In a 26‑year‑old woman with a low glycated hemoglobin (HbA1c) of 4.6% and an elevated mean corpuscular volume (MCV) of 100 fL, what are the likely causes of a low HbA1c and what initial work‑up should be performed?

Low HbA1c in a Young Woman with Elevated MCV

In this 26-year-old woman with HbA1c 4.6% and MCV 100 fL, the most likely cause is hemolytic anemia, which falsely lowers HbA1c by shortening red blood cell lifespan.

Primary Mechanism

Any condition that shortens RBC survival or decreases mean RBC age falsely lowers HbA1c test results compared with actual mean glycemia, regardless of the assay method used 1. The elevated MCV of 100 fL in this patient is a critical clue pointing toward a hemolytic process or other RBC turnover disorder.

Likely Causes in This Clinical Context

Hemolytic Anemia (Most Likely)

• Shortened erythrocyte lifespan directly reduces the time available for hemoglobin glycation, producing spuriously low HbA1c values 1, 2
• The elevated MCV suggests either:
  • Reticulocytosis from compensatory bone marrow response to hemolysis
  • Underlying macrocytic anemia contributing to altered RBC dynamics

Other RBC Turnover Disorders

• Recovery from acute blood loss
• Conditions affecting mean RBC age 1

Essential Initial Work-Up

Order these tests immediately:

1. Complete Blood Count with differential

   • Hemoglobin level (expect anemia if hemolysis)
   • Reticulocyte count (elevated in hemolysis) 2, 3

2. Hemolysis markers:

   • Total and indirect bilirubin (elevated in hemolysis) 2
   • Haptoglobin (decreased or undetectable in hemolysis) 2
   • Lactate dehydrogenase (LDH) - elevated in hemolysis

3. Peripheral blood smear - look for spherocytes, schistocytes, or other RBC morphology abnormalities

4. Alternative glycemic markers:

   • Fasting glucose
   • Glycoalbumin or fructosamine - these reflect shorter-term glycemic control (2-3 weeks) and are not affected by RBC turnover 1, 2

Critical Interpretation Points

HbA1c cannot be reliably used for glucose monitoring in this patient if hemolytic anemia is confirmed 1. The guideline explicitly states that HbA1c measurements in individuals with disorders affecting red blood cell turnover provide spurious (generally falsely low) results regardless of the method used, and glucose testing will be necessary for screening, diagnosis, and management 1.

Mean Corpuscular Hemoglobin (MCH) Effect

Research shows that elevated MCV and MCH correlate negatively with HbA1c 4. For at least 25% of patients, high MCV levels are associated with increased risk of erroneous HbA1c-based identification of glycemia status 4. After adjustment for fasting glucose, a 1 pg increase in MCH reduced the odds of HbA1c-defined dysglycemia by 10-14% 4.

Additional Considerations

If Hemolysis is Confirmed:

• Consider hereditary spherocytosis (especially relevant given case reports in young patients) 2
• Perform osmotic fragility testing if spherocytosis suspected
• Consider Coombs testing to rule out autoimmune hemolytic anemia 5

Other Potential Causes to Exclude:

• Iron deficiency (paradoxically increases HbA1c, so unlikely here) 1
• Vitamins C and E supplementation (may falsely lower HbA1c) 1
• Liver disease (10/17 patients with HbA1c <4% had hepatopathy in one series) 3
• Hypoglycemia (rare but consider insulinoma if recurrent hypoglycemic episodes) 6

Common Pitfalls to Avoid

1. Do not assume good glycemic control based solely on low HbA1c in the presence of elevated MCV
2. Do not use HbA1c for diabetes screening or monitoring until RBC turnover disorder is excluded or ruled out
3. Do not delay ordering alternative glycemic markers - glycoalbumin or fructosamine should be ordered simultaneously with the hemolysis work-up 1, 2

Switch to glycoalbumin or fructosamine for ongoing glucose monitoring if hemolytic anemia is confirmed, as these assays of other glycated proteins reflect a shorter period of average glycemia and are not compromised by RBC turnover abnormalities 1.