The diagnosis of type 2 diabetes mellitus can be made by either a plasma glucose or a 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.

When HbA1c is used for the diagnosis of diabetes:

• 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 (see below)

Conditions where HbA1c may not be used for diagnosis: #link11

• haemoglobinopathies
• haemolytic anaemia
• untreated iron deficiency anaemia
• suspected gestational diabetes
• children
• HIV
• chronic kidney disease
• people taking medication that may cause hyperglycaemia (for example corticosteroids)