What is microcytosis?

What is Microcytosis?

Microcytosis is a laboratory finding defined as a mean corpuscular volume (MCV) below the lower limit of the normal range (typically <80 fL in adults), indicating that red blood cells are smaller than normal. 1, 2, 3

Definition and Measurement

Microcytosis is identified through automated cell counters that measure red blood cell size. The condition is defined as MCV lower than the normal laboratory range, with most laboratories using <80 fL as the cutoff in adults 1, 4, 3. This is a descriptive finding, not a diagnosis in itself—it represents a red cell morphological change that requires further investigation to determine the underlying cause.

Clinical Significance and Common Causes

Microcytosis is characteristic of iron deficiency but occurs in several other important conditions 1, 2:

Most Common Causes:

• Iron deficiency anemia (most frequent cause)
• Thalassemia trait (when red cell count is usually elevated, with MCV reduced out of proportion to anemia) 1, 2
• Anemia of chronic disease (due to inability to use iron) 1, 2

Less Common Causes:

• Haemoglobinopathies (particularly in certain ethnic groups) 1, 2
• Sideroblastic anemia 2, 4
• Lead toxicity 3

Important Clinical Caveats

Microcytosis may be absent even when iron deficiency exists 1. This occurs in:

• Combined deficiencies (e.g., concurrent folate or vitamin B12 deficiency, which may normalize the MCV)
• Early iron deficiency before MCV has decreased
• These situations may be recognized by an elevated red cell distribution width (RDW) 1

Diagnostic Approach

When microcytosis is identified, the diagnostic pathway should prioritize:

1. Serum ferritin measurement (the most powerful single test for iron deficiency) 1, 2

   • <15 μg/L is highly specific for iron deficiency (specificity 0.99) 2
   • <30 μg/L generally indicates low body iron stores 2
   • In inflammation, ferritin up to 45-100 μg/L may still represent iron deficiency 2, 5

2. If ferritin is normal or elevated with microcytosis, consider:

   • Hemoglobin electrophoresis (particularly with appropriate ethnic background) to exclude thalassemia or hemoglobinopathies 2, 3
   • Transferrin saturation, serum iron, and total iron-binding capacity 3
   • Chronic disease evaluation 2

3. Mean corpuscular hemoglobin (MCH) may be a more reliable marker than MCV, as it is less dependent on storage conditions and counting machines, and shows reduction in both absolute and functional iron deficiency 2

Clinical Implications

Even microcytosis without anemia warrants attention in certain populations. In males particularly, microcytosis has been identified as a risk marker for underlying malignancy, with cancer incidence of 6.2% in males with microcytosis versus 2.7% without, even when hemoglobin is normal 6. This underscores that microcytosis should not be dismissed as clinically insignificant.

The specificity of microcytosis for iron deficiency is limited because it also occurs in thalassemia (where MCV is typically reduced out of proportion to anemia level), sideroblastic anemia, and some cases of chronic disease 2. Therefore, hemoglobin electrophoresis is recommended in those with microcytosis and normal iron studies to prevent unnecessary gastrointestinal investigation 2.