Type 1 diabetes can be diagnosed at any age, with a peak around 10-14 years. It commonly presents with polyuria, polydipsia, weight loss, and generalised weakness. Other symptoms, such as blurred vision, may occur. Many patients present with diabetic ketoacidosis, an acute complication of type 1 diabetes. These patients have symptoms of dehydration and acidosis such as nausea, vomiting, abdominal pain, tachypnoea, tachycardia, lethargy, and altered mental status. When the initial presentation of type 1 diabetes occurs in adulthood, it is known as latent autoimmune diabetes in adults (LADA). It is important to distinguish LADA from type 2 diabetes, as treatment with insulin is required. 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 can be made on the basis of any of the following:
In a symptomatic patient, random plasma glucose of >11 mmol/L (>200 mg/dL)
Fasting plasma glucose >7.0 mmol/L (>126 mg/dL)
Plasma glucose ≥11 mmol/L (≥200 mg/dL) 2 hours after 75 g oral glucose
Glycosylated haemoglobin (HbA1c) ≥48 mmol/mol (≥6.5%).
In an asymptomatic patient, results should be confirmed by repeating the test. In symptomatic patients, measurement of plasma glucose if sufficient to diagnose diabetes. 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.
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. 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. Suspicion should be higher in antibody-negative, ketone-negative paediatric diabetes with a family history of diabetes in multiple generations. 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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