Investigations

1st investigations to order

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Confirms diagnosis in the presence of symptoms of polyuria, polydipsia, and unexplained weight loss.[1] Bear in mind that a repeat test is required in most cases.

Result

≥11.1 mmol/L (≥200 mg/dL)

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Fasting is defined as no caloric intake for at least 8 hours.[1] Bear in mind that a repeat test is required in most cases. 

Result

≥7.0 mmol/L (≥126 mg/dL)

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Plasma glucose is measured 2 hours after 75 g oral glucose load.[1] Bear in mind that a repeat test is required in most cases.

Result

≥11.1 mmol/L (≥200 mg/dL)

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Result
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Reflects degree of hyperglycaemia over the preceding 3 months. Bear in mind that a repeat test is required in most cases.

Result

≥6.5% (≥48 mmol/mol)

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In adults, diagnose type 1 diabetes on clinical grounds if the patient presents with hyperglycaemia, bearing in mind that people with type 1 diabetes typically (but not always) have one or more of: ketosis, rapid weight loss, age <50 years, BMI <25 kg/m², personal and/or family history of autoimmune disease.[37]

Result

clinical diagnosis

Investigations to consider

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In the presence of hyperglycaemia suggest type 1 diabetes.

Result

medium or high quantity

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The National Institute for Health and Care Excellence (NICE) in the UK recommends only considering measuring C-peptide in the following circumstances:

1. In a child or adult: there is difficulty distinguishing type 1 diabetes from other types.[37][35]

2. In an adult: you suspect type 1 diabetes but the presentation includes atypical features (e.g., age >50 years, BMI > 25 kg/m², slow evolution of hyperglycaemia or long prodrome).[37]

3. In an adult: type 1 diabetes has been diagnosed and treatment started but you have a clinical suspicion that the person may have a monogenic form of diabetes, and C-peptide may guide the use of genetic testing.[37]

If C-peptide testing is indicated, bear in mind that it has better discriminative value the longer the test is done after initial presentation.[37][35]

In clinical practice, C-peptide testing should only be done with a paired glucose. In practical terms, this can be achieved by using C-peptide on a single non-fasting random blood or urine sample after the patient has eaten one of their own meals.[42] Otherwise, C-peptide might be suppressed, making a false positive result more likely. This is a particular concern if the patient has been started on therapy that can cause hypoglycaemia (e.g., insulin).

C-peptide is a by-product formed when proinsulin is processed to insulin. Therefore, its levels reflect insulin production. Half life of C-peptide is 3 to 4 times longer than that of insulin. Low or undetectable C-peptide level indicates absence of insulin secretion from pancreatic beta cells.

Result

low or undetectable

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These include autoantibodies to glutamic acid decarboxylase, insulin, islet cells, islet antigens (IA2 and IA2-beta), and the zinc transporter ZnT8.

The National Institute for Health and Care Excellence (NICE) recommends only considering measuring diabetes-specific autoantibody titres in an adult in the following circumstances:[37]

1. You suspect type 1 diabetes but the presentation includes atypical features (e.g., age >50 years, BMI > 25 kg/m², slow evolution of hyperglycaemia or long prodrome).

2. Type 1 diabetes has been diagnosed and treatment started but you have a clinical suspicion that the person may have a monogenic form of diabetes, and C-peptide and/or autoantibody testing may guide the use of genetic testing.

3. Classification is uncertain, and confirming type 1 diabetes would have implications for availability of therapy (for example, continuous subcutaneous insulin infusion [CSII or 'insulin pump'] therapy).

If diabetes-specific autoantibody titres are indicated, bear in mind that they have their lowest false negative rate at the time of diagnosis; the false negative rate rises thereafter.[37] Carrying out tests for two different diabetes-specific autoantibodies, with at least one being positive, reduces the false negative rate.[37]

Presence indicates autoimmune beta-cell destruction.

Result

positive

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