Key Takeaways

The two most common causes of microcytosis are iron deficiency and thalassemia. Both may occur in the same patient.

Iron deficiency anemia may be associated with microcytosis, hypochromia, and thrombocytosis. Thalassemia trait is also associated with microcytosis, though the red cells tend to appear well hemoglobinized with normal or near normal mean corpuscular hemoglobin concentration (MCHC), and the platelet count is normal.

For a given mean cell volume (MCV), thalassemia is associated with a higher red cell count compared to iron deficiency anemia. This is captured by the Mentzer index, one of dozens of discriminant formulas used to differentiate between the two conditions.

The fact that there are so many discriminant formulas to distinguish between iron deficiency anemia and thalassemia is testament to the fact that none is perfect!

A low ferritin is diagnostic of iron deficiency.

A high HbA2 on hemoglobin electrophoresis suggests a diagnosis of beta thalassemia, though this may be masked by concomitant iron deficiency.

Hemoglobin electrophoresis is normal in alpha thalassemia trait.

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