What is the recommended fasting blood glucose level to diagnose diabetes?

Diagnostic Criteria for Diabetes Using Fasting Blood Glucose

A fasting blood glucose level of ≥126 mg/dL (7.0 mmol/L) on two separate occasions is diagnostic for diabetes mellitus, provided there is no unequivocal hyperglycemia with classic symptoms. 1, 2

Diagnostic Methods for Diabetes

The diagnosis of diabetes can be established using any of the following criteria:

1. Fasting Plasma Glucose (FPG):

   • ≥126 mg/dL (7.0 mmol/L) on two separate occasions
   • Requires at least 8 hours of fasting before testing 1, 2

2. Hemoglobin A1C:

   • ≥6.5% (48 mmol/mol) on two separate occasions
   • Must be performed in a laboratory using a method certified by the National Glycohemoglobin Standardization Program (NGSP) 1

3. 2-hour Plasma Glucose during OGTT:

   • ≥200 mg/dL (11.1 mmol/L) during a 75-g oral glucose tolerance test 1

4. Random Plasma Glucose:

   • ≥200 mg/dL (11.1 mmol/L) with classic symptoms of hyperglycemia (polyuria, polydipsia, weight loss) 1

Confirmation Requirements

In the absence of unequivocal hyperglycemia with classic symptoms, results should be confirmed with repeat testing 1, 2:

• Repeat the same test on a different day
• Use a different test on a separate occasion
• Measure two different tests (e.g., glucose and A1C) on the same day

Categories of Increased Risk for Diabetes (Prediabetes)

• Impaired Fasting Glucose (IFG): 100-125 mg/dL (5.6-6.9 mmol/L) 1, 3
• Impaired Glucose Tolerance (IGT): 2-hour plasma glucose during OGTT of 140-199 mg/dL (7.8-11.0 mmol/L) 1
• A1C: 5.7-6.4% 1, 3

Special Considerations

Limitations of Fasting Glucose Testing

• 12-15% day-to-day variance in fasting blood glucose values 1
• Requires 8-hour fast before testing 1
• May miss some cases of diabetes that would be detected by OGTT or A1C 4

A1C Considerations

• Not recommended in conditions with increased red blood cell turnover:
  • Sickle cell disease
  • Pregnancy (second and third trimesters)
  • Hemodialysis
  • Recent blood loss or transfusion
  • Erythropoietin therapy 1, 2
• Marked discrepancies between A1C and plasma glucose levels should raise suspicion of hemoglobin variants 1, 2

Clinical Pitfalls to Avoid

1. Relying on a single elevated fasting glucose measurement for diagnosis (except when symptoms are present with random glucose ≥200 mg/dL) 2

2. Using point-of-care A1C testing for diagnosis instead of laboratory testing 2

3. Diagnosing diabetes based on incidental hyperglycemia during acute illness, especially in children 2

4. Failing to confirm abnormal results with a second test on a different day 1, 2

5. Not considering the limitations of A1C in certain populations with hemoglobinopathies or conditions affecting red blood cell turnover 1

Practical Approach to Diagnosis

1. Measure fasting plasma glucose after at least 8 hours of fasting
2. If FPG ≥126 mg/dL (7.0 mmol/L), repeat the test on a different day
3. If both tests show FPG ≥126 mg/dL, diagnose diabetes
4. If results are discordant, consider using A1C or OGTT as an alternative test
5. For blood sample collection, use tubes containing glycolytic inhibitors or immediately place in ice-water slurry to minimize glycolysis 2

By following these diagnostic criteria and considerations, clinicians can accurately diagnose diabetes and initiate appropriate management to reduce the risk of complications and improve patient outcomes.