The diagnosis of type 2 diabetes mellitus can only be made by either a plasma glucose of HbA1c sample. Diagnostic criteria vary according to whether the patient is symptomatic (polyuria, polydipsia etc) or not.
If the patient is symptomatic:
• fasting glucose greater than or equal to 7.0 mmol/l
• random glucose greater than or equal to 11.1 mmol/l (or after 75g oral glucose tolerance test)
If the patient is asymptomatic the above criteria apply but must be demonstrated on two separate occasions.
In 2011 WHO released supplementary guidance on the use of HbA1c on the diagnosis of diabetes:
• a HbA1c of greater than or equal to 6.5% (48 mmol/mol) is diagnostic of diabetes mellitus
• a HbAlc value of less than 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 (see below)
Conditions where HbA1c may not be used for diagnosis:
• haemoglobinopathies
• haemolytic anaemia
• untreated iron deficiency anaemia
• suspected gestational diabetes
• children
• HIV
• chronic kidney disease
Impaired fasting glucose and impaired glucose tolerance
A fasting glucose greater than or equal to 6.1 but less than 7.0 mmol/l implies impaired fasting glucose (IFG)
Impaired glucose tolerance (IGT) is defined as fasting plasma glucose less than 7.0 mmol/l and OGTT 2-hour value greater than or equal to 7.8 mmol/l but less than 11.1 mmol/l
Diabetes UK suggests:
• ‘People with IFG should then be offered an oral glucose tolerance test to rule out a diagnosis of diabetes. A result below 11.1 mmol/l but above 7.8 mmol/l indicates that the person doesn’t have diabetes but does have IGT.’