Approach

Clinical presentation

Type 1 diabetes can be diagnosed at any age, with a peak around 10 to 14 years.[18] It commonly presents with polyuria, polydipsia, weight loss, and generalized weakness.[40] Other symptoms, such as blurred vision, may occur.[40] Many patients present with diabetic ketoacidosis, an acute complication of type 1 diabetes.[41][42][43] These patients have symptoms of dehydration and acidosis such as nausea, vomiting, abdominal pain, tachypnea, tachycardia, lethargy, and altered mental status.[41][42][43] When the initial presentation of type 1 diabetes occurs in adulthood, it is known as latent autoimmune diabetes in adults (LADA).[44] It is important to distinguish LADA from type 2 diabetes, as treatment with insulin is required.[6] 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 of diabetes can be made on the basis of any of the following:[1]​​

  1. Glycosylated hemoglobin (HbA1c) ≥6.5% (≥48 mmol/mol); OR

  2. Fasting plasma glucose ≥126 mg/dL (≥7.0 mmol/L); OR

  3. Plasma glucose ≥200 mg/dL (≥11.1 mmol/L) 2 hours after 75 g oral glucose; OR

  4. Random plasma glucose of ≥200 mg/dL (≥11.1 mmol/L) in the presence of symptoms of hyperglycemia or hyperglycemic crisis.

In an asymptomatic patient, results should be confirmed by ensuring that two tests (e.g., HbA1c and fasting glucose) are abnormal at the same time, or by repeating the abnormal test (or performing an additional test) at a different time point.[1]​ In symptomatic patients, a random plasma glucose value of ≥200 mg/dL (≥11.1 mmol/L) is 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 classified based on clinical and/or laboratory criteria.[45]

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.[46] 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 hyperglycemia suggests type 1 diabetes, but can occasionally be seen at presentation in a patient with type 2 diabetes. As an example, when a teenager with obesity and 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 important that people with diabetes are classified appropriately to ensure provision of optimal and individualized management.[1]​ When type 1 diabetes is suspected, it is important to consider the possibility of monogenic diabetes as a differential, since this accounts for up to 4% of pediatric diabetes, and insulin treatment may be inappropriate in these cases.[1][46] The two main forms of monogenic diabetes are maturity-onset diabetes of the young (MODY) and neonatal diabetes.[1]​ Suspicion for monogenic diabetes should be higher in autoantibody-negative pediatric and younger adult patients (<35 years) who do not display the typical signs of type 1 or type 2 diabetes, and who have a family history of diabetes in successive generations.[1]​ The American Diabetes Association (ADA) recommends genetic testing for MODY in these patients.[1]​ They also recommend that all individuals (regardless of current age) that have been diagnosed with diabetes in the first 6 months of life should have genetic testing for neonatal diabetes.[1]​​ HbA1c <7.5% (<58 mmol/mol) at diagnosis is another feature suggestive of monogenic diabetes, and the person may have additional features of a specific monogenic cause (e.g., renal cysts, partial lipodystrophy).[1]​ Genetic testing is definitive and can be used to counsel the patient and family members.[1]​ C-peptide testing can also be used when there is uncertainty about the diagnosis to help identify people who should have genetic testing.[1]

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