Is hemoglobin A1c (HbA1c) falsely low in patients with anemia?

HbA1c and Anemia: The Critical Relationship

Yes, HbA1c is falsely LOW in most types of anemia that shorten red blood cell lifespan, including hemolytic anemia and acute blood loss, but is falsely HIGH in iron-deficiency anemia. This is a critical clinical pitfall that can lead to missed diabetes diagnoses or inappropriate diabetes management.

The Mechanism Behind the Effect

The direction of HbA1c alteration depends entirely on red blood cell turnover dynamics:

Conditions Causing FALSELY LOW HbA1c:

Any condition that shortens RBC survival or decreases mean RBC age falsely lowers HbA1c test results, compared with mean glycemia, regardless of the assay method 1. This occurs because:

• HbA1c formation requires time—glycation accumulates as red blood cells age over their typical 120-day lifespan
• When RBCs die prematurely, there is less time for glucose to glycate hemoglobin
• The result: HbA1c underestimates true glycemic control

Specific conditions include:

• Hemolytic anemia (autoimmune, hereditary spherocytosis, G6PD deficiency) 2, 3, 4, 5
• Recovery from acute blood loss 1, 3
• Sickle cell disease 6, 7
• Pregnancy (second and third trimesters due to increased RBC turnover) 8, 6
• Hemodialysis 8, 6
• Recent blood transfusion 8, 6
• Erythropoietin therapy 8, 6

Conditions Causing FALSELY HIGH HbA1c:

Iron-deficiency anemia is reported to increase test results 1, 2, 3. The mechanism is opposite:

• Iron deficiency prolongs erythrocyte lifespan
• Longer RBC exposure to glucose allows more glycation
• Result: HbA1c overestimates glycemic control

Aplastic anemia also increases HbA1c by creating an older erythrocyte cohort 3.

Clinical Implications and Management Algorithm

When to Suspect Unreliable HbA1c:

1. HbA1c < 4.0% should serve as a clinical "red flag" requiring immediate investigation 4
2. Marked discordance between HbA1c and plasma glucose levels 1, 8, 6
3. Known hemolytic conditions or unexplained anemia
4. Reticulocytosis suggesting increased RBC turnover

Recommended Action Steps:

For patients with disorders affecting RBC turnover:

1. Do NOT use HbA1c for screening, diagnosis, or management 1
2. Use plasma glucose testing instead (fasting glucose, 2-hour OGTT) 1, 8, 6
3. Consider alternative glycated protein assays:
   • Fructosamine 1, 7
   • Glycated albumin 1, 9
   • Note: These reflect shorter periods of glycemia (2-3 weeks vs 2-3 months)

Diagnostic Workup When HbA1c is Unexpectedly Low:

If HbA1c is abnormally low (especially <4.0%):

1. Perform 75g OGTT immediately to assess true glucose tolerance 4
2. Order hematologic workup:
   • Complete blood count with reticulocyte count
   • Peripheral blood smear (look for spherocytes, schistocytes)
   • Direct antiglobulin test (Coombs test) 4, 5
   • Haptoglobin (will be low/undetectable in hemolysis) 9
   • Total and indirect bilirubin (elevated in hemolysis) 9
   • Serum ferritin and iron studies

Critical Clinical Pitfalls to Avoid

Pitfall #1: Missing Diabetes in Hemolytic Patients

Recent case reports demonstrate patients with HbA1c as low as 2.02% who actually had gestational diabetes or established diabetes with complications 4, 7. The falsely reassuring HbA1c masked significant hyperglycemia.

Pitfall #2: Hemoglobin Variants

Beyond anemia effects, hemoglobin variants (HbS, HbC, HbE) can cause spuriously high OR low results depending on the assay method 3, 6. African Americans with sickle cell trait may have HbA1c 0.3% lower than expected for their true glycemia 6.

Pitfall #3: Iron Deficiency Goes the Opposite Direction

Do not assume all anemias lower HbA1c—iron deficiency anemia falsely elevates it 1, 2, 3. This can lead to overtreatment of diabetes.

Pitfall #4: Pregnancy Considerations

Pregnancy naturally lowers HbA1c due to increased RBC turnover, independent of glucose levels 1, 3. An HbA1c <4.0% in pregnancy mandates OGTT to rule out gestational diabetes 4.

Special Populations

Chronic kidney disease patients: More frequent hemoglobin monitoring is indicated, particularly in diabetic patients who have higher anemia prevalence at all stages of CKD 10. Hemodialysis patients should not rely on HbA1c at all 8, 6.

Patients on growth hormone therapy: Require glucose monitoring, but hemolytic anemia can confound HbA1c interpretation; consider glycoalbumin as alternative 9.

Patients with compound hemoglobinopathies: (e.g., sickle cell disease + thalassemia) have multiple factors affecting HbA1c accuracy and absolutely require alternative monitoring 7.

Bottom Line for Clinical Practice

In any patient with anemia, verify the TYPE before interpreting HbA1c:

• Hemolytic/blood loss anemia → HbA1c falsely LOW → use glucose testing
• Iron deficiency anemia → HbA1c falsely HIGH → may still use cautiously
• When in doubt → confirm with fasting glucose or OGTT, not HbA1c alone

The 2023 Diabetes Care guidelines are unequivocal: glucose testing will be necessary for screening, diagnosis, and management in patients with disorders affecting red blood cell turnover 1.