Is a patient with a Fasting Blood Sugar (FBS) level of 146 and a Hemoglobin A1c (HbA1c) of 6 considered to have diabetes?

Diagnosis: Yes, This is Diabetes

Based on the fasting blood sugar of 146 mg/dL alone, this patient meets diagnostic criteria for diabetes mellitus, as values ≥126 mg/dL establish the diagnosis. The HbA1c of 6.0% does not meet the diabetes threshold (≥6.5%), creating a discordant result that requires careful interpretation 1.

Diagnostic Criteria Analysis

Fasting Blood Sugar Assessment

• FBS of 146 mg/dL exceeds the diabetes diagnostic threshold of ≥126 mg/dL, which is one of the established criteria for diabetes diagnosis 1.
• This single elevated fasting glucose value requires confirmation with repeat testing unless the patient has classic symptoms of hyperglycemia (polyuria, polydipsia, unexplained weight loss) or is in hyperglycemic crisis 1.

HbA1c Assessment

• The HbA1c of 6.0% falls into the prediabetes range (5.7-6.4% per American Diabetes Association criteria), not the diabetes range (≥6.5%) 1, 2.
• This creates diagnostic discordance between the two tests 1.

Resolving Discordant Results

Confirmation Strategy

• When two different tests show discordant results (one above and one below diagnostic thresholds), the test exceeding the diagnostic threshold should be repeated 1.
• In this case, repeat the fasting blood sugar to confirm the diagnosis of diabetes 1.
• If the repeat FBS is ≥126 mg/dL, diabetes is confirmed regardless of the HbA1c value 1.

Why the Discordance Matters

• The HbA1c of 6.0% may indicate predominantly postprandial hyperglycemia (elevated glucose after meals) with relatively preserved fasting glucose control, though the FBS of 146 contradicts typical fasting preservation 3.
• Alternatively, this discordance suggests the patient may be in early diabetes with fasting hyperglycemia that hasn't yet significantly elevated the 2-3 month average reflected by HbA1c 1, 4.

Critical Factors to Exclude Before Finalizing Diagnosis

Conditions Affecting HbA1c Reliability

Before accepting the HbA1c as accurate, rule out 1:

• Hemoglobin variants or hemoglobinopathies (can falsely lower or raise HbA1c)
• Increased red blood cell turnover: recent blood loss, transfusion, hemolytic anemia
• Iron deficiency anemia (falsely elevates HbA1c)
• Chronic kidney disease (affects HbA1c reliability)
• Pregnancy (second and third trimesters affect HbA1c)

Verification Steps

• Confirm the FBS sample was properly collected after ≥8 hours of fasting with no caloric intake 1.
• Verify the HbA1c was measured using an NGSP-certified laboratory method 1.
• Check complete blood count to assess for anemia or conditions affecting red cell turnover 3.

Clinical Recommendation

Repeat the fasting blood sugar without delay. If the second FBS is ≥126 mg/dL, the diagnosis of diabetes is confirmed even with the HbA1c of 6.0% 1. The test result above the diagnostic threshold takes precedence when confirmed 1.

If Repeat FBS Confirms Diabetes (≥126 mg/dL):

• Diagnose diabetes mellitus and initiate management 1, 5.
• The relatively low HbA1c suggests early disease or predominantly fasting hyperglycemia without severe postprandial excursions 3, 4.
• Consider obtaining a 2-hour post-meal glucose to assess postprandial control 3.

If Repeat FBS is <126 mg/dL:

• The patient has prediabetes based on both tests (FBS 100-125 mg/dL and HbA1c 5.7-6.4%) 2.
• Prediabetes carries 10% annual progression risk to diabetes and increased cardiovascular risk 2.
• Initiate intensive lifestyle modification (≥150 min/week physical activity, calorie restriction, weight loss) or consider metformin, particularly if BMI ≥35, age <60, or FBS ≥110 mg/dL 1, 2.

Common Pitfalls to Avoid

• Do not dismiss the elevated FBS simply because HbA1c is <6.5%—the fasting glucose criterion stands independently 1, 5.
• Do not delay repeat testing—confirmation should occur promptly, not in 3-6 months 1.
• Do not assume laboratory error without verification—both values may be accurate, reflecting early or discordant diabetes patterns 1, 4.
• Do not ignore ethnic differences—African American patients may have HbA1c values 0.3-0.4% higher than non-Hispanic whites at identical glucose levels 3.