Investigations

HbA1c test:

HbA1c gives an estimate of the average glucose concentration over the previous 3 months. This test is not appropriate in:

• All children and young people,
• Women who are currently pregnant or have been pregnant in the past two months,
• People with suspected type 1 diabetes, regardless of age

Guidance:

• a HbA1c of greater than or equal to 48 mmol/mol (6.5%) is diagnostic of diabetes mellitus
• a HbAlc value of less than 48 mmol/mol (6.5%) does not exclude diabetes (i.e. it is not as sensitive as fasting samples for detecting diabetes)
• in patients without symptoms, the test must be repeated to confirm the diagnosis
• it should be remembered that misleading HbA1c results can be caused by increased red cell turnover

Overnight fasting blood glucose:

After an overnight fast the only sources of glucose is gluconeogenesis and net glycogenolysis in  the liver. Since insulin inhibits gluconeogenesis  and activates glycogen synthesis, a raised o/n fasting blood glucose gives an indication of liver insulin resistance

Oral Glucose Tolerance Test (OGTT):

performed in the morning pre-breakfast

Method: A baseline fasting plasma glucose result is taken, then a 75g glucose drink is given and plasma glucose is measured after 2 hours