Differentials
Transient hyperglycaemia (e.g., from stress, corticosteroids, parenteral/enteral nutrition)
SIGNS / SYMPTOMS
In corticosteroid-induced hyperglycaemia, there is a clear history of corticosteroid use (including possible intramuscular injections of corticosteroids).
INVESTIGATIONS
HbA1c normal (reflecting normal blood glucose before the illness).
Type 1 diabetes mellitus
SIGNS / SYMPTOMS
Onset often at age <35 years, but it can occur in older individuals.[1] Many patients are not obese.[1] More commonly presents with symptoms (polyuria, polydipsia, weight loss, generalised weakness, blurred vision) and ketosis, rather than being detected by screening.[16]
INVESTIGATIONS
Urine ketones are often present in type 1 diabetes, but they may be positive in type 2 diabetes if there is severe volume depletion.
Low (<0.2 nanomol/mL) or absent C-peptide level.[17]
One or more autoantibodies (anti-glutamic acid decarboxylase 65 [GAD65] antibodies, islet cell antibodies [ICA], insulin autoantibodies, autoantibodies to the tyrosine phosphatase-related islet antigen-2 [IA-2 and IA-2beta], and zinc-transporter-8 [ZnT8] antibodies) are present in 85% of patients with type 1 diabetes at the time of diagnosis, but may disappear within a few years.[1][18][19]
Type 1 diabetes is defined by the presence of one or more of these autoimmune markers, but testing is usually not required for diagnosis.[1]
Glucose screening criteria cannot be used to differentiate type 1 and type 2 diabetes, as they are identical.[1]
Type 2 diabetes mellitus
SIGNS / SYMPTOMS
Typically, signs of insulin resistance (such as acanthosis nigricans) should be sought and in their absence clinical suspicion of type 1 diabetes is greater.
Signs of more marked insulin deficiency (for example, glycaemic lability as well as susceptibility to ketosis) raise suspicion of type 1 diabetes.
Older age and slow onset, obesity, a strong family history, absence of ketoacidosis, and initial response to oral antidiabetic drugs are typical of type 2 diabetes.
INVESTIGATIONS
C-peptide present.
Autoantibodies absent.
Testing for C-peptide and autoantibodies usually not required.
Glucose screening criteria cannot be used to differentiate type 1 and type 2 diabetes, as they are identical.[1]
Pre-diabetes
SIGNS / SYMPTOMS
Risk factors and history similar to those of type 2 diabetes.
INVESTIGATIONS
Impaired fasting glucose: fasting plasma glucose 5.6 to 6.9 mmol/L (100-125 mg/dL).
Impaired glucose tolerance: 75 g oral glucose tolerance test 2-hour plasma glucose 7.8 to 11.0 mmol/L (140-199 mg/dL).
HbA1c of 39-47 mmol/mol (5.7% to 6.4%) indicates pre-diabetes or high risk of future diabetes.[1]
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