Approach

Clinical presentation

Type 1 diabetes presents with polyuria, polydipsia, weight loss, generalised weakness, and blurred vision. Some patients present with diabetic ketoacidosis, the acute complication of type 1 diabetes. These patients have symptoms of dehydration and acidosis such as nausea, vomiting, abdominal pain, tachypnoea, tachycardia, and lethargy. Rarely, a patient is diagnosed with type 1 diabetes during routine blood tests. The condition is diagnosed long before its chronic complications have developed.

Diagnosis

Diagnosis can be made on the basis of any of the following:[1]

  1. In a symptomatic patient, random plasma glucose of >11 mmol/L (>200 mg/dL)

  2. Fasting plasma glucose >6.9 mmol/L (>126 mg/dL)

  3. Plasma glucose ≥11 mmol/L (≥200 mg/dL) 2 hours after 75 g oral glucose

  4. Glycosylated haemoglobin (HbA1c) ≥48 mmol/mol (≥6.5%).

In an asymptomatic patient, results should be confirmed by repeating the test (from the same sample or in two separate test samples).[1] In symptomatic patients, measurement of plasma glucose if sufficient to diagnose diabetes.[1] Diabetes is the overall diagnostic term applied to people satisfying these criteria, with type 1 and type 2 being further sub-classes based on clinical and/or laboratory criteria.[34]

The diagnosis of type 1 diabetes is often obvious from the clinical presentation, but can be confirmed through additional testing. Low C-peptide levels and presence of one or more autoimmune markers are consistent with a diagnosis of type 1 diabetes.[35] Autoimmune markers include autoantibodies to glutamic acid decarboxylase (GAD), insulin, islet cells, islet antigens (IA2 and IA2-beta), and the zinc transporter ZnT8.

Elevated plasma or urine ketones in the presence of hyperglycaemia suggests type 1 diabetes, but is occasionally seen at presentation in a patient with type 2 diabetes. As an example, when an obese teenager with a positive family history of type 2 diabetes is found to have high plasma glucose levels on routine blood tests, the diagnosis of type 1 versus type 2 diabetes may not be clear. If C-peptide levels are very low or undetectable relative to the plasma glucose and anti-GAD antibodies are positive in such a patient, a diagnosis of type 1 diabetes can be made.

It is also important to consider monogenic diabetes in the differential for type of diabetes, since this accounts for up to 4% of paediatric diabetes and insulin treatment is inappropriate in these cases.[35] Suspicion should be higher in antibody-negative, ketone-negative paediatric diabetes with a family history of diabetes in multiple generations.[1] The two main forms of monogenic diabetes are maturity-onset diabetes of the young (MODY) and neonatal diabetes. Genetic testing is definitive and can be used to counsel the patient and family members.


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