3.4 Diagnosis of diabetes
Once the diagnostic criteria (see Box 3) are reached, type 2 diabetes is the most likely diagnosis (see Figure 1).
Box 3. Diagnostic criteria for type 2 diabetes
- FBG ≥7.0 mmol/L (on two separate occasions)
- 2 hour postprandial ≥11.0 mmol/L on OGTT (on two separate occasions)
- HbA1c ≥6.5% (48 mmol/mol) (on two separate occasions)
Figure 1. Screening and diagnosis algorithm
HbA1c has recently been endorsed as a diagnostic test for diabetes by the World Health Organization. The Australian Diabetes Society, the Royal College of Pathologists of Australasia, and the Australasian Association of Clinical Biochemists have reviewed the available evidence and confirmed that HbA1c can be used to establish the diagnosis of diabetes.31
At the time of publication, the HbA1c assay was not funded by Medicare as a diagnostic or screening test for diabetes, although approval for this purpose is being sought. Note that HbA1c may be artificially normal in people with haemoglobinopathy or haemolysis, and that it may be artificially high in people with iron deficiency.
Other diagnostic possibilities include unusual presentations of:
- type 1 diabetes
- latent autoimmune diabetes of adults (LADA)32
- maturity onset diabetes of the young (MODY)33
- GDM (in pregnancy).
Consider type 1 diabetes if there are no other features of the metabolic syndrome and there is the presence of:
- ketonuria (which may be absent)
- polyuria, polydipsia
- weight loss or BMI <25 kg/m2
- young age
- family history of autoimmune disease
- rapid onset of symptoms.
If suspicious, test for glutamic acid decarboxylase (GAD) and/or insulinoma antigen 2 (IA-2) antibodies. These will be present in 90% of patients with type 1 diabetes. Alternatively C-peptide levels will determine those patients with absence of or minimal insulin production.
Note that GAD antibodies are common in LADA. GAD and IA-2 antibodies are absent in MODY, where there is often an autosomal dominant inheritance within the immediate family.
Common forms are MODY 3 (incidence of 69% of MODY types), which has a hepatocyte nuclear factor 1 alpha gene mutation, and MODY 2 (14%) which has a glucokinase mutation.
- d’Emden MC, Shaw JE, Colman PG, et al. The role of HbA1c in the diagnosis of diabetes mellitus in Australia. Med J Aust 2012;197:220–1.
- Naik RG, Brooks-Worrell BM, Palmer JP. Latent autoimmune diabetes in adults. J Clin Endocrinol Metab 2009;94:4635–44.
- Kavvoura FK, Owen KR. Maturity onset diabetes of the young: clinical characteristics, diagnosis and management. Pediatr Endocrinol Rev 2012;10:234–42.