General considerations

Diagnostic tests for diabetes include glycated haemoglobin (HbA1c), venous blood glucose concentration and the oral glucose tolerance test. Each test has benefits and limitations. Diagnostic thresholds for diabetes gives the tests and the diagnostic thresholds for diabetes, which are based on the thresholds of glycaemia associated with microvascular complications of diabetes, especially retinopathy. If two different tests are performed and the results are discordant, the test with a result above the diagnostic threshold should be repeated. The diagnosis can then be made based on the confirmed test.

If the patient is symptomatic, a diagnosis of diabetes can be confirmed based on a single positive result from one test. However, if the patient is asymptomatic and has a positive test result (eg from screening for type 2 diabetes), confirm the diagnosis with a repeat measurement (preferably using the same test) performed on a different day. Laboratories may use different guidelines with different thresholds to guide repeat confirmatory testing; if so, follow their protocol.

Note: Diabetes can be diagnosed clinically in a symptomatic patient and confirmed by a single positive result from one test.

In a patient with suspected type 1 diabetes, waiting for confirmatory tests such as fasting blood glucose concentration or HbA1c delays treatment and could result in development of diabetic ketoacidosis. A simple point-of-care test—a capillary (finger-prick) blood glucose concentration test or a urine dipstick test—can be used initially. Elevated blood glucose concentration or significant glycosuria and ketonuria is a medical emergency. Immediate specialist assessment and management is required. An oral glucose tolerance test has no place in the diagnosis of type 1 diabetes, or in patients who have already met the diagnostic thresholds for diabetes based on other tests.

Note: Measuring insulin concentrations has no role in the diagnosis of diabetes.

Measuring blood insulin concentrations has no role in the diagnosis of diabetes. Measuring diabetes-associated autoantibodies or C-peptide matched with a glucose (as a marker of endogenous insulin production) may be useful in some situations when the classification of diabetes is unclear; however, they are not routine diagnostic tests.

Table 1. Diagnostic thresholds for diabetes
[NB1] [NB2] [NB3]
glycated haemoglobin (HbA1c) venous BGC oral glucose tolerance test
glycated haemoglobin (HbA1c) [NB4]
diabetes threshold	48 mmol/mol (6.5%) or more [NB6] [NB7]
prediabetes threshold [NB5]	adults: 42 to 46 mmol/mol (6.0 to 6.4%) [NB6] [NB7] children and adolescents: no Australian HbA1c values have been defined for prediabetes [NB8]
venous BGC [NB9]
fasting
diabetes threshold	7 mmol/L or more
prediabetes threshold [NB5] (ie impaired fasting glucose)	adults: 6.1 to 6.9 mmol/L children and adolescents: 5.6 to 6.9 mmol/L
random (nonfasting)
diabetes threshold	11.1 mmol/L or more
prediabetes threshold [NB5]	no random venous glucose concentration values have been defined for prediabetes; however, consider further diagnostic testing if between 7.8 and 11 mmol/L
oral glucose tolerance test [NB10]
venous BGC 2 hours after a 75 g oral glucose tolerance test [NB9]
diabetes threshold	11.1 mmol/L or more
prediabetes threshold [NB5] (ie impaired glucose tolerance)	7.8 to 11 mmol/L
Note: BGC = blood glucose concentration NB1: For information on diagnosing and screening for gestational diabetes, see Detection and diagnosis of hyperglycaemia in pregnancy. NB2: Consider the following: If two different tests are performed and the results are discordant, the test with a result above the diagnostic threshold should be repeated. The diagnosis can then be made based on the confirmed test. If the patient is symptomatic, a diagnosis of diabetes can be made based on a single positive result from one test. However, if the patient is asymptomatic, confirm the diagnosis with a repeat measurement performed on a different day. Laboratories may use different guidelines with different thresholds to guide repeat confirmatory testing; if so, follow their protocol. NB3: A presentation of hyperglycaemia with or without symptoms of diabetes (eg polyuria, polydipsia, weight loss) in a child or adolescent is a medical emergency. Immediate specialist assessment and management is required to avoid development of life-threatening complications of acute insulin deficiency, such as diabetic ketoacidosis (DKA). Refer the patient to the local emergency department, or phone the nearest children’s hospital or major healthcare service for specialist endocrinology advice. NB4: Caution is needed in interpreting HbA1c if the patient has a condition that affects red blood cell turnover (see Limitations of HbA1c tests). NB5: For discussion of prediabetes, see text below. Prediabetes thresholds are not relevant to patients with type 1 diabetes. NB6: HbA1c less than 48 mmol/mol (6.5%) does not refute a diagnosis of diabetes made based on venous blood glucose concentrations or an oral glucose tolerance test. NB7: The accepted prediabetes and diabetes thresholds for HbA1c do not define patients with an HbA1c of 47 mmol/mol. In Australia, laboratories provide dual reporting of HbA1c in both mmol/mol and % units, and generally an HbA1c of 47 mmol/mol would be rounded up to an equivalent HbA1c of 6.5%. NB8: An elevated HbA1c has been defined to identify Aboriginal, Torres Strait Islander, Māori and Pacific children and adolescents who are at increased risk of developing type 2 diabetes; see Interpreting screening results in children and adolescents. NB9: Venous blood samples should be taken and tested using validated laboratory methods; do not use point-of-care capillary (finger-prick) blood testing to diagnose diabetes. NB10: An oral glucose tolerance test has no place in the diagnosis of type 1 diabetes, or in patients who have already met the diagnostic thresholds for diabetes based on other tests.

There is a spectrum of dysglycaemia ranging from normal blood glucose concentrations to threshold values sufficient to diagnose diabetes. The term ‘prediabetes’ is often used to define a patient who has not yet met the thresholds for type 2 diabetes but has blood glucose concentrations above the normal range (see Diagnostic thresholds for diabetes). It includes those with impaired fasting glucose, impaired glucose tolerance and elevated HbA1c. A patient may meet the threshold for prediabetes on one or multiple tests; however, meeting the threshold for prediabetes on any test does not refute a diagnosis of diabetes made based on another test. Prediabetes is not in itself a disease, but it does help define patients at risk of developing type 2 diabetes and at higher risk of cardiovascular disease. Patients with prediabetes need more frequent follow-up to assess and manage their cardiovascular disease risk factors and risk of developing type 2 diabetes—see Management of people at risk of developing type 2 diabetes for information on management of prediabetes. Prediabetes thresholds are not relevant to patients with type 1 diabetes.

Hyperglycaemia can be precipitated by an acute concurrent stressor (eg infection, cardiovascular event, surgery or other stress) or by medications (eg high-dose glucocorticoids). In an acute event, it may be difficult to distinguish between hyperglycaemia from these causes and previously undiagnosed diabetes. An HbA1c may help gauge the duration of hyperglycaemia, see Glycated haemoglobin (HbA1c) testing. After the acute event has resolved, the patient should have definitive testing for diabetes. However, even if the patient’s blood glucose concentrations have reduced to normal, they should be considered at greater risk for developing diabetes (see Screening and management of people at risk of developing type 2 diabetes).