What other lab test should be performed to confirm changes in glycemic control if Hemoglobin A1c (HbA1c) has been completed?

Fasting Plasma Glucose (FPG) and 2-hour Postprandial Glucose (2-h PG) Tests Are the Most Appropriate Lab Tests to Confirm Changes in Glycemic Control After HbA1c

When HbA1c has been completed and you need another lab test to confirm changes in glycemic control, fasting plasma glucose (FPG) and/or 2-hour postprandial glucose (2-h PG) should be used as they provide complementary information about glycemic status and have established diagnostic thresholds. 1

Primary Options for Confirming Glycemic Control Changes

Fasting Plasma Glucose (FPG)

• Requires patient to fast for at least 8 hours
• Diagnostic threshold: ≥126 mg/dL (7.0 mmol/L) for diabetes
• Advantages:
  • Standardized collection protocol
  • Well-established reference ranges
  • Less day-to-day variability than postprandial measurements
• Limitations:
  • May not capture postprandial excursions
  • Correlation with HbA1c is moderate (r = 0.61) 2

2-hour Postprandial Glucose (2-h PG)

• Measured 2 hours after meal or during oral glucose tolerance test (OGTT)
• Diagnostic threshold during OGTT: ≥200 mg/dL (11.1 mmol/L) for diabetes
• Advantages:
  • Better correlation with HbA1c (r = 0.68) than FPG 2
  • More sensitive for detecting glycemic abnormalities
  • Better predictor of overall glycemic control 3
• Limitations:
  • Requires standardized timing and conditions
  • More inconvenient for patients

Algorithm for Test Selection

1. If concerned about overall glycemic control:

   • Choose 2-h PG as it shows stronger correlation with HbA1c and better sensitivity for detecting glycemic abnormalities 2, 3

2. If concerned about baseline glucose levels:

   • Choose FPG for its standardized protocol and established diagnostic thresholds

3. For most comprehensive assessment:

   • Perform both FPG and 2-h PG tests, as the concordance between these tests and HbA1c is imperfect 1

Special Considerations

When to Avoid Relying on HbA1c Alone

In certain conditions, HbA1c may be unreliable and plasma glucose measurements become essential:

• Hemoglobinopathies (e.g., sickle cell disease/trait)
• Altered red blood cell turnover conditions:
  • Pregnancy (second and third trimesters)
  • Glucose-6-phosphate dehydrogenase deficiency
  • Hemodialysis
  • Recent blood loss or transfusion
  • Erythropoietin therapy
  • Iron-deficient anemia
  • HIV treated with certain medications 1

Patterns of Discordance

Be aware that discordance between HbA1c and plasma glucose values is common:

• 21.7% of patients may show postprandial hyperglycemia despite normal FPG 4
• 36.7% may have high pre-meal and post-meal glucose with normal FPG 4

Implementation Tips

• Ensure adequate carbohydrate intake (at least 150 g/day) for 3 days prior to glucose testing 1
• For FPG, samples must be centrifuged and separated immediately after collection to prevent falsely low results 1
• When results are near diagnostic thresholds, repeat testing in 3-6 months 1
• For discordant results between different tests, repeat the test that shows abnormal values 1

Common Pitfalls to Avoid

• Don't rely solely on FPG: Studies show postprandial glucose values are better predictors of glycemic control than fasting values alone 3
• Don't interchange HbA1c and glucose values: While correlated, they provide different information about glycemic status 5
• Don't ignore racial/ethnic variations: African Americans may have higher HbA1c for the same mean glucose concentration 1
• Don't neglect proper sample handling: Glucose samples left at room temperature can produce falsely low results 1

By using both FPG and 2-h PG alongside HbA1c, you'll gain a more comprehensive picture of glycemic control, allowing for better-informed treatment decisions and improved patient outcomes.