Blood Tests for Monitoring Blood Sugar When HbA1c is Already Prescribed
Fasting plasma glucose (FPG) is the most appropriate blood test to check blood sugar levels when HbA1c is already prescribed, as it provides complementary information about glycemic control and can help identify discrepancies between fasting levels and average glycemia. 1
Primary Testing Options
When HbA1c is already being used for long-term glycemic monitoring, the following blood glucose tests can provide additional valuable information:
1. Fasting Plasma Glucose (FPG)
- Requires 8 hours of fasting (no caloric intake)
- Diagnostic threshold: ≥126 mg/dL (7.0 mmol/L) for diabetes
- Provides information about baseline glucose control
- Helps identify overnight hyperglycemia or the dawn phenomenon
2. Postprandial Plasma Glucose (PPG)
- Measured 1-2 hours after meals
- Diagnostic threshold: ≥200 mg/dL (11.1 mmol/L) for diabetes
- Particularly useful when:
- HbA1c and FPG values show discrepancies
- Postprandial hyperglycemia is suspected
- Patient is on medications targeting postprandial glucose
Complementary Nature of Different Tests
HbA1c and blood glucose measurements provide different but complementary information:
- HbA1c: Reflects average blood glucose over 2-3 months 1
- FPG: Captures baseline glucose control and overnight regulation
- PPG: Identifies postprandial excursions that may be missed by FPG alone
When to Consider Alternative Testing
In certain conditions, HbA1c may be unreliable and more frequent blood glucose testing becomes essential:
- Hemoglobinopathies (sickle cell disease, hemoglobin variants)
- Conditions with altered red blood cell turnover:
- Pregnancy (second and third trimesters)
- Hemodialysis
- Recent blood loss or transfusion
- Erythropoietin therapy
- Iron-deficiency anemia
- HIV treated with certain medications
- Glucose-6-phosphate dehydrogenase deficiency 1
Relationship Between Different Glycemic Measures
The correlation between FPG and HbA1c varies depending on overall glycemic control:
- At lower HbA1c levels (<7.3%), postprandial glucose contributes more significantly to HbA1c
- At higher HbA1c levels (>9.3%), fasting glucose becomes the dominant contributor 2
- A moderate correlation (r = 0.65) exists between FPG and estimated average glucose derived from HbA1c 3
Important Considerations
- When marked discordance exists between HbA1c and plasma glucose levels, consider potential HbA1c assay interference 1
- FPG samples must be processed promptly (spun and separated immediately after collection) to avoid falsely low results 1
- For comprehensive assessment, both fasting and postprandial measurements may be necessary, especially in patients with suspected glycemic variability
Clinical Pitfalls to Avoid
- Don't rely solely on HbA1c: While convenient, HbA1c doesn't capture daily glucose fluctuations or hypoglycemic events
- Don't ignore discrepancies: When FPG and HbA1c values don't align, investigate potential causes rather than dismissing one test
- Don't overlook the timing of blood draws: Ensure proper fasting status for FPG (minimum 8 hours) and appropriate timing for PPG (1-2 hours post-meal)
- Don't forget to consider mean plasma glucose: The average of FPG and PPG [(FPG + PPG)/2] can be a useful metric when standardized HbA1c testing is unavailable 4