Interpretation and Management of Discordant A1c and Fasting Glucose Results
You must repeat the fasting plasma glucose test immediately to confirm the diagnosis before making any treatment decisions, because a single fasting glucose of 7.5 mmol/L meets the diabetes threshold but requires confirmation per ADA standards, while your A1c of 6% falls in the prediabetes range and creates diagnostic discordance. 1, 2
Understanding the Discordance
Your results show:
- Fasting plasma glucose: 7.5 mmol/L — this exceeds the diabetes diagnostic threshold of ≥7.0 mmol/L 1
- A1c: 6% — this falls in the prediabetes range (5.7–6.4%), not the diabetes range (≥6.5%) 1
This discordance means one test suggests diabetes while the other suggests prediabetes, creating diagnostic uncertainty that mandates immediate repeat testing. 1, 2
Mandatory Confirmation Protocol
Why Confirmation is Required
- In the absence of unequivocal hyperglycemia (hyperglycemic crisis or classic symptoms with random glucose ≥11.1 mmol/L), all abnormal results must be confirmed by repeat testing to exclude laboratory error. 1, 2
- A single fasting glucose measurement has moderate test-retest variability due to biological fluctuation and potential laboratory error. 2, 3
- You cannot diagnose diabetes or initiate diabetes medications without confirmed diagnostic testing — doing so violates ADA standards and may expose you to unnecessary treatment if the initial result was spurious. 2, 3
Immediate Next Steps
Repeat the fasting plasma glucose within days to weeks (not months) using these conditions: 2, 3
- Ensure a true 8-hour fast with absolutely no caloric intake 1, 2
- Use the same certified laboratory and same methodology 2, 3
- Perform the test in a certified laboratory, not with a point-of-care glucometer 2, 4
Simultaneously obtain an A1c if not already done recently, as concordant abnormal results from two different tests can confirm the diagnosis on the same day. 1, 2
Interpretation Algorithm for Repeat Testing
| Repeat FPG Result | A1c Result (if obtained) | Diagnosis | Management |
|---|---|---|---|
| ≥7.0 mmol/L | ≥6.5% | Diabetes confirmed | Initiate diabetes management including lifestyle modification and consider metformin [2,3] |
| ≥7.0 mmol/L | 5.7–6.4% | Diabetes confirmed (FPG repeated above threshold) | Initiate diabetes management; investigate causes of A1c-glucose discordance [1,2] |
| 5.6–6.9 mmol/L | 5.7–6.4% | Prediabetes | Intensive lifestyle intervention (diet, exercise); annual monitoring; not metformin as first-line [1,2] |
| <5.6 mmol/L | <5.7% | Normal | Rescreen in 3 years [1,2] |
Causes of A1c-Glucose Discordance
When A1c and glucose results diverge, consider: 1
Conditions that falsely lower A1c:
- Hemoglobin variants (sickle cell trait, other hemoglobinopathies) 1
- Increased red blood cell turnover (recent blood loss, hemolysis, pregnancy second/third trimester) 1
- Glucose-6-phosphate dehydrogenase deficiency 1
- Erythropoietin therapy or hemodialysis 1
In these conditions, use only plasma glucose criteria for diagnosis. 1
Factors that may elevate fasting glucose acutely:
- Inadequate fasting (less than 8 hours or any caloric intake) 1, 2
- Recent illness or acute stress 1
- Medications (corticosteroids, thiazides) [general medical knowledge]
Common Pitfalls to Avoid
- Do not diagnose diabetes on your single fasting glucose of 7.5 mmol/L without confirmation, despite it exceeding the threshold. 2, 3
- Do not start metformin or other diabetes medications before confirming the diagnosis — this violates ADA standards. 2, 3
- Do not delay confirmation testing for 3–6 months — that interval is only appropriate after proper repeat testing yields persistently borderline results. 1, 2
- Do not use point-of-care glucose meters for diagnostic confirmation — only certified laboratory plasma glucose measurements are acceptable. 2, 4
- Do not assume the A1c is "wrong" — both tests measure different aspects of glycemia (long-term average vs. current fasting state), and discordance requires investigation. 1
If Repeat Testing Confirms Diabetes
Once diabetes is confirmed with repeat FPG ≥7.0 mmol/L (or concordant A1c ≥6.5%), initiate: 1
- Intensive lifestyle modification (diet, physical activity counseling with multiple contacts over extended periods) 1
- Consider metformin as first-line pharmacotherapy [general medical knowledge, ADA standards]
- Screen for cardiovascular risk factors (lipids, blood pressure, smoking) 1
- Establish monitoring schedule (A1c every 3–6 months initially) [general medical knowledge]
If Repeat Testing Shows Prediabetes
If repeat FPG falls to 5.6–6.9 mmol/L with A1c 5.7–6.4%: 1, 2