Tests
1st tests to order
HbA1c
Test
Confirm an abnormal result in asymptomatic individuals with a repeat HbA1c or another diabetes diagnostic test.[1] Reflects the average degree of hyperglycemia over the preceding 2-3 months.[1] Result can be affected by factors affecting hemoglobin concentrations or red blood cell turnover, and may not be a suitable diagnostic test in those with some hemoglobin variants, anemia (or recent blood loss or transfusion), or those undergoing treatment with hemodialysis, erythropoietin, or for HIV.[1] Pregnancy (second and third trimesters and the postpartum period) can also affect the result.[1] In such instances plasma glucose criteria should instead be used for diagnosis.[1] An HbA1c result that does not appear concordant with other blood glucose results should prompt consideration of potential problems or interference with either test.[1]
Result
≥6.5% (≥48 mmol/mol)
fasting plasma glucose
2-hour post-glucose load plasma glucose
Test
Plasma glucose is measured 2 hours after 75 g oral glucose load.[1]
Patients should be advised to consume a varied diet with at least 150 g of carbohydrate on the 3 days prior to testing, as fasting and carbohydrate restriction can falsely increase plasma glucose levels.[1]
Confirm an elevated result with an HbA1c (which can be done on the same sample), a second 2-hour plasma glucose, or another diabetes diagnostic test.[1]
Result
≥200 mg/dL (≥11.1 mmol/L)
random plasma glucose
Test
Confirms diagnosis in the presence of symptoms of hyperglycemia or hyperglycemic crisis (e.g., polyuria, polydipsia, blurred vision, and unexplained weight loss.[1]
Result
≥200 mg/dL (≥11.1 mmol/L)
Tests to consider
plasma or urine ketones
Test
In the presence of hyperglycemia suggest type 1 diabetes.
Result
medium or high quantity
random C-peptide
Test
C-peptide is a byproduct formed when proinsulin is processed to insulin. Therefore, its levels reflect insulin production. Half life of C-peptide is 3-4 times longer than that of insulin.
Low or undetectable C-peptide level indicates absence of insulin secretion from pancreatic beta cells.
C-peptide testing can be used when there is uncertainty about type 1 diabetes diagnosis and can help identify people who should have genetic testing for monogenic diabetes.[1]
Result
low or undetectable (<0.6 ng/mL [<200 pmol/L])
autoimmune markers
Test
These include autoantibodies to glutamic acid decarboxylase (GAD), insulin, islet tyrosine phosphatase 2 (IA-2), and the zinc transporter ZnT8.[1]
Presence of autoantibodies in an individual presenting with diabetes is in keeping with a diagnosis of type 1 diabetes.[1] However, the absence of autoantibodies does not exclude type 1 diabetes. Approximately 5% to 10% of white European people with new-onset type 1 diabetes have negative islet antibodies.[72] Furthermore, antibodies may disappear over time.[72] Further investigation is therefore necessary in patients who are suspected to have type 1 diabetes but who test negative for islet autoantibodies.
Result
positive
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