What are the diagnostic tests for diabetes in an 18-year-old patient?

Diagnostic Testing for Diabetes in an 18-Year-Old

For an 18-year-old, diagnose diabetes using any one of three tests: fasting plasma glucose ≥7.0 mmol/L (126 mg/dL), 2-hour plasma glucose ≥11.1 mmol/L (200 mg/dL) during a 75-g oral glucose tolerance test, or A1C ≥6.5% (48 mmol/mol), with confirmation required by repeat testing unless the patient has unequivocal hyperglycemia. 1, 2

Primary Diagnostic Tests

All three tests are equally appropriate for diagnosis in this age group 1:

• Fasting Plasma Glucose (FPG): ≥7.0 mmol/L (126 mg/dL) after no caloric intake for at least 8 hours 1, 2, 3

  • This is the preferred screening test because it is faster, easier to perform, more convenient, and less expensive than OGTT 1, 3
  • Must be performed on venous plasma samples in an accredited laboratory 2

• 2-Hour Plasma Glucose During 75-g OGTT: ≥11.1 mmol/L (200 mg/dL) 1, 2, 3

  • The test uses 75 grams of anhydrous glucose dissolved in water 1
  • Patient must fast for 8 hours before testing 2
  • More sensitive than FPG alone—detects approximately 66% of new diabetes cases compared to only 41% with FPG criterion 4

• A1C: ≥6.5% (48 mmol/mol) 1, 2, 3

  • Must be performed using a method certified by the National Glycohemoglobin Standardization Program (NGSP) and standardized to the Diabetes Control and Complications Trial (DCCT) assay 1, 2
  • Offers greater convenience as fasting is not required 1

Confirmation Requirements

In the absence of unequivocal hyperglycemia, a second test is required for confirmation 1, 2, 3:

• Repeat the same test on a different day (e.g., if A1C is 7.0% and repeat is 6.8%, diabetes is confirmed) 1
• Alternatively, perform a different test above the diagnostic threshold (e.g., if A1C is 7.0% and FPG is ≥7.0 mmol/L, diagnosis is confirmed) 1, 2
• If two different tests show discordant results, repeat the test that was above the diagnostic threshold 1, 2

Exception: If the patient has classic symptoms of hyperglycemia (polyuria, polydipsia, unexplained weight loss) or is in hyperglycemic crisis with a random plasma glucose ≥11.1 mmol/L (200 mg/dL), diagnosis is confirmed immediately without need for repeat testing 1, 2, 3

Critical Situations Where A1C Should NOT Be Used

Use only plasma glucose criteria (FPG or OGTT) in the following conditions 1, 2, 3:

• Sickle cell disease or other hemoglobinopathies 1, 2
• Pregnancy (second and third trimesters) 1, 2
• Recent blood loss or transfusion 1, 2
• Hemodialysis 1, 2
• Erythropoietin therapy 1, 2
• Conditions with increased red blood cell turnover 1

Important Caveats and Pitfalls

• Marked discordance between A1C and plasma glucose levels should raise suspicion of A1C assay interference due to hemoglobin variants—consider using an assay without interference or switch to plasma glucose criteria 1, 2, 3

• The three tests do not necessarily detect diabetes in the same individuals—concordance between FPG, 2-hour PG, and A1C is imperfect 1, 2

• Age considerations: While the epidemiological studies supporting A1C diagnostic criteria included only adults, it remains unclear whether the same A1C cutpoint should be used in adolescents 1. However, current guidelines support using A1C ≥6.5% for diagnosis in 18-year-olds 1

• Race/ethnicity factors: A1C may be affected by race/ethnicity independently of glycemia, which should be considered when interpreting results 1

• Sample handling: Blood must be collected in tubes with glycolytic inhibitors and processed promptly to avoid falsely low glucose values 3