibodies against islet cell antigens (ICA), insulin and GAD may be of assistance. In IDDM, insulin measurement taken during the OGTT are low for the accompanying blood glucose levels.