Investigations
1st investigations to order
random plasma glucose (children)
Test
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)
fasting plasma glucose (children)
Test
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)
2-hour plasma glucose (children)
Test
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)
HbA1c (children)
Test
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)
clinical diagnosis (adults)
Test
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
plasma or urine ketones
Test
In the presence of hyperglycaemia suggest type 1 diabetes.
Result
medium or high quantity
C-peptide
Test
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
autoimmune markers
Test
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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