Diagnosis Confirmation: Repeat Fasting Glucose
Based on the discordant test results (fasting glucose 7.4 mmol/L meeting diabetes criteria but HbA1c 6.3% in the prediabetes range), you should repeat the fasting glucose test to confirm the diagnosis of diabetes (Answer D). 1
Rationale for Repeat Testing
When two different diagnostic tests yield discordant results, guidelines mandate that the test whose result exceeds the diagnostic threshold should be repeated for confirmation. 1
- The fasting glucose of 7.4 mmol/L exceeds the diabetes diagnostic threshold of ≥7.0 mmol/L (126 mg/dL) 1
- The HbA1c of 6.3% falls in the prediabetes range (5.7-6.4%) and does not meet the diabetes diagnostic criterion of ≥6.5% 1
- In the absence of unequivocal hyperglycemia, any single abnormal test result must be confirmed by repeat testing 1
Why Not the Other Options
Option A (Repeat HbA1c after 6 weeks) is incorrect because:
- The HbA1c is already below the diagnostic threshold for diabetes 1
- When results are discordant, you confirm the test that exceeds the diagnostic cutpoint, which is the fasting glucose in this case 1
- HbA1c reflects 2-3 months of glycemic control, so repeating it after only 6 weeks provides limited additional information 1
Option B (Random blood glucose) is incorrect because:
- Random glucose is only diagnostic when ≥11.1 mmol/L (200 mg/dL) with classic symptoms of hyperglycemia 1
- This patient is asymptomatic and presenting for screening 1
- Random glucose adds no value when you already have a fasting glucose result that needs confirmation 1
Option C (2-hour 75g OGTT) is less appropriate because:
- While OGTT is a valid diagnostic test, it is more cumbersome and resource-intensive 1
- The most straightforward approach when a fasting glucose exceeds the diagnostic threshold is to simply repeat the fasting glucose 1
- OGTT would be reserved for situations where fasting glucose and HbA1c remain inconclusive after repeat testing 1
Clinical Interpretation of Discordant Results
The 2010 ADA guidelines specifically address this scenario: "If a patient meets the diabetes criterion of the A1C (two results ≥6.5%) but not the FPG (≥126 mg/dL or 7.0 mmol/L), or vice versa, that person should be considered to have diabetes." 1
- However, this applies when both tests have been confirmed with repeat testing 1
- In your patient, neither test has been repeated yet, and only one (fasting glucose) exceeds the diagnostic threshold 1
- The healthcare professional should repeat the abnormal test within 3-6 months to establish the diagnosis 1
Practical Considerations
Factors that may explain the discordance include: 1
- Preanalytical variability in glucose measurement (glycolysis in the sample tube) 1
- Recent acute illness or stress affecting glucose levels 1
- Racial/ethnic differences in HbA1c levels for a given glycemia (African Americans may have HbA1c 0.4% higher than whites for the same glucose levels) 1
- Conditions affecting erythrocyte lifespan that alter HbA1c (iron deficiency, hemolytic anemia, chronic kidney disease) 1
For the repeat fasting glucose: 1
- Ensure the patient fasts for at least 8 hours 1
- Draw blood in the morning 1
- Use a tube with rapidly effective glycolytic inhibitor (granulated citrate buffer) and process within 15-30 minutes 1
- Measure in an accredited laboratory, not point-of-care 1
Next Steps After Confirmation
If the repeat fasting glucose is ≥7.0 mmol/L: 1
- Diagnose diabetes even though HbA1c is <6.5% 1
- The patient has a family history of diabetes, which is a significant risk factor 1, 2
- Initiate diabetes management including lifestyle modifications and consider metformin 3, 4
If the repeat fasting glucose is <7.0 mmol/L but ≥5.6 mmol/L: 1