When should FPG be used instead of HbA1c for diabetes screening in a healthy adult?

When to Use FPG vs. HbA1c for Diabetes Screening in a Healthy 35-Year-Old

For diabetes screening in a healthy 35-year-old, fasting plasma glucose (FPG) should be used when there are conditions that affect red blood cell turnover or hemoglobin variants, while HbA1c is preferred in most other cases due to greater convenience. Using both tests together provides the most comprehensive assessment for individuals with multiple risk factors.

General Screening Recommendations

The American Diabetes Association (ADA) recommends beginning diabetes screening at age 35 for all adults, regardless of risk factors 1. For a healthy 35-year-old:

• Either FPG, HbA1c, or 2-hour plasma glucose (2-h PG) during oral glucose tolerance test (OGTT) are appropriate screening tests 1
• If initial results are normal, testing should be repeated at minimum 3-year intervals 1
• More frequent testing is recommended if additional risk factors develop

When to Use FPG Instead of HbA1c

FPG should be used instead of HbA1c in the following situations:

1. Conditions affecting red blood cell turnover:

   • Pregnancy (especially second and third trimesters)
   • Recent blood loss or transfusion
   • Erythropoietin therapy
   • Hemolysis 1, 2

2. Hemoglobinopathies and anemias:

   • Sickle cell disease/trait
   • Thalassemias
   • Hemolytic anemias
   • Iron deficiency anemia 1, 2

3. Ethnic considerations:

   • In certain populations where hemoglobinopathies are common
   • When there's marked discordance between measured A1C and plasma glucose levels 1

When to Use HbA1c Instead of FPG

HbA1c is preferred in these scenarios:

1. Convenience factors:

   • When fasting is impractical or difficult for the patient
   • Greater preanalytical stability
   • Less day-to-day perturbations during periods of stress and illness 1

2. Patient compliance concerns:

   • When patient is unlikely to return fasting for follow-up testing 1

When to Use Both Tests Together

Both FPG and HbA1c should be ordered in the following situations:

1. Multiple risk factors present:

   • BMI ≥25 kg/m² (≥23 kg/m² in Asian Americans)
   • First-degree relative with diabetes
   • High-risk race/ethnicity (African American, Latino, Native American, Asian American, Pacific Islander)
   • History of cardiovascular disease
   • Hypertension (≥130/80 mmHg or on therapy)
   • HDL cholesterol <35 mg/dL and/or triglycerides >250 mg/dL
   • Polycystic ovary syndrome
   • Physical inactivity
   • Other conditions associated with insulin resistance 1

2. Discordant results:

   • When previous test results are inconsistent or borderline 3

3. Highest diagnostic accuracy:

   • Combined use of HbA1c and FPG provides better detection of individuals at highest risk 3

Important Considerations

• HbA1c has slightly lower sensitivity but higher specificity than FPG for detecting diabetes 4
• An HbA1c >6.0% has been shown to be 100% specific for diabetes diagnosis, while <5.2% reliably excludes it 5
• Point-of-care A1C assays should not be used for diagnosis due to potential accuracy limitations 1, 2
• FPG requires proper fasting (no caloric intake for at least 8 hours) 1
• When using OGTT, adequate carbohydrate intake (at least 150 g/day) should be assured for 3 days prior to testing 1

Diagnostic Criteria

• Diabetes: FPG ≥126 mg/dL (7.0 mmol/L) or HbA1c ≥6.5% (48 mmol/mol)
• Prediabetes: FPG 100-125 mg/dL (5.6-6.9 mmol/L) or HbA1c 5.7-6.4% (39-47 mmol/mol) 1

In the absence of unequivocal hyperglycemia, diagnosis requires abnormal results on two different tests or two abnormal results from samples obtained on different days 1.