What test should be performed to recheck blood sugar (BS) levels after a Hemoglobin A1c (HbA1c) test?

Blood Sugar Testing After HbA1c

For rechecking blood sugar after a HbA1c test, fasting plasma glucose (FPG) is the preferred test due to its wide availability, low cost, and established diagnostic value. 1

Recommended Testing Options (In Order of Preference)

1. Fasting Plasma Glucose (FPG)

   • Requires 8+ hours of fasting
   • Diagnostic threshold: ≥126 mg/dL (7.0 mmol/L) for diabetes
   • Advantages: Inexpensive, widely available in most laboratories 1, 2
   • Sample must be processed promptly to prevent glycolysis 1

2. 2-hour Plasma Glucose (2-h PG) via Oral Glucose Tolerance Test (OGTT)

   • More sensitive than FPG for detecting diabetes 3
   • Diagnostic threshold: ≥200 mg/dL (11.1 mmol/L) for diabetes
   • Requires 75g glucose load and 2-hour wait time
   • More time-consuming and expensive than FPG 1

3. Random Plasma Glucose

   • Only diagnostic if ≥200 mg/dL (11.1 mmol/L) AND patient has classic symptoms of hyperglycemia 1
   • No fasting required, but less reliable for diagnosis without symptoms

Important Considerations

• Confirmation is necessary: Unless there is unequivocal hyperglycemia with classic symptoms, diagnosis requires two abnormal test results, either from the same test on different days or from two different tests 1

• When to avoid HbA1c and use glucose tests only:

  • Conditions with altered red blood cell turnover (pregnancy, hemolytic anemia, recent blood loss/transfusion)
  • Hemoglobinopathies (sickle cell disease/trait)
  • Glucose-6-phosphate dehydrogenase deficiency
  • Hemodialysis
  • Erythropoietin therapy 1

• Test discordance: If results from different tests (e.g., FPG and HbA1c) are discordant, the test with results above diagnostic threshold should be repeated 1

Diagnostic Thresholds to Remember

Test	Diabetes	Prediabetes
FPG	≥126 mg/dL (7.0 mmol/L)	100-125 mg/dL (5.6-6.9 mmol/L)
2-h PG (OGTT)	≥200 mg/dL (11.1 mmol/L)	140-199 mg/dL (7.8-11.0 mmol/L)
Random PG	≥200 mg/dL (11.1 mmol/L) with symptoms	N/A
HbA1c	≥6.5% (48 mmol/mol)	5.7-6.4% (39-47 mmol/mol)

Common Pitfalls to Avoid

• Improper sample handling: Plasma must be separated immediately after collection to prevent glycolysis which can cause falsely low glucose values 1
• Inadequate fasting: Not strictly following fasting requirements can lead to falsely elevated glucose levels 2
• Relying solely on one test: Multiple tests may be needed for confirmation, especially when values are near diagnostic thresholds 1
• Ignoring clinical context: In symptomatic patients with classic hyperglycemia, a single random glucose ≥200 mg/dL is sufficient for diagnosis 1

The evidence strongly supports using FPG as the primary follow-up test after HbA1c due to its practicality, cost-effectiveness, and established diagnostic value in clinical settings.