Laboratory Tests and Fasting Requirements for Diagnosing Diabetes
For diagnosing diabetes, order a fasting plasma glucose (FPG) requiring at least 8 hours of fasting, with a diagnostic threshold of ≥126 mg/dL (7.0 mmol/L), or alternatively use HbA1c ≥6.5% which requires no fasting. 1
Primary Diagnostic Tests
The American Diabetes Association recognizes four laboratory methods for diagnosing diabetes, each with specific fasting requirements:
Fasting Plasma Glucose (FPG) - Preferred Test
- Fasting requirement: At least 8 hours with no caloric intake 1
- Diagnostic threshold: ≥126 mg/dL (7.0 mmol/L) 1
- Timing: Blood should be drawn in the morning after overnight fast 1
- Sample handling: Use a tube with rapidly effective glycolytic inhibitor (granulated citrate buffer); if unavailable, immediately place in ice-water slurry and centrifuge within 15-30 minutes 1
- Critical caveat: Standard sodium fluoride tubes are insufficient to prevent glycolysis and should not be relied upon 1
Hemoglobin A1c (HbA1c)
- Fasting requirement: None - can be performed at any time 1
- Diagnostic threshold: ≥6.5% (48 mmol/mol) 1
- Laboratory requirement: Must be NGSP-certified and standardized to DCCT assay 1
- Important limitation: Point-of-care assays should NOT be used for diagnosis 1
2-Hour Oral Glucose Tolerance Test (OGTT)
- Fasting requirement: At least 8 hours 1
- Glucose load: 75 grams anhydrous glucose dissolved in water 1
- Diagnostic threshold: 2-hour plasma glucose ≥200 mg/dL (11.1 mmol/L) 1
- When to use: Consider when FPG is normal but clinical suspicion remains high 2
Random Plasma Glucose
- Fasting requirement: None 1
- Diagnostic threshold: ≥200 mg/dL (11.1 mmol/L) 1
- Required context: Only diagnostic when patient has classic symptoms (polyuria, polydipsia, unexplained weight loss) or hyperglycemic crisis 1
Confirmation Requirements
All positive results must be confirmed on a subsequent day with repeat testing, unless the patient presents with unequivocal hyperglycemia (≥200 mg/dL with symptoms). 1
- Confirmation can use the same test or a different diagnostic test 1
- This requirement prevents false-positive diagnoses due to laboratory error or biological variation 1
Screening Tests (Different from Diagnosis)
For screening asymptomatic high-risk individuals:
- Options: HbA1c, FPG, or 2-hour OGTT 1
- Normal screening values:
- Repeat interval: Every 3 years if screening is normal 1
Critical Technical Requirements
Sample Collection and Handling
- Venous plasma must be used for diagnostic testing - capillary blood glucose meters are inadequate for diagnosis 1
- Accredited laboratory required for all diagnostic and screening tests 1
- Proper glycolysis prevention is essential to avoid falsely low glucose readings 1
Analytical Performance Standards
- Laboratory glucose measurement should have analytical imprecision ≤2.4%, bias ≤2.1%, and total error ≤6.1% 1
- These standards minimize misclassification at diagnostic thresholds 1
Common Pitfalls to Avoid
Do not diagnose diabetes based on point-of-care glucose meters - these lack the precision required for diagnosis and should only be used for monitoring 1, 3
Do not rely on sodium fluoride tubes alone - they contain only enolase inhibitors which are too slow-acting to prevent glycolysis; use citrate buffer tubes instead 1
Do not skip confirmation testing - approximately 60% of patients diagnosed solely by the FPG criterion of 126-139 mg/dL have normal HbA1c levels, suggesting potential false positives 4
Be aware that FPG has lower sensitivity (41%) compared to 2-hour OGTT (67%) for detecting new diabetes cases, particularly in women and less obese individuals 5
Special Populations
Gestational Diabetes Mellitus (GDM)
- Timing: Screen at 24-28 weeks gestation 1
- Fasting requirement: 8 hours for diagnostic OGTT 1
- Two approaches available: